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HomeMy WebLinkAbout07272004 BSC Agenda Item 1 RECOMMENDED SCOPE OF GEOTECHNICAL ENGINEERING SERVICES FOR THE ' CITY OF WEST UNIVERSITY MAY 2004 By David Eastwood,P.E. Updated 07-27-04 Foundations Foundations for residences in the City of West University will consist of either a structural slab or slab- on-fill type foundation systems. The structural slab foundation system may consist of a pier and beam with a crawl space or slab with void underneath it and supported on piers. In all cases,piers will be used to support the superstructure. Geotechnical Exploration The geotechnical exploration for residences in West University should be conducted by an accredited geotechnical engineering firm. The firm should be accredited by American Association of Laboratory Accreditation (A2LA) in geotechnical engineering. The accreditation should be current. A copy of the accreditation in geotechnical engineering should be placed as an appendix of the report. The firm should have a minimum of$500,000 errors and omissions insurance (professional liability insurance). Geotechnical Borings There shall be a minimum of two borings required for houses with slabs of up to about 3,000 sq. ft. Additional borings should be conducted for any additional 1500 sq. ft. of the floor slab areas. This includes the garage area as well. All borings will be a minimum of 25-ft deep. However, 20-ft borings are allowed if a portable rig has to be used. The geotechnical borings for additions to an existing residence will consist of one boring for additions of 1000 sq. ft. or less. Additions greater than 1000 sq. ft. will require two borings. The boring depth will be the same as above. Laboratory Testing The laboratory testing will be determined by the geotechnical engineer. Some of these tests shall include Atterberg limits, unconfined compression, torvane, hand penetrometer, soil suction, grain size analyses, etc. Minimum testing will be as follows: Minimum Number Test Type ASTM Standard of Tests per Boring Moisture Content ASTM D-2216 4 Atterberg Limits ASTM D-4318 2 Torvane ---- 5 Hand Penetrometer ---- 5 Unconfined Compression ASTM D-2166 2 Swell Tests ASTM D4546 As needed Suction ASTM 5298 As needed Sieve Analysis ASTM D-422 As needed Engineering Report The geotechnical engineering report should include recommendations on the recommended foundation types. In addition, the report should discuss various foundation risks. The recommended pier depths should be specifically spelled out. Furthermore, the recommended fill thickness should clearly be presented. In addition, on sites where trees are present, the effected tree removal and/or dying needs to be considered in the evaluation of the recommended foundation system and its components. Furthermore, a picture of the site should be presented in the geotechnical report. In general, the geotechnical report should include the following: o Introduction. o Field Exploration Procedures. o Laboratory testing. o Site conditions. o Soil Stratigraphy. o Groundwater. o Foundations and Risks. o Foundation Types, depth and allowable by pressures. This includes structural slab supported on piers, slab-on-fill supported on piers, floating slab recommendations. o Estimated settlements and potential vertical rise (PVR). o Construction considerations, including earthwork and testing requirements. o Vegetation control, including the effects of existing trees, tree removal,tree dying and new trees. o Site drainage. o Use of on-site soils for fill. o Limitations. o Site plan o Boring logs, including the depth of root fibers. o Homeowner maintenance program. o Project site pictures. The geotechnical report should also include a copy of the A2LA Accreditation certificate in geotechnical engineering and a copy of errors and omissions insurance certificate. • 1 Quality Control and Testing • The quality control and testing during construction for a residence should be conducted by the same firm that conducted the initial geotechnical report. This work will include earthwork testing, drilled footing observations, and concrete testing. The subgrade soils should be proofrolled and compacted to 95 percent maximum dry density degree by ASTM D698 at moisture content with optimum and+3 percent. All fill soils placed underneath the floor slabs should consist of structural fill with liquid limit less than 40 and plasticity index between 12 and 20. These soils should be compacted to a minimum of 95% maximum dry density (ASTM D698). Moisture contents between optimum and +3%. All the fill soils should be tested, at a minimum of five locations within the floor slab areas at each eight-inch lifts. The subgrade soils should be proofrolled and tested for density at five locations prior to fill placement. The subgrade soils should have the same level of compaction as the fill soils. This information should be provided at the completion to the client, the city building official as well as the structural engineer. The construction of all the piers should be observed by the geotechnical engineer of the record or the structural engineer of the record. The piers should be check for proper depth, soils stratum, proper soil strength, the correct bell size and steel placement, steel size and grade. The concrete for the drilled footings slab should be tested in compression. A minimum of four concrete cylinders should be made for every 50 yards of pour or portions of. Two of the cylinders will be broken at 7 days and two at 28 days. Submittals A copy of the geotechnical report together with all field testing and observations should be sent to the structural engineer of the record, the client, as well as the building official. All failed tests should be retested until passing results are achieved. The reports should include the following: o Initial geotechnical report o Proofrolling observations report of subgrade o Proctor test reports o Subgrade density reports o Fill compaction reports per all lifts o Drilled footing installation report o Concrete compression test reports for the drilled footing and the slab o A2LA Accreditation certificates in geotechnical engineering o A2LA Accreditation certificates in materials engineering o Errors and omission insurance certificates for a minimum of$500,000 annual claims made. All reports should be signed and sealed by a licensed professional engineer in the State of Texas. The laboratory conducting the testing should be accredited by the American Association of Laboratory Accreditation (A2LA) in Construction Materials Testing. A copy of the accreditation certificate should accompany the submittals. Word\marketing\westuniversity A RE: Draft West U Foundation.Ordinance Page 1 of 3 1s B. B. S l i m p From: Ron Kelm [RonKelm @ForensicEng.com] Sent: Monday, July 26, 2004 2:39 PM To: B. B. Blimp Subject: RE: Draft West U Foundation Ordinance Dear Bryant, At your request, I attended your meeting on 1 July 2004. As since requested, I summarized the changes I recommended to the proposed ordinance you discussed during that meeting. Attached are my proposed changes in both Word and Acrobat formats. Notice under 3.h.1, I have a question for you on whether you specify somewhere in your ordinances what design life is required of residential structures in West U. As I stated in your 1 July meeting, you will not be able to limit movement of the soil to 1". It is theoretically possible to limit movement of the foundation(not the soil)to 1", but you will no longer be able to allow slab on grade foundations in your city, only slabs on deep footings. I will be happy to provide you further details on this if needed. You also requested I provide a source and cost of obtaining a soil report in accordance with Document No. FPA-SC-04-0. Three years ago I asked several local geotechnical engineers to estimate the cost for doing a geotechnical investigation in accordance with this document. I no longer have the estimates but seem to remember it costing about $400Qfor single family custom homes like yours. I remember that both Geotech Engineering and Henderson Engineering said they could provide services using this document. Geotech has provided these services to me since then, however it was a forensic job and out of town. I did recently have a Dallas geotechnical engineer(Bryant Consultants)provide the services for a new home design in Houston and have asked and obtained permission from them and the homeowner to send you their soil report. This attached report follows the FPA-SC-04-0 document but also provides additional information that I feel is necessary today, and which is included in my attached comments in section 3.b.4 and 3.b.5. Note that their report also has other information provided including an external benchmark and geophysical testing (i.e., GMMIR). I do not feel these are necessary in all cases and should therefore not be part of your requirements. The cost for Bryant Consultants to duplicate the services in their report on a house by house basis in your city is between $3750 and $4000. This estimate from them does not include mobilization fees from Dallas nor the extra geophysical testing and benchmark. Thus it gives you a real cost of locally obtaining geotechnical information in accordance with FPA-SC-04 but with my additional criteria of deeper borings and a aerial photo. If you hear higher estimates, it may be because some of them have to finance suction equipment and swell test equipment that they do not already own. This equipment can be purchased for about$4000 to $8000. I applaud you for taking these honorable steps to make the foundation design and construction industry do the work we need to do so we can mitigate foundation problems in West U. I know you will have to withstand criticism from those wishing to keep the status quo. Thank you for standing up to this criticism and for taking this giant step to serve the public and preserve their welfare. I and other structural engineers appreciate your time and dedication to help us get the information we need in order 7/26/2004 RE: Draft West U Foundation Ordinance Page 2 of 3 to do a proper job in design foundations. If you would like me to attend a future workshop on this subject,please send me an email. I do not think your city's website shows your meeting schedule and I am unable to come by regularly to view the physical posting you may have outside city hall. Best Regards, Ron Kelm,P.E. Forensic Engineers Inc. 9930 Shadow Wood Dr. Houston, Texas 77080-7110 USA T: 713-468-8100 F: 713-468-8184 E: ronkelm @forensiceng.com From: "B. B. Slimp" <bbslimp @swbell.net> To: "Ron Kelm" <ronkelm @forensiceng.com> Subject: Draft West U Foundation Ordinance Date: Thu, 1 Jul 2004 08:23:07-0500 X-Priority: 3 (Normal) Importance: Normal X-Loop-Detect:1 Status: RON- - - This is the latest draft of the revisions to the foundation sections of the Standard Code that we are considering for West University in the Building Standards Commission. It is the topic of my phone messages to you. I would appreciate your comments on these as a structural engineer. Please feel free to call me at the numbers below. We are having a BSC meeting tonight at 6:00 PM in the Watson conference room (across the hall from the City Council chambers in the West U City Hall) if you would like to come. We will be discussing these revisions as an agenda item, and I welcome your comments there if you would like to attend. If you can not attend, I would like to meet you in person or via phone at a time that would be convenient to you to obtain your comments on this draft ordinance. Regards, Bryant Slimp(BSC Chair) Office: 713 / 839 - 9910 Fax: 713 / 839 - 9920 Cell: 713 / 725 - 7805 Pager: 713 /602 - 4758 7/26/2004 - RE: Draft West U Foundation Ordinance Page 3 of 3 Attachment converted: 17" PB-80:54 o foundation 6-28-04.pdf(PDF /CARO) (001B8FF7) 7/26/2004 Comments by Ron Kelm, RE. 26Juiy 2004 Page 1 of 15 Comments to West University Ordinance Proposed by Building Standards Commission and Discussed at 1 July 04 Meeting 3. All foundations for new buildings (or additions to existing buildings) with a gross floor area 485 square feet or more must meet the following criteria, as applicable. a. Engineering. Foundations must be constructed in accordance with complete plans and specifications prepared, signed and sealed by an licensed or registered professional engineer who is currently licensed by the Texas Board of Professional Engineers and who is: (1) employed by a registered engineering firm currently registered by the Texas Board of Professional Engineers and (2) covered by professional liability (errors and omissions)insurance with limits of a least $500,000 per year, aggregate ("RLPE"). The plans and specifications must be prepared specifically for the site of the work, and they must meet criteria as to scope, content and form specified by the building official. b. Geotechnical Report. The plans and specifications for each foundation must be based on a written geotechnical report prepared, signed and sealed by an engineer who is currently licensed by the Texas Board of Professional Engineers and who is:a licensed (1) employed by a engineering firm currently registered by the Texas Board of Professional Engineers and--- ; - ; - • -- - -_- . . -- (2) covered by professional liability (errors and omissions) _ . insurance with limits of at least$500,000 per year, aggregate. - -% The site investigation,testing and report must meet all applicable criteria in "Recommended Practice for Geotechnical Explorations and Reports" published by the Structural Committee of the Foundation Performance Association,Houston, Texas (Document# FPA-SC- 04-0, Rev#0, 11 April 2001, issued for website publishing), a copy of which is on file in the City Secretary's office. However,the following additional criteria shall also be met: (3) The minimum depth of all borings is 20 feet-in all cases. (4) At least one boring shall be 35 feet deep if the foundation is to be supported below existing grade at a depth of 6 feet or greater. (5) The report shall include a recent aerial photo that shows prior land use and the location of the proposed foundation (6) The validity of the report shall be as specified in the geotechnical report but no more than 6 months. If the foundation construction is to begin after this validity period, sealed certification letters are required by both the geotechnical engineer of record and the foundation design engineer of record that the report recommendations and design are either still applicable or else new designs are to be submitted. (7) All tests and other laboratory work must be performed by a laboratory accredited for such work by the American.Association for Laboratory Accreditation on the basis of ISO/IEC 17025:1999 ("general requirements Comments by Ron Kelm, RE. 26July 2004 Page 2 of 25 for the competence of testing and calibration laboratories"). (8) The potential vertical movement of the soil, both up and down, shall be estimated for both normal (expected) and extreme(boundary)conditions, using at least two of the following methods: e. Foundation Performance Standard. The plans a - a - -: a -a. a musfbe-prepafed-to--ac- - - 4-potential of-one4n a - , - - by the estimated depth-of the-aetive zo ion with at 1-ast two of the 41■. (4)Potential vertical rise (PVR)determined in accordance with Test Method Tex-124-E, Rev. January 1, 1978/December 1982, Texas State Department of Highways and Public Transportation, Materials and Test Division, "Method for Determining the Potential Vertical Rise, PVR"(a copy of which is on file in the office of the City Secretary). For this purpose,the"dry" moisture condition(from which little shrinkage is experienced, but where volumetric swell potential is greatest) shall be used for each sample and test. (ii.2) Swell tests performed in accordance with ASTM D4546-03, "Standard Test Methods for One-Dimensional Swell or Settlement Potential of Cohesive Soils" as last revised prior to June 1, 2004. (iii ) Suction and hydrometer swell tests performed in accordance with A.STM D5298-03 "Standard Test Method for Measurement of Soil Potential (Suction)Using Filter Paper"and ASTM D6836-02 "Standard Test Methods for Determination of the Soil Water Characteristic Curve for Desorption Using a Hanging Column, Pressure Extractor, Chilled Mirror Hygrometer, and/or Centrifuge," as such methods were last revised prior to June 1, 2004. c. Foundation Drawings. The foundation design engineer of record shall show on the design plans and specifications: (1) The Geotechnical Report number, report date, and date of the site exploration (2) The estimated depth of the active zone, i.e. the depth of zero movement (3) The weighted plasticity index of the upper 15 ft of soil computed in accordance with B.R.A.B. #33 (4) The maximum differential movement(as measured with a level)the foundation design can withstand in centerlift and edgelift conditions as defined by the Post-Tensioning Institute in their latest edition of"Design and Construction of Post-Tensioned Slab-On-Ground," whether the foundation is founded at grade or at depth and independent of the type of reinforcing used in the design. d. Foundations, Basic Type. Each foundation must be of an approved basic type. Approved basic types are listed below. In this list,types of foundations are defined and described in "Foundation Design Options For Residential and Other Low-Rise Buildings on Expansive Soils"published by the Structural Committee of the Foundation Performance Association, Houston, Texas (Document# FPA-SC--01-0X, Comments by Rory Kelm, P.E. 26July 2004 Page 3 of 35 Rev#0X, -1-A r-i 130 June 2004, issued for FPA-peer reviewwebsitepublishing), a copy of which is on file in the City Secretary's office. (1) Structural slab with void space and deep foundations. (2) Structural floor with crawl space and deep foundations. (3) Stiffened structural slab with deep foundations. (4) Stiffened non-structural slab with deep foundations. (5) Grade-supported stiffened structural slab. (6)Grade-supported stiffened non-structural slab. (7)Grade-supported non-stiffened slab of uniform thickness (approved one- story buildings--or additions to buildings--containing only storage space, not living space). (8)Mixed-depth system for all new building construction. (9)Mixed-depth system for building additions with deep foundations. (10)Mixed-depth system for building addition with shallow foundations. (11)Another type approved by special exception issued by the BSC. below. e. Foundations, Deep Support Components. Deep support components must be of an approved type. Approved types are listed below. In this list, types support components are defined and described in"Foundation Design Residential and Other Low-Rise Buildings on Expansive Soils"published by the Structural Committee of the Foundation Performance Association, Texas (Document#FPA-SC-01-0#,Rev#0X, 30 Junei---April 2004, issued for reviewwebsite publishing), a copy of which is on file in the City Secretary's office. (1)Drilled and underreamed concrete piers. (2)Drilled straight-shaft concrete piers. (3)Auger-cast concrete piles. (4)Another type approved by special exception issued by the BSC. below. f. Foundations, reinforcement. Reinforcement for each foundation must of an approved type. Approved types are listed below. In this list, types reinforcement are defined and described in "Foundation Design Options Residential and Other Low-Rise Buildings on Expansive Soils"published Structural. Committee of the Foundation Performance Association, Texas (Document# FPA-SC-01-X0, Rev#OX, 1 Apri130 June 2004, issued for review), a copy of which is on file in the City Secretary's office. (1)Deformed bar reinforcing. (2) Welded wire fabric reinforcing(approved for one-story buildings--additions to buildings--containing only garage or storage space). (3)Another type approved by special exception issued by the BSC. below. Foundations, Observation & Certification. Each foundation must be professionally observed and must be certified by an RLPE, as more described below: (1) Observations must: (i) be performed either by the certifying RLPE or by-a that RLPE's direct supervision and control whose professional qualifications are approved by the RLPE, Comments by Ron Kelm, P.E. 26July 2004 Pane 4 of 46 (ii) include actual measurement of pier holess, fill, compaction, reinforcement, forms, materials, dimensions, structural elements, stressing, tendons,tensions, attachments, etc. before the work is covered or concrete is placed, (iii) be performed continuously during placement of concrete and any stressing or tensioning operations, and (iv) be documented in a form and manner approved by the building official (which may include photographs). (2)Certifications must: (i) refer to and be based upon the professional observations required by this section, (ii) state that the work complies with the plans and specifications last approved by the building official (with any field changes that are ordered by the RLPE and reported to the building official and that comply with applicable regulations), (iii) state that the work complies with sound engineering practices, (iv) comply with criteria as to form and content as may be specified by the building official, (v) be signed and sealed by the certifying RLPE, and (vi) be filed with the building official. Before framing or other work commences atop a foundation(and before the foundation is otherwise covered), the permittee must obtain written acknowledgment from the Building official that the certification for the foundation was duly filed as required above. h. The BSC may issue a special exception from any requirement in subsection"a" through"g," above, but only upon a showing that: (1) the requirement will not affect life safety or the performance of a structure (for its estimated useful life HOW MANY YEARS IS THIS? WHERE IS IT DEFINED OR SPECIFIED?; ), or (2) an alternate requirement to be imposed by the special exception will provide equal or better protection for life safety and long-term structural performance. in connection with any such special exception,the BSC may require that the applicant provide supporting engineering data and opinion, and the BSC may impose conditions to carry out the purpose and intent of applicable regulations. 4. All concrete piers, footings and foundations must be cured for at least 72 hours before any significant load is placed on them. S. All walls and ceilings ..... Comments by Ron Kelm, P.E. 26July 2004 Page 5 of 55 Respect!idly submitted by: Ron Kelm, P.E. # 44898, State of Texas FORENSIC ENGINEERS INC. 9930 Shadow Wood Dr. Houston TX 77080 W: 713-468-8100 F: 713-468-8184 C.. 713-443-6323 E: ronkelm@f6rensieeng.com RECOMMENDED SCOPE OF GEOTECHNICAL ENGINEERING SERVICES FOR THE CITY OF WEST UNIVERSITY MAY 2004 By David Eastwood, P.E. Updated 07-27-04 Foundations Foundations for residences in the City of West University will consist of either a structural slab or slab- on-fill type foundation systems. The structural slab foundation system may consist of a pier and beam with a crawl space or slab with void underneath it and supported on piers. In all cases,piers will be used to support the superstructure. Geotechnical Exploration The geotechnical exploration for residences in West University should be conducted by an accredited geotechnical engineering firm. The firm should be accredited by American Association of Laboratory Accreditation (A2LA) in geotechnical engineering. The accreditation should be current. A copy of the accreditation in geotechnical engineering should be placed as an appendix of the report. The firm should have a minimum of$500,000 errors and omissions insurance (professional liability insurance). Geotechnical Borings There shall be a minimum of two borings required for houses with slabs of up to about 3,000 sq. ft. Additional borings should be conducted for any additional 1500 sq. ft. of the floor slab areas. This includes the garage area as well. All borings will be a minimum of 25-ft deep. However, 20-ft borings are allowed if a portable rig has to be used. The geotechnical borings for additions to an existing residence will consist of one boring for additions of 1000 sq. ft. or less. Additions greater than 1000 sq. ft. will require two borings. The boring depth will be the same as above. Laboratory Testing The laboratory testing will be determined by the geotechnical engineer. Some of these tests shall include Atterberg limits, unconfined compression, torvane, hand penetrometer, soil suction, grain size analyses, etc. Minimum testing will be as follows: Minimum Number Test Type ASTM Standard of Tests per Boring Moisture Content ASTM D-2216 4 Atterberg Limits ASTM D-4318 2 Torvane ---- 5 Hand Penetrometer ---- 5 Unconfined Compression ASTM D-2166 2 Swell Tests ASTM D4546 As needed Suction ASTM 5298 As needed Sieve Analysis ASTM D-422 As needed " Engineering Report The geotechnical engineering report should include recommendations on the recommended foundation types. In addition, the report should discuss various foundation risks. The recommended pier depths should be specifically spelled out. Furthermore, the recommended fill thickness should clearly be presented. In addition, on sites where trees are present, the effected tree removal and/or dying needs to be considered in the evaluation of the recommended foundation system and its components. Furthermore, a picture of the site should be presented in the geotechnical report. In general, the geotechnical report should include the following: o Introduction. o Field Exploration Procedures. o Laboratory testing. o Site conditions. o Soil Stratigraphy. o Groundwater. o Foundations and Risks. o Foundation Types, depth and allowable by pressures. This includes structural slab supported on piers, slab-on-fill supported on piers, floating slab recommendations. o Estimated settlements and potential vertical rise (PVR). o Construction considerations, including earthwork and testing requirements. o Vegetation control, including the effects of existing trees, tree removal, tree dying and new trees. o Site drainage. o Use of on-site soils for fill. o Limitations. o Site plan o Boring logs, including the depth of root fibers. o Homeowner maintenance program. o Project site pictures. The geotechnical report should also include a copy of the A2LA Accreditation certificate in geotechnical engineering and a copy of errors and omissions insurance certificate. y t • Quality Control and Testing The quality control and testing during construction for a residence should be conducted by the same firm that conducted the initial geotechnical report. This work will include earthwork testing, drilled footing observations, and concrete testing. The subgrade soils should be proofrolled and compacted to 95 percent maximum dry density degree by ASTM D698 at moisture content with optimum and+3 percent. All fill soils placed underneath the floor slabs should consist of structural fill with liquid limit less than 40 and plasticity index between 12 and 20. These soils should be compacted to a minimum of 95% maximum dry density (ASTM D698). Moisture contents between optimum and +3%. All the fill soils should be tested, at a minimum of five locations within the floor slab areas at each eight-inch lifts. The subgrade soils should be proofrolled and tested for density at five locations prior to fill placement. The subgrade soils should have the same level of compaction as the fill soils. This information should be provided at the completion to the client, the city building official as well as the structural engineer. The construction of all the piers should be observed by the geotechnical engineer of the record or the structural engineer of the record. The piers should be check for proper depth, soils stratum, proper soil strength, the correct bell size and steel placement, steel size and grade. The concrete for the drilled footings slab should be tested in compression. A minimum of four concrete cylinders should be made for every 50 yards of pour or portions of. Two of the cylinders will be broken at 7 days and two at 28 days. Submittals A copy of the geotechnical report together with all field testing and observations should be sent to the structural engineer of the record, the client, as well as the building official. All failed tests should be retested until passing results are achieved. The reports should include the following: o Initial geotechnical report o Proofrolling observations report of subgrade o Proctor test reports o Subgrade density reports o Fill compaction reports per all lifts o Drilled footing installation report o Concrete compression test reports for the drilled footing and the slab o A2LA Accreditation certificates in geotechnical engineering o A2LA Accreditation certificates in materials engineering o Errors and omission insurance certificates for a minimum of$500,000 annual claims made. All reports should be signed and sealed by a licensed professional engineer in the State of Texas. The laboratory conducting the testing should be accredited by the American Association of Laboratory Accreditation (A2LA) in Construction Materials Testing. A copy of the accreditation certificate should accompany the submittals. Word\marketing\westuniversity Rev. 6-28-04 Standard Codes Schedule Adoption. Subject to the amendments and deletions indicated beneath each code, each of the following codes, including all of its published appendices and attachments, is adopted, ordained and made a part of the Code of Ordinances of the City and of each chapter where it is referenced, except as otherwise expressly provided. Procedure for amendments,etc. The procedure for adopting new codes, updated codes, local amendments and provisions for administration and enforcement of these codes is as follows: (1)proposal by the building official or other appropriate City official, (2)referral to the Building&Standards Commission, (3) consideration by the City Council, after giving required meeting notices, and(4) adoption and publication, as required by Article II of the City Charter. International Building Code, 2000 Ed.,International Code Council, Inc.. 1. The administrative officer is the building official. All hearings, variances etc. are handled by the BSC. 2. All roofs must have Class C or better fire resistance, as determined under Sec. 1505.1. 3. All foundations for new buildings and foundation repairs shall be designed by a registered a illustrated in complete plans and specifications signed and sealed by the RPE; l/7 based on a soils report from a recognized and reputable firm or agency (Exception: no soils report is required for a single-story accessory building with less than 450 sq. ft. of gross floor arca); and inspected by an RPE who certifies proper construction, bcforc work proceeds further. All foundations for new buildings (or additions to existing buildings) with a gross floor area 485 square feet or more must meet the following criteria, as applicable. 4AQATUS.,9 a. Engineering. Foundations must be constructed in accordance with complete plans and specifications prepared, signed and sealed by a licensed or registered professional engineer who is: (1) employed by a registered engineering firm and (2) covered by professional errors and omissions insurance with at -g. . limits of at least $500,000 per year, aggregate ("RLPE"). (ic.,.1 The plans and specifications must be prepared specifically for the site of the work, and they must meet criteria as to scope, content and form specified by the building official. , b. Geotechnical Report. The plans and specifications for each q>./ foundation must be based on a written geotechnical report prepared, signed and sealed by a licensed or registered geotechnical engineer who is: (1) employed by a registered engineering firm; and (2) covered by professional errors and omissions insurance with limits of at least $500,000 per year, aggregate. diAk * The report must meet all applicable criteria in "Recommendtk—e Practice for Geotechnical Explorations and Reports" published by the Structural Committee of the Foundation Performance Association, - Houston, Texas (Document # FPA-SC-04-0, Rev #0, 11 April 2001, issued for website publishing), a copy of which is on file in the City Secretary's office. However, the minimum depth of borings i 20 f--t in all cases. All tests and other laboratory work must be performed ry p n p by a laboratory accredited for such work by the American Association for Laboratory Accreditation on the basis of ISO/IEC 17025:1999 ("general requirements for the competence of testing and calibration laboratories"). c. Foundation Performance Standard The plans and specifications for �..ch foundation must be prepared to achieve a soil movement . y -" potential of one inch or less, determined by the estimated depth of the C'''' active zone in combination with at least two of the following methods: (1) Potential vertical rise (PVR) determined in accordance with Test Method Tex-124-E, Rev. January 1, 1978/December 1982, Texas State Department of Highways and Public Transportation, Materials and Test Division, "Method for Determining the Potential Vertical Rise, PVR" (a copy of which is on file in the office of the City Secretary). For this purpose, the "dry" moisture condition (from which little shrinkage is experienced, but where volumetric swell potential is greatest) shall be used for each sample and test. (2) Swell tests performed in accordance with ASTM D4546-03, "Standard Test Methods for One-Dimensional Swell or Settlement Potential of Cohesive Soils" as last revised prior to June 1, 2004. \,.? (3) Suction and hydrometer swell tests performed in accordance with ASTM D5298-03 "Standard Test Method for I*le Measurement of Soil Potential Suctio ) Using Filter Paper" j\j`O and ASTM D6836-02 "Standard Test Methods for 'Ct Determination of the Soil Water Characteristic Curve for Desorption Using a Hanging Column, Pressure Extractor, Chilled Mirror Hygrometer, and/or Centrifuge," as such methods were last revised prior to June 1, 2004. d. Foundations, Basic Type. Each foundation must be of an approved basic type. Approved basic types are listed below. In this list, types of foundations are defined and described in "Foundation Design Options For Residential and Other Low-Rise Buildings on Expansive Soils" published by the Structural Committee of the Foundation Performance Association, Houston, Texas (Document # FPA-SC-01- X, Rev #X, 1 April 2004, issued for FPA peer review), a copy of which is on file in the City Secretary's office. ;M • 61)(\ic, t � (1) Structural slab with void space and deep foundations. Structural floor with crawl space and deep foundations.' (3) Stiffened structural slab with deep foundations. (4) Stiffened non-structural slab with deep foundations. (5) Grade-supported stiffened structural slab. (6) Grade-supported stiffened non-structural slab. (7) Grade-supported non-stiffened slab of uniform thickness (approved for one-story buildings--or additions to buildings-- containing only garage or storage space, not living space). (8) Mixed-depth system for all new building construction. (9) Mixed-depth system for building additions with deep foundations. (1 , ) 7ITh -deyficte9for b di , addi •on ith hall• founda ions. (11) Another type approved by special exception issued by the BSC. See (h), below. \/\ :`") ) \k e. Foundations, Deep Support Components. Deep support components must be of an approved type. Approved types are listed below. In this list, types of deep support components are defined and described in "Foundation Design Options For Residential and Other Low-Rise Buildings on Expansive Soils" published by the Structural Committee of the Foundation Performance Association, Houston, Texas (Document # FPA-SC-01-X, Rev #X, 1 April 2004, issued for FPA peer review), a copy of which is on file in the City Secretary's office. (1) Drilled and underreamed concrete piers. (2) Drilled straight-shaft concrete piers. (3) Auger-cast concrete piles. (4) Another type approved by special exception issued by the BSC. See (h), below. f. Foundations, reinforcement. Reinforcement for each foundation must be of an approved type. Approved types are listed below. In this list, types of reinforcement are defined and described in "Foundation Design Options For Residential and Other Low-Rise Buildings on Expansive Soils" published by the Structural Committee of the Foundation Performance Association, Houston, Texas (Document # FPA-SC-01-X, Rev #X, 1 April 2004, issued for FPA peer review), a copy of which is on file in the City Secretary's office. (1) Deformed bar reinforcing. (2) Welded wire fabric reinforcing (approved for one-story buildings--or additions to buildings--containing only garage or storage space, not living space). (3) Another type approved by special exception issued by the BSC. See (h), below. g. Foundations, Observation & Certification. Each foundation must be professionally observed and must be certified by an RLPE, as more fully described below: (1) Observations must: (i) be performed either by the certifying RLPE or by a person under that RLPE's direct supervision and control whose professional qualifications are approved by the RLPE, (ii) include actual measurement of piers, fill, compaction, reinforcement, forms, materials, dimensions, structural elements, stressing, tendons, tensions, attachments, etc. before the work is covered or concrete is placed, (iii) be performed continuously during placement of concrete and any stressing or tensioning operations, and (iv) be documented in a form and manner approved by the building official (which may include photographs). (2) Certifications must: (i) refer to and be based upon the professional observations required by this section, (ii) state that the work complies with the plans and specifications last approved by the building official (with any field changes that are ordered by the RLPE and reported to the building official and that comply with applicable regulations), (iii) state that the work complies with sound engineering practices, (iv) comply with criteria as to form and content as may be specified by the building official, (v) be signed and sealed by the certifying RLPE, and (vi) be filed with the building official. Before framing or other work commences atop a foundation (and before the foundation is otherwise covered), the permittee must obtain written acknowledgment from the Building official that the certification for the foundation was duly filed as required above. h. The BSC may issue a special exception from any requirement in subsection "a" through "g," above, but only upon a showing that: (1) the requirement will not affect life safety or the performance of a structure (for its estimated useful life); or (2) an alternate requirement to be imposed by the special exception will provide equal or better protection for life safety and long-term structural performance. In connection with any such special exception, the BSC may require that the applicant provide supporting engineering data and opinion, and the BSC may impose conditions to carry out the purpose and intent of applicable regulations. d4__„INAW, "Aiqj L 300 4. All concrete iers, footings and foundations must be cured for at least 72 hours before any g Y significant load is placed on them. 5. All walls and ceilings within a R-1, R-2, R-3 and R-4 type occupancy shall be sheathed with Type X gypsum board at least 5/8-inch(15.9 mm)thick. Exception: Where this code (IBC) requires otherwise for moisture protection. 6. Delete: Appendices A(Employee Qualifications), B (Board of Appeals) and D (Fire Districts). International Energy Conservation Code, as it existed on May 1,2001, International Code Council,Inc. 1. The administrative officer is the building official. All hearings, variances etc. are handled by the BSC. 2. In lieu of inspection by City employees, the building official may require a written certification that a building meets or exceeds minimum requirements, if the certification is: (i) signed by a code-certified inspector(as defined in Section 388.02, TEX. HEALTH & SAFETY CODE)not employed by the city, and(ii) accompanied by an approved inspection checklist, properly completed, signed and dated by the inspector. If the fees of the code-certified inspector are paid by the City, the amount shall be added to the building permit fees otherwise payable. With approval from the building official, a permittee may pay such fees directly to an independent inspection firm. Only code-certified inspectors may perform inspections and enforce this code in the City. International Fire Code,2000 Ed., International Code Council, Inc. 1. The fire official shall be the fire chief or acting fire chief, who may detail other members of the fire department or the building inspection division to act as inspectors. Chapter 6 of this Code shall apply to enforcement and administration of the fire code in the same manner as it applies to the building code (except that the fire official shall have the powers and duties of the building official under such articles). 2. The BSC shall have the same jurisdiction and authority with respect to the fire code as it has with respect to the building code. 3. The limits of the fire district referred to in Section 902.1.1 are coextensive with the City limits. 4. Explosives and fireworks, as defined in Chapter 33, are prohibited within the City limits. 5. Notwithstanding Section 2206.7.6 (relating to service stations), "latch-open"type devices are prohibited. 6. Section 603.8.4 (hours for burning) is amended to read in its entirety as follows: "An incinerator shall not be used or allowed to remain with any combustion inside it: (i) at any time from an hour preceding sunset on one day until sunrise the following day; or(ii) at any time when unattended." (7) Delete: Appendices FA (Board of Appeals), FE (Hazard Categories), FF (Hazard Ranking) and FG (Cryogenic Fluids - Weight and Volume Equivalents). International Fuel Gas Code,2000 Ed., International Code Council, Inc. 1. The administrative officer is the building official. Chapter 6 of this Code shall apply to enforcement and administration of this code in the same manner as it applies to the building code. The BSC shall have the same jurisdiction and authority with respect to this code as it has with respect to the building code. 2. Delete Sections FG103, FG106 and FG10. Jt STATE OF PRACTICE OF GEOTECHNICAL ENGINEERING FOR DESIGN OF CUSTOM HOMES IN THE HOUSTON AREA BETWEEN 1990 AND 2001 BY BY DAVID A. EASTWOOD, P.E.' BY FRANK ONG, P.E.' PRESENTED AT FOUNDATION PERFORMANCE ASSOCIATION MEETING ON JUNE 20, 2001 Abstract Practice of geotechnical engineering in the Houston area has been quite interesting during the past decade, depending on the firm, recommendations for design of custom residential homes vary quite a bit. The purpose of this paper is to look at various design approaches and recommendations. This paper summarizes the state of practice for the past decade. Furthermore, the paper recommends procedures to conduct better geotechnical exploration for custom residential projects in the Houston area. Introduction Due to the strong economy, custom homes are being constructed all over the Houston area. Most of these homes are supported on drilled footing type foundations. The focus of this paper is primarily on the design of homes on drilled footings. Many odd shaped homes (U, L shaped or houses with large slabs with notches) are supported on drilled footings. Furthermore, some of these houses have major foundation problems. The purpose of this paper is to review and summarize 99 geotechnical reports in the Houston area, look at various soil types, discussion of risks, heave computations, drilled footing depths, various slab designs, evaluation of environmental conditions, etc. These reports were conducted uy 11 dale'cut 11.ins located in the Houston area. About 1v'6 of these tepuii were conducted by Geotech Engineering and Testing. This paper also develops recommendations on how to better conduct geotechnical exploration for custom residential projects in the Houston area. Report Research In order to develop a State of Practice Report, research was conducted to find geotechnical reports for custom homes conducted by various Geotechnical firms in the Houston area. A total of 99 reports were located from GET's library and from various structural engineers throughout the Houston area. These reports were used as a basis of development of our findings. A map showing where these various soils reports for custom homes were conducted is shown on the site plan, Plate 1 of this paper. As indicated on this map, the concentration of these reports are located within 610 Loop area, near West University, Bellaire, Medical Center, and Kirby area. Furthermore, some of these reports are located along Memorial Drive, Hunters Creek Village, Piney Point Village and Bunker Hill area. A few data points are located outside the Loop. Principal Engineer,Geotech Engineering and Testing,800 Victoria Drive,Houston,Texas 77022,713-699-4000 2 Chief Engineer,Geotech Engineering and Testing, 800 Victoria Drive, Houston,Texas 77022, 713-699-4000 1 of 11 Definitions The data developed during this research study are summarized on Plate 2 of this paper. The definition of each term used in this paper is presented below: Map I.D. - The map I.D. indicates where a specific soils report was conducted. The map 1.D. is key to the site plan, Plate 1. Year - This indicates the year the soils report was conducted. Company - This term identifies which company did the soils report. We have companies A, B, C, D, EF, G, H, J, K, L, M, N, 0, P, Q, and R, a total of17. Report No. - This term designates the report number for each specific company. Expansive Soils - This term signifies whether or not expansive soils are present at the job site. Expansive soils should have minimum plasticity index of 20 (Ref. 1). Trees on Site/Site Conditions - This term signifies whether or not trees were present on the site or the firm who conducted the geotechnicaI report had a Site Condition Section in, the report. Sometimes, these soils reports did not even discuss the site conditions. In this case, a dashed line is put in this space for the segment "trees on site/site conditions". Effective Plasticity Index (PI) - This column presents the effective plasticity index of the soils developed by BR.AB method (Ref. 2). Discussion of Expansive Soils - This column signifies whether or not the geotechnical report discussed the presence of expansive soils on the site. Discussion of Risks - This column describes whether or not the geotechnical report discussed various risks that are associated with different types of foundations used for residential foundations built on expansive soils. For example, a structural slab with void would be low risk foundation. However, a slab-on-fill pier foundation will have a higher risk than a structural slab with void type foundation system. A discussion of foundations and risk is given on Plate 3. Heave Computations - Most of the heave computations for residential projects (if computed at all) were computed by using Potential Vertical Rise method (Ref. 3). This column signifies whether or not a heave computation was computed for the soils report. 2of11 1 Drilled Footing Depth - This column signifies what was the recommended pier depth for the specific soils report. Structural. Slab - This column signifies whether or not recommendations on a low risk foundation, which is a structural slab with voids/crawl space, was given in a specific geotechnical report. Furthermore, if recommendations on structural slab was given, whether or not recommendations on voids under the floor slabs was given. Therefore, this column signifies whether or not recommendations on structural slabs were present and if recommendations on structural slab were present what was the recommended void size under the floor slab (not under the grade beams). Slab on I ill - This column defines whether or not recommendations on slab-on-fill were given in the specific geotechnical report. Furthermore, whether or not a specific fill thickness was given. Some reports presented a very vague fill thickness recommendations. Void Box - This column signifies whether or not recommendations on void boxes were given under the grade beams. Drainage - This column signifies whether or not recommendations on site drainage around the house were given. Sprinkler -- This column signifies whether or not recommendations on the presence of a sprinkler system, and their location around the house were given. Trees - This column signifies whether or not recommendations were given on planting a tree next to the foundation. furthermore, it indicates whether or not recommendations were given on the existing trees next to the foundation. Tree Root Removal - This column signifies whether or not the specific geotechnical report discussed how to treat the tree removal from a specific site. What would be the ramifications of tree removal (heave). Construction Monitoring - This column signifies whether or not the geotechnical report gave specific recommendations on quality control such as conducting of testing including drilled footing observations, concrete testing, earthwork testing by the design geotechnical engineer. Review of Foundation Drawings - This column signifies whether or not the geotechnical engineer of record required that for the foundation drawings be reviewed by him/her to make sure his/her design recommendations are properly interpreted by the structural engineer and other design team members. 3 of 11 II Soil Variability - This column indicates whether or not the geotechnical report indicated that the soils across the site could be variable (from a standpoint of stratigraphy or properties) and there might be a need for design modifications if different soil conditions were encountered. Analy is of the Data General. The contents of all of these reports were reviewed and analyzed. The specific recommendations on analysis are presented in the following sections. Expansive Soils. Many of' the reports acknowledged that expansive soils were present on the site. In general when the effective soil plasticity index is above 20, expansive soils are present on the site. Trees on Site/Site Conditions. About 64% of the reports did not discuss site conditions. Specifically, they did not discuss whether or not trees were on the site or any other site features were located on the property. This is an extremely important part of a geotechnical report where many firms failed to discuss. Effective Plasticity Index. About 82% of the reports discussed how expansive the soils were at the specific site and gave plasticity index data in the report. About 91% of the sites reviewed had expansive soils on them. Discussion of Expansive Soils. About 82% of the soils report reviewed did have a discussion of expansive soils within the body of the report. Discussion of' Risks. About 40% of the reports did not have a discussion on risks of using different types foundations. For example, they did not discuss whether a structural slab was better than a slab-on-fill type foundation supported on piers. Again, this is extremely important because, by discussing the risk associated with each different type of foundation, the soils engineer brings in the architect, the structural engineer, the builder and the owner into the decision making process. Heave Computations. About 74% of the reports did not discuss or calculate the heave. It is not customary to estimate the heave for design of custom residential foundations in the Houston area. One of the reasons for that is, it's not a consensus on the correct method for estimation of heave. Furthermore, it could be expensive. Therefore, most geotechnical consultants use their experience in specifying how much fill is required under the floor slabs to reduce heave on various types of residential foundations. The required fill thickness is usually determined based on the experience and engineering computations of the heave, such as Potential Vertical Rise, PVR (Ref.3). Drilled .Footing Depth. An average pier depth of about 9-ft was specified after reviewing all of'these reports. Review of' reports from 1996 to 2001 indicated an average pier depth of 10.5-ft. Depth of drilled footings is very important in areas where expansive soils are present. Shallow piers can push up against the grade beams and lift the foundation system, if expansive soils are present at the site. The pier should be placed below the zero movement line. The zero movement line is a line below which no movement (heave) of expansive soils occurs due to weight of the colum of the soils. The piers should be anchored below the zero movement line. Currently, we recommend piers to be placed 12-ft to 15.-ft in the Houston area where expansive soils are present. 4of11 Structural Slab Recommendations, About 55% of the reports discussed structural slab systems. The other 45% did not discuss it at all. Furthermore if they were discussed, most of them did not specify any kind of a void space that should go under the structural slab system. We believe that there should be a detailed discussion about the use of a structural slab. Furthermore, the recommended void size should be specified. In addition, recommendations on venting of the air underneath the slab should also be discussed.. To limit moisture migration through the slab. This is only applicable to a structural slab with a void/crawl space. Slab-on-Fill. About 97% of the reports did specify a slab-on-fill type foundation or drilled footings system as the type of foundation that should be used to support the structural loads for a typical custom residential foundation. Some of these reports were vague, because they specifically say whether or not fill is required under the floor slabs. The vague statements given did not specify exactly how much fill should be placed under the floor slabs. Majority of the soils reports reviewed indicated the required fill thicknesses of about 24-inches or less. In general, a maximum of 48- inches of select structural fill was specified in the areas where the soils were highly expansive. Our experience indicates that about four-fl of fill will generally reduce the movements to an acceptable level, provided positive environmental controls (drainage, trees, sewer/plumbing leak, etc.) are implemented. Void Boxes. About 38% of the reports specified the required void box size under the grade beams. Void boxes are recommended by many geotechnical firms in Houston as a way of reducing foundation movements. Expansive soils once swelled up can theoretically move into a void space area (void box) without lifting the grade beams. The discussion on whether or not void boxes should be used under grade beams on residential foundations was conducted by the Foundation Performance Association. It is generally believed that void boxes under grade beams provide channels for water to flow underneath the foundation system. Therefore, the use of them are discouraged. This discussion and idea was developed in 1996. Drainage. About 93% of the reports discussed that positive drainage was extremely important to the performance of the custom foundation system. Drainage was discussed in a majority of the reports. Sprinkler Systems. About 86% of the reports reviewed did not discuss the sprinkler system. They did not discuss how the sprinkler system (if used at all) should be placed around the structure to minimize moisture variations and therefore, differential movements. Trees. About 59% of the reports did discuss trees. Specifically, all the reports that discussed trees, described planting trees next to the foundation system and how they would affect the foundation system. It is understood that if the tree is left in place or planted next to a foundation system, it may cause the soil to shrink and the foundation to settle if clays are present. In all of the reports the word planted was synonymous with trees left in place. Tree Removal. About 70% of' the reports did not discuss the tree root removal in their reports. This is an extremely important section of a report, because tree removal in areas where expansive soils are present, can cause significant heave. Therefore, the reports should address this condition and warn the client. This has not been customary in the Houston area until the year 1995. A detailed study of tree removal and its effect on foundation systems was presented in 1997 by Eastwood and Peverley (Ref. 4). 5 of11. Construction Monitoring_ About 85% the reports suggested following up the design with construction monitoring. Construction monitoring is an important part of any design. Review of the Plans and Specifications. About 75% of the reports suggested review of the foundation drawings after the design was completed. The review of the foundation design drawings by the geotechnical engineer of the record is important. This review will provide the client with the confidence that the initial designer (architect, structural engineer, owner) understood the soils report and followed the recommendations. It is possible once the drawings are reviewed by the geotechnical engineer of the record, mistakes are found that are reported to the structural engineer. Furthermore, foundation and risks are discussed. Soil Variability_ About 99% of the reports discussed the potential variation of soil stratigraphy and properties across the site. This is a true condition. Subsoils may vary across the lot from a standpoint of stratigraphy and soil properties. Conclusions and Recommendations Based on the review of the 99 reports written between 1990. and 2001 for custom homes in the Houston area, the following conclusions and recommendations can be made: o The soil reports reviewed from 17 companies represent a cross section of the geotechnical firms in the Houston area that do residential work for custom homes. o Many of these soils reports indicated the presence of expansive soils in the vicinity of the project site. Most of them discussed the presence of expansive soils. o Only 36% of the reports reviewed discussed site conditions. The rest of the reports did not even address site conditions. The site conditions should be discussed in all reports. Perhaps a picture of the site should be included in the report. o Foundation types and risks must be discussed. o A review of the reports indicates an average pier depth of about nine-ft. Pier depths have been increasing in depth in the Houston area since 1995. In the 1.970's and early 80's, piers were placed at a depth of about eight-ft. However, due to new understanding of the active zone depth and the effect of trees on foundations, deeper piers have been recommended. However, most geotechnical firms in Houston are not taking account the effect of tree removal in their foundation system. This condition was known to the Houston area after 1995 or 1996. Furthermore, the use of deeper piers is resisted by some designers, builders, owners, etc., because this may add some to the cost of the construction of the foundation system. However, we believe that increasing the depth of the piers by a few feet, the cost of the foundation system should not increase significantly. Furthermore, the risks of putting shallow piers in the area where expansive soils are present are too great. These shallow piers can actually be grabbed by the expansive soils and be pushed against the foundation system, resulting in floor slab heave. Considering that most of the distress of the newly constructed foundations in Houston is heave, the piers should be deep enough to resist uplift due to expansive soils. 6of11 o Almost all of the reports discussed the slab-on-fill on drilled footings. However, many of them did not suggest the required till thickness. We believe that if this type of foundation system is recommended, the fill thickness should be clearly defined. Unfortunately, some soils reports are very vague about the required fill thickness. This has caused foundation problems, because inadequate fill thickness has been placed underneath the floor slabs. o About one-third of the reports did discuss the use of void boxes underneath the grade beams. Many of them did not. The use of void boxes is a controversial issue today. We do not recommend the use of void boxes under the grade beams. o Nearly all of the geotechnical reports discussed positive drainage as a major component of foundation design in their report. This should be covered in all reports. o Not very many reports discussed the presence of sprinkler systems around a house. We believe that the sprinkler system, if used, should be placed all around the house to provide uniform moisture conditions at the edge of the foundation. A non-uniform moisture condition will result in differential movement of the slab and foundation distress. o The area of the geotechnical reports that requires most improvement is the section that has to do with removal of existing trees. This issue must be further discussed in geotechnical reports. Currently, most geotechnical reports in Houston do not even discuss the effect of tree removal. In the event that a drilled footing foundation system is to be used, minimum boring depths should be 20-ft. Root fiber depth should be logged in the borings. Furthermore, the depth of active zone should be estimated. The effect of tree removal should be clearly discussed in the report. o Structural engineers who are designing a custom home must make sure the effects of tree removal have been considered in the geotechnical report prior to conducting a design of a residential slab. Some structural engineers blindly disregard this issue and they claim, they followed an erroneous soil report. Knowing the site conditions is also the responsibility of the structural engineer of the record. o None of the reports reviewed had any suction data or used suction to estimate heave. o The authors hope that in the future, the concepts, such as the use of suction will be implemented in the design of lightly loaded structures. Currently, most firms in the Houston area do not run suction tests on their soils samples due to costs associated with conducting this type of test and analyzing the data. Due to the extremely competitive nature of doing residential geotechnical work, it is almost impossible to do a detail geotechnical exploration and testing for residential projects. We believe that by providing a minimum standard in conducting geotechnical explorations, more firms will be interested in conducting more advanced and up to date geotechnical explorations and therefore, saving the client from the risks and spending too much money during construction and repair. Having a minimum standard is the only way, we believe, that the practice of design of a custom foundation system for lightly loaded structure in Houston can be improved. 7 oft1 References 1. David Eastwood and Others "Methodology for Foundations on Expansive Clays" Published in December, 1980 Edition of ASCE Journal of Geotechnical Engineering Division. 2. Building Research Advisory Board, National Research Council, "Criteria for Selection and design of Residential Slabs on Grounds," National Academy of Science Publication 1571, 1968. 3. "Method for Determining the Potential Vertical Rise, PVR," State Department of Highways and Public Transportation, Test Method Tex 124-E, Austin, Texas. 4. D. Eastwood and D. Peverley "Design of Foundations with Trees in Mind", presented before the ASCE, Texas Section, Spring Meeting in Houston, April 1997. 8of11 01 i ig i \\ „,,,...le•e::;-- ' 1 r. - i.:'•• r,7” \ i ■ I r"-,,,,, l' 1 ‘-':::,..-7,--.,„......------------ t,..,■ 1 :„.......,,..........__,e if „Li z 14 l'il,' -;-= -': 14 ..- ' -..-',""■, , 0, , 1 ell i 4 , i i!r"'"' 7::7;■ " ibr , , - p ■-,4 ..... , • ' , s, ,.. , ' ' Ve"''''' I . tz,i yk ..4tc. ' ,... , .0. „47'it ff. v ',\ ,,,....k ; i ,,2,_ I — - ,swarime. "..'-'-', ___,..-- --...., 1 3,,D.rtlu ..T ;LI: 1 •+ . 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Vold I ft 1'n1ms. FO I Boo/Op.' Splink- - Roo;T'ueflal !Famish.(0140 _1. Y'ul lC on Nn. _ Sells/ 5007 nde_PI Sod? 01Ri5ks 1003, D-159. delions Size.n. 0030,? -1N040150s,In. Svq n.j leo Trees RenmralI I.JO0'_g Urawin•s 09 --1 1103 2 209-TOE Yxs - 32 No _No Nu R o - T Ye!; 1'L 4_ xs_ No Nn NO Yes' Yes _2_ 1990__K 20-1020 Yes - --40 Yes 'fes_ I 1 4 Yes 4A - Too Yes Ves No Y. Yes 3 1990 H 090.330 Yes --_49 'Yes Yos Yes - Too -. Yns _No Yes_ Ya::_I__Yes Yes_•--4_ 1001 8 0161024 Yes Y. 4'i Yes No - Yes Yes No Too_No Yes - Yes 5 1591 H 091-40%_ Yes -20 No No a �� = 95 - No No 140 Yes Y«_ _6_ IMi H_ 091-150 _Yes - _ 21_ No Nn 8 e �� Yes No No No Yes Yxs_ Y % _ 1991 H 0!11-110 Yes ���A��rr.;.����-Yes Yes Yes Nn Yes Yes Yes Yes _0 1991 L 1910 0 Yes_�M. Yes fl No 0 Yes 24 0 Yes No No Nom Toe __- 9� 1591 N 001-120 Yes -- 37 Yes Yes Ves O Yes _- _ Yes No Yes Yas_ Yes Yes- '111 1991 II 286-05200-1 Yes - 62 Yes Yes Yes ID Yes - 25 ye {ea 'ras 140 'tes Yes Yes 11 1991 H - 091-202 ® -- 59 Yes _--Yes Yes - -4 --NO Yev_ Yes Yes Yes 12 1991 L 1911:76 Yas Nc - 24 6 J.Nn Nn IJ-ri No No No 13 G 11154-I OiR ' No No - - 1 Yes NO Nu No Hu Yes MU f HE!I1-0%'1 IIIIMEENIIIIMMIIINENIINLM 0 MINIM 0 24 0 Yes No Yes Yee ■ Yes 0 Yes IS 1!0192 A 920259E O� - Yes No 1U � 24 4 Yes Yes yes 1 Yes Yes a Yes 16 102 0 9202028 yes No 9 1 4 Yes a yes yes Yes 111111=1111 Yes 1% IM% _14 0211 3026 Yes - 40 Yes Yes No Yes Yes 24 No __No No No No � Ya5 -155 IOB+ H 092248 _Ye_ 65 Vx_ 205 Yes Ves - Yes I -- Yes No Yet 1 Yes Yes Yes 19 1,0299999 02_05/0 Yes - 'S 'Yes No No Yes 4 Yes 16 4 No No No No Yes Yes 'Yas~ 20 1952_ J 115092E Vus ® No No No No - Yes 12 -- Yes No No No Yes WI= Ye 21 10924 0 000-215 0n W� No Nu 0 -- Yes -_ �11 Yes No No NO Yeaa P25 22 IM2 F ® Yes ® ��� Yas 1 - 1 'Yes No Yes Yes Y. ME= Yes 23 IIMMI H 1 G92-289 Yes j - 41 ® - _'{= I_ -- '!es Nu Yes r Yes 1 le:.s Yas 24 1092 ,I 1800-92E Yes 1 -- 40 111171111.211 Yes 12 4 Yes No No No Yes _ Yes 20 _59`_ G 9206-1010 Yes i-__ 42 Mia® Yes I6 © Yes No No No Yes 20 Yos 1992 L 192057 Yes -- 47 �� Yes 24 yes NO Tess 1 No Yes Yes 2/ 1,9:2, ' J 209-92E Yes - 41 No No No Yes 12 Yes 140 No No Yes Yas Yn 28 1002 F_ 92940-1 _ Yes No 32 Yes Yes Vex I Yes No No Ve_ Yes Yes Yes 29 1992 M 300.92 19 No Pen No a Yes - Yes No No �No Yex =MI Yes -3(1 1992 _ ,5 218092E Yes 42 Yes No No 10 No ® Yes 24 - 4 ties No No No Yes No Yes 11 1992 N. 92.2680 Yes 32 Yes Yes I 8 Ye s Yew 20 0 Vol. No No No Yes Yes Yes , 32 1902 14 092-_R3 Yes -- 51 Ves_ Yes Yes 0 Yes Yes _I -- Yes No Yes Yes Yes Yea Yes 0 33 1092 M_ 323.92 Yes-_ 45 Yes No I 8 No Yew 12 4 Yes No No No , No No Yes 34 1992 H 0,92.122 Yes No 51 Yes yes Yes B Yes -- Yes _ 1 - Yes No P.O Yes Yes Yes P.O 30 1:13 G 9:510-1011 Yes 30 Yes Yes NO 6-11 Yes - Yes 24 -- _ No No No No NO No Yes 36 1993 0 0303867 Yes Yes 42 Yes Yes No 9 Yes -- Yes 24 - Yes No Yes ' No Yes Yes Yes 37 1900 H G9=211 Nn II Nn No No P No Yes - -- _'Yes No No No Yes Yes Yes ' _38 1993 _ F 03034-1 Yes ® Yes No Yes 0 140 MN Vet - - Yes No No NO No • No Yes Yes 30 4 Yes No Yes No _ Yes Yes Yes 40 19113 9 9303021 Ves Yea o 9 Yes Yes '� Ill 4 Yes No Yos No Yes Yes Yes i 41 1093 B 9303562 Yes Yes No 9 Yes -. Yes 24 4 Yes No Yes No 4 Yes Yes Yes 42 1993 0 0304002 EN= Yes Yes No 9 Yes - Yes 53.24 4 Yes No Yes No Yes Yes Yes 43 1!03 6935// No _ 0 I No No No No -- Yes - Yes No No No Yes Yes Yes 44 1993 - 9:.034/1_ Yos Yes 30 _ Yes Yes I No Yee -- Yes 18 4 Yes No Yes No I Yes Yes Yes 45 1993 0__ 9311-1021 Yes -- 56 _Yes Yes No Yes - Yes 24 -- Yes No No No Yes Yes Yes , _10 1:153 B_ 9:03609 Yes No 32 " Yes No 9 Yes -- Yes 24 4 Yes No Yes No Yes Yes Yes 7 1!k53 J 200-03E Yes -- •15 Yes No No 0 No - Yes - - Yes No No No Yes No Yes 44 1993 J 254-93E Yes -- 21 No No No B No -- Yes - - Yes No No No Yea No 1993 J 118093E Yes -- 41 Yes No Na 8 No -. Yes - - Yes _It No No Yes Yes Yes 9......,99 2.0.03E__. Yes 11,.11 Y_- -�.._.-.._._� ._ Sp 1003 J 121093E Yes -- 39 Yes No No B No -- Yes _ 12 4 Yes No Nu No 1 Y. 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Yes No Yes`No , Yes Yes Yes 10.4 H 094-359 No -, __15 No No _ No ��8�e�s No - Yes _ - I - Yee__NO No No Yes Yes Yes 11.14 H 944;4901 Yes 1a_I _ Yes No No - No -- Yes �18 -1-4 Yes No Yen NO Yes Yes Yes 6l 1994 0 940-1003 Yes -- 27 Yes Yes No 8 Yes -- Yes 18 3 Yes No Yes No , Yes No Yes 51 40 61 14 F 9473 No -- 16 Yes Na No 7 No - Yes - - Yes No No No No No Yes 52 5944 G 9405-101/ Yes - _ 37 Yes - B Yes Yes 18 7. sts No No elo Nc Na Ye- 63 1094 B 94.4518 �� No Q No -- Yes - Yes No Yee No Yes Yes Yes 64 1194 B 9404511 NMI No 40 ®® Yas - Yes Yes No Yes No r Yes Yes Yes 1 86 100__ B 9405009 No 51 Yes - Yes �� Yes No Yes No Yes _Yes Yes 641 1494 II 094-403 0 0 Yes Yea © Yes - Yee 00 yea No Yes\Too Yea Yex i Yas 5/ 10.0.4 H_�� Yes Yes O� Yxs No Yes Yes Yes Yes Ye., 00 1999' _ B �� Yes NO ®W1 Yes No 0e_s Nu Yes Yes Yes '■ 69 1994 B 034400_ Nn In No -. Yes - a Yes No Yes No Yes Yes Vol ' _'I1 190_ A X9944.11134E _V Yes s Y. Yes No N_ Yes 8 Yes 36 = Yes Yos Yes Y. Yes r Yos Yea 72 1905 E 95(:155/ Yes =MEM Yes No WM No - Yes - - Yes No ',,es I No y Yes No I Yes 7 _ 1905 B 95(:6210 Yes No No No -- Yes -- Teo No Yes No 1 Yes Yes Yes _04 1595© ® Nx 21 1 N1, No No -- Yxs a - Yes Nn Yes No Yes Yes Yes 75 1:45 F 90 740-1 aril No 50 MEE Ye_ _Yes IIIIMIE Yes -- Yes 12 WM Yoe No Yes No No No Yes ON 1105 F 054184-1 Yes No 28 Yes _ No Yes 111 No -- Yes ® Tom, No No No No 140 Yos T7 1105 H 096-221 Y. - 05 Yes Yes_ Yes Yea Yes I Yes No Yes Yes Yes Yes Yes IN 1!195 E 9501406 Yes - 23 No No No 1 No_ -- Yes - - Yes No Yes _ No Yes Yes Yes 1905 A OlnMSE _Yes _- 45 Yes _ Yes No 11 Yee 6 Yes 36 4 Yes Yes Y. _Yes Yes Yes Vex 5 ' 9007137 TOO No 50 Yes No 040 03 No - Yes 24 4 Yes 140 Yes No Vas Yes Yea 1096 11 096206 Yes - 26 Yes Yes NO B Yes - Yes - - Yes Yes Yes Yes Yes Yes Yes 108/6 F 969901 Y. - 25 Yes No Be U No Yes - - Yes No No No NO No Yos 1996 F 96%761 Yes - 35 Yes Nu Yes B No -- Yes 12 Yes No No No Na No Yes PA 1996 F 91935.1 Yes_ -- 30 No Yes No -- , Yes 10 ax MC No 00 No No Yes_ RS 1996 H 096590 Yes 1101 30 No No No -- Yes __ Yes No No No 'Yes Yes Yes 2 6 1996 _A 96-6580 Yes 'Y 71 Yes Yes No Yes 8 Yes 40 Yos _Yes Yes Yes 1 Yes Yes Yes I? 199'7 1, 9/030'06 - - Yes Yes No 10 Yes - Yes 30 Yes No Yes Yes Y. Yes Yes Ap ■rn1'7}}}}---- el 296-7513/ - 40 Yes Yes Yes 8 No -- - 24 vex ye. Yes N', Yes Yes , Yes 89 1 0 r A 90 0410E Ye, Yes 52 Yes Yes No ME Yes 6 Yes _40 Yes Yee Yes Yes Yes Yes On 1 nR A (0+100 Yes Yes SP TOo SO, No 15 Yes R Y 00 V. Yes Teo Tess Ye, vex YYOS 91 197j ir 9.35-1----Yos Yos 31 Yes Yes Yes 0 Yes - _Yes 24 140 Yes Yos No No. Yes 92 f!l_l_ 990;-10 Y 1 N 55 Yes Yes Nu _ 10 No - .---Yes 30 No Y. No Yes Na-_Yes -03 10!19 E.- 9 Y^!.1 9 8 Yes Ves 28 - Yes— Yes No 9 Yes _�----Yes 24 Yes N o YesNo Yes Yes Yes 94 1909 E 9905225 Yes_ No_41 Yes Yes No N No - Yes 24 Yox �Nn I!�.Yxs Na Yes No Yes 95 ® A-_- 06534E Yos Yes 55 Yes Yes No 12 Y. 0 Yes_._ 40 -- Yes •"a ,.,,. Yes Yes Yes Yee 00 210;(1 C _061'/ Yos Yes No No j fU , Too -- Yes - No No No NO Y. Yes Yes 90 2000 D 201-04 No - 10 Yes Yes No 10 Yes 4 Yos IC No No NO No Yes Yes_ Yes 98 2IeIT A 00-822E Yes Yes 32 Yes Yes No 1ti Yes 6 Yes 42 Yes Yes 1 Yes Yes Yos Yos Yes -99 20111 01009C- Yes Ye Os {es Yes No 19 Yes N Yew 48 s Yes i Yes Yes Yes Yes Y. NOT[1.M8,4:04il0 reconenenl90ons no heave vn o111pulalinn and 6e 061.Nn055. Plate 2 10 of 1 1 FOUNDATIONS AND RISKS Many lightly loaded foundations are designed and constructed on the basis of economics, risks, soil type, foundation shape and structural loading. Many times, due to economic considerations, higher risks are accepted in foundation design. Most of the time, the foundation types are selected by the owner/builder, etc. It should be noted that some levels of risk are associated with all types of foundations and there is no such thing as a zero risk foundation. All of these foundations must be stiffened in the areas where expansive soils are present and trees have been removed prior to construction. It should be noted that these foundations are not designed to resist soil and foundation movements as a result of sewer/plumbing leaks, excessive irrigation, poor drainage and water ponding near the foundation system. The followings are the foundation types typically used in the area with increasing levels of risk and decreasing levels of cost: FOUNDATION TYPE REMARKS Structural Slab with Piers This type of foundation(which also includes a pier and beam foundation with a crawl space)is considered to be a low risk foundation if it is built and maintained with positive drainage and vegetation control. A minimum crawl space of six-inches or larger is required. Using this foundation,the floor slabs are not in contact with the subgrade soils. This type of foundation is particularly suited for the area where expansive soils are present and where trees have been removed prior to construction. The drilled footings must be placed below the potential active zone to minimize potential drilled footing upheaval due to expansive clays. In the areas where non-expansive soils are present, spread footings can be used instead of drilled footings. Slab-On-Fill Foundation This foundation system is also suited for the area where expansive soils are present. This Supported on Piers system has some risks with respect to foundation distress and movements, where expansive soils are present. However,if positive drainage and vegetation control are provided, this type of foundation should perform satisfactorily. The fill thickness is evaluated such that once it is combined with environmental conditions(positive drainage, vegetation control)the potential vertical rise will be reduced. The structural loads can also be supported on spread footings if expansive soils are not present. Floating(Stiffened)Slab The risk on this type of foundation system can be reduced.sizably if it is built and Supported on Piers. The Slab can maintained with positive drainage and vegetation control.. Due to presence of piers,the either be Conventionally- stab cannot move down. However, if expansive soils are present,tlx:slab may move Reinforced or Post-Tensioned up,behaving like a floating slab. In this case,the steel from the drilled piers should not he dowelled into the grade beams. The structural loads can also be supported on spread footings if expansive soils are not present. Floating Super-Structural Slab The risk on this type of foundation system can be reduced significantly if it is built and Foundation(Conventionally- maintained with positive drainage and vegetation control. No piers are used in this type Reinforced or Post-Tensioned of foundation. Many of the lightly-loaded structures in the state of Texas are built on this Slab) type of foundation and are performing satisfactorily. In the areas where trees have been removed prior to construction and where expansive clays exists,these foundations must be significantly stiffened to minimize the potential differential movements as a result of subsoil heave due to tree removal. The beauty of this foundation system is that as long as the grade beams penetrate a minimum of six-inches into the competent natural soils or properly compacted structural fill,no compaction of subgrade soils are required. The subgrade soils should;however,be firm enough to support the floor slab loads during construction. The structural engineer should design the floor slabs such that they can span in between the grade beams. The subsoils within which the grade beams are placed must have a minimum shear strength of 1000 psf and a minimum degree of compaction of 95 percent standard proctor density(ASTM D 698-91)at a moisture content within :1.2%optimum moisture content. Floating Slab Foundation The risk on this type of foundation can be reduced significantly if it is built and (Conventionally-Reinforced maintained with positive drainage and vegetation control. No piers are used in this type or Post-Tensioned Slab) of foundation. Many of the lightly-loaded structures in the state of Texas are built on this type of foundation and are performing satisfactorily. In the area where trees have been removed prior to construction and where expansive clays exists,these foundations must he significantly stiffened to minimize the potential differential movements as a result of subsoil heave due to tree removal. However, foundation tilt can still occur even if the foundation system is designed rigid. The above recommendations, with respect to the best foundation types and risks, are very general. The best type of foundation may vary as a function of structural loading and soil types. For example, in some cases, a floating slab foundation may perform better than a drilled footing type foundation. Plate 3 11 of 1.1 • RECOMMENDED SCOPE OF GEOTECHNICAL ENGINEERING SERVICES FOR THE 1 CITY OF WEST UNIVERSITY MAY 2004 By David Eastwood,P.E. Foundations Foundations for residences in the City of West University will consist of either a structural slab or slab- on-fill type foundation systems. The structural slab foundation system may consist of a pier and beam with a crawl space or slab with void underneath it and supported on piers. In all cases, piers will be used to support the superstructure. Geotechnical Exploration The geotechnical exploration for residences in West University should be conducted by an accredited geotechnical engineering firm. The firm should be accredited by American Association of Laboratory Accreditation (A2LA) in geotechnical engineering. The accreditation should be current. A copy of the accreditation in geotechnical engineering should be placed as an appendix of the report. The firm should have a minimum of$1,000,000 errors and omissions insurance (professional liability insurance). Geotechnical Borings There shall be a minimum of two borings required for houses with slabs of up to about 3,000 sq. ft. Additional borings should be conducted for any additional 1500 sq. ft. of the floor slab areas. This includes the garage area as well. All borings will be a minimum of 25-ft deep. However, 20-ft borings are allowed if a portable rig has to be used. Laboratory Testing The laboratory testing will be determined by the geotechnical engineer. Some of these tests may include Atterberg limits, unconfined compression, torvane, hand penetrometer, soil suction, grain size analyses, etc. Engineering Report A geotechnical engineering report should include recommendations on the recommended foundation types, specifically the above-mentioned foundations. In addition, the report should discuss various foundation risks. The recommended pier depths should be specifically spelled out. Furthermore, the recommended fill thickness should clearly be presented. In addition, on sites where trees are present, the effected tree removal and/or dying needs to be considered in the evaluation of the recommended foundation system and its components. Furthermore, a picture of the site should be presented in the geotechnical report. Quality Control and Testing The quality control in testing for residences should be conducted by the same firm that conducted the initial geotechnical report. This work will include earthwork testing, drilled footing observations, and concrete testing. All fill soils placed underneath the floor slabs should consist of structural fill with liquid limit less than 40 and plasticity index between 12 and 20. These soils should be compacted to a minimum of 95%maximum dry density (ASTM D698). Moisture contents between optimum and +3%. All the fill soils should be tested, at a minimum of four locations within the floor slab areas at each eight-inch lifts. The subgrade soils should be proofrolled and tested for density at four locations prior to fill placement. The subgrade soils should have the same level of compaction as the fill soils. This information should be provided at the completion to the city building official as well as the structural engineer. The construction of all the piers should be observed by the geotechnical engineer of the record or the structural engineer of the record. The piers should be check for proper depth, soils stratum, proper soil strength, the correct bell size and steel placement. The concrete for the drilled footings slab should be tested in compression. A minimum of four concrete cylinders should be made for every 50 yards of pour or portions of. Two of the cylinders will be broken at 7 days and two at 28 days. Submittals A copy of the geotechnical report together with all field testing and observations should be sent to the structural engineer of the record, the client, as well as the building official. All failed tests should be retested until passing results are achieved. The tests should include the following: o Proofrolling observations report o Proctor test reports o Subgrade density report o Fill compaction reports o Drilled footing installation report o Concrete compression test reports for the drilled footing and the slab All reports should be signed and sealed by a licensed professional engineer in the State of Texas. Word\marketing\westun iversity FPA-SC-01-X(CRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 1 of 42 FOUNDATION DESIGN OPTIONS FOR RESIDENTIAL AND OTHER LOW-RISE BUILDINGS ON EXPANSIVE SOILS by The Structural Committee of The Foundation Performance Association wwvv.foundationperformance.org Houston, Texas Document # FPA-SC-01-X (DRAFT) ISSUE HISTORY (Only includes issues outside the Structural Committee) Rev Date Description Subcommittee Subcommittee Co-Chairs Members A 27 Jan 00 First Issue George Wozny Ron Kelm V 25 Mar 04 For Committee Comments Michael Skoller Bill Polhemus X 1 Apr 04 For FPA Peer Review Lowell Brumley Jon Monteith Mari Mes George Cunningham John Clark Toshi Nobe Nicole Wylie Denis Hanys Jim Austin a FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 2 of 42 PREFACE This document was written by the Structural Committee and has been peer reviewed by the Foundation Performance Association (FPA). This document is published as FPA-SC-01 Revision 0 and is made freely available to the public at www.foundationperformance.org so all may have access to the information. To ensure this document remains as current as possible, it may be periodically updated under the same document number but with higher revision numbers such at 1, 2, etc. The Structural Committee is a permanent committee of the Foundation Performance Association. Please direct suggestions for improvement of this document to the current chair of the Structural Committee. If sufficient comments are received to warrant a revision, the committee will form a new subcommittee to revise this document.If the revised document successfully passes FPA peer review, it will be published on the FPA website and the previous revision will be deleted. This document was created with generously donated time in an effort to improve the performance of foundations. The Foundation Performance Association and its members make no warranty regarding the accuracy of the information contained herein and will not be liable for any damages, including consequential damages, resulting from the use of this document. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 3 of 42 TABLE OF CONTENTS 1.0 INTRODUCTION 5 2.0 PROBLEM DEFINITION 6 3.0 GENERAL DESIGN CONSIDERATIONS 7 4.0 FOUNDATION SYSTEM DESIGN OPTIONS 7 4.1 DEEP SUPPORT SYSTEMS 8 4.1.1 Isolated Structural Systems with Deep Foundations 8 4.1.1.1 Structural Slab with Void Space and Deep Foundations 8 4.1.1.2 Structural Floor with Crawl Space and Deep Foundations 11 4.1.2 Stiffened Structural Slab with Deep Foundations 13 4.1.3 Stiffened Non-Structural Slab with Deep Foundations 14 4.1.4 Non-Stiffened Slab-on-Grade with Deep Foundations 15 4.2 SHALLOW SUPPORT SYSTEMS 15 4.2.1 Grade-Supported Stiffened Structural Slab 16 4.2.2 Grade-Supported Stiffened Non-Structural Slab 17 4.2.3 Grade-Supported Non-Stiffened Slab of Uniform Thickness 18 4.3 MIXED DEPTH SYSTEMS 19 4.3.1 Mixed Depth System for All-New Building Construction 19 4.3.2 Mixed Depth System for Building Additions with Deep Foundations 20 4.3.3 Mixed Depth System for Building Addition with Shallow Foundations 20 5.0 FOUNDATION COMPONENT DESIGN OPTIONS 21 5.1 DEEP SUPPORT COMPONENTS 21 5.1.1 Drilled and Underreamed Concrete Piers 21 5.1.2 Drilled Straight-Shaft Concrete Piers 23 5.1.3 Auger-Cast Concrete Piles 24 5.1.4 Displacement Piles 25 5.1.5 Helical Piers 26 5.2 SLAB AND GRADE BEAM REINFORCING 27 5.2.1 Post-Tensioned Reinforcing 28 5.2.2 Deformed Bar Reinforcing 29 5.2.3 Welded Wire Fabric Reinforcing 30 5.2.4 Fiber Reinforced Concrete 31 5.2.5 Unreinforced Concrete 31 5.3 VOID SYSTEMS UNDER GRADE BEAMS AND PIER CAPS 31 5.4 VAPOR RETARDERS 32 FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 4 of 42 5.5 GRADE-BEAM-TO-PIER CONNECTIONS 32 5.5.1 Grade-Beam-to-Pier Connections with No Restraints 32 5.5.2 Grade-Beam-to-Pier Connections with Horizontal-Only Restraints 33 5.5.3 Grade-Beam-to-Pier Connections with Horizontal and Vertical Restraints 33 6.0 FOUNDATION SITE DESIGN OPTIONS 34 6.1 MOISTURE CONTROL SYSTEMS 34 6.1.1 Site Drainage Systems 34 6.1.1.1 Site Grading 34 6.1.1.2 French Drains 34 6.1.1.3 Area Drains 35 6.1.2 Moisture Retarder Systems 35 6.1.2.1 Horizontal Moisture Retarders 36 6.1.2.2 Vertical Moisture Retarders 36 6.1.3 Watering Systems 36 6.1.3.1 Sprinkler Systems 37 6.1.3.2 Soaker Hose Systems 37 6.1.3.3 Under-Slab Watering Systems 37 6.1.3.4 Drip Watering Systems 38 6.2 VEGETATION CONTROL SYSTEMS 39 6.2.1 Root Retarder Systems 39 6.2.1.1 Vertical Root Retarders 39 6.2.1.2 Horizontal Root Retarders 41 6.2.2 Root Watering Wells 41 6.3 TREE AND PLANT SELECTION 42 FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 5 of 42 1.0 INTRODUCTION The scope of this document is to provide guidance in the selection of design options for residential and other low-rise building foundations, typically called light foundations, which are founded on expansive soils. Low-rise buildings are defined as one to three stories in height and have masonry, wood, cement-board, or stucco exterior walls. These buildings include houses, garages, small apartment and condominium buildings, restaurants, schools, churches, and other similar structures. Design options for foundation systems, foundation components, and moisture and vegetation control methods are reviewed and compared. There are no absolute design rules for choosing a design. This document provides a list of advantages and disadvantages for each of the many commonly used foundation design options to assist the designer in selecting the most suitable option The intended audiences for the use of this document are structural, civil, and geotechnical engineers, builders, architects, landscape architects, owners, and others that may be involved in the design or purchase of foundations and building sites that are located in the southeast region of the state of Texas, and primarily within the City of Houston and the surrounding metropolitan area. However, many of the advantages and disadvantages discussed for each of the foundation and site design options may also apply to other geographical areas with expansive soils. Geographical areas that potentially have foundation problems similar to those in the Houston area may be identified as those having large surface deposits of clays with a climate characterized by alternating wet and dry periods. A brief overview of the design problems associated with expansive soils is provided in Section 2.0, and general design considerations are presented in Section 3.0. The foundation design options are categorized into three separate sections. Section 4.0 covers foundation system design options considering the structural foundation system as a whole. The foundation systems are subdivided into two groups: deep support systems and shallow support systems. Section 5.0 addresses design options for various individual structural components of the foundation systems that are discussed in Section 4.0. Section 6.0 discusses design options for site moisture and vegetation control systems. Foundation design options for heavily loaded structures such as mid-to high-rise buildings or large industrial structures that usually require deep foundations or thick large mat foundations are not addressed.Nor are design options for lightly loaded structures that are not susceptible to significant damage due to differential vertical movements from soil moisture changes. Examples of these types of buildings include relatively flexible light gage metal buildings with exterior metal siding and roofing and wide open interior spaces with no interior partition walls. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 6 of 42 2.0 PROBLEM DEFINITION The challenge with designing building foundations on moderate to highly expansive clay soils is the potential detrimental effects of differential movements of the foundation structural elements due to volumetric changes of the underlying and surrounding soils. In simple terms, expansive clay soils swell and can cause heave when they become moist, and can cause shrinkage and can subside when they dry out. Movement of expansive soils is directly caused by fluctuations in the moisture content of the soil particles.Because homogeneous expansive clay soils have very low permeability, fluctuations in the moisture content of the soils might normally be expected to occur over a very long period. However, geotechnical phenomena such as ground faults, surface fractures due to desiccation of clays, and decomposition of tree roots cause fissures and cracks that become widely disseminated over time. Due to the repeated wetting, swelling, drying, and shrinking of the clay as it weathers, the fissures often fill with silt and sand, and create pathways for water that can speed up the infiltration process. Water can also spread easily through naturally occurring sand strata, sand seams, and micro-cracks in clay soil caused by previous shrinkage. High negative pressures in clay soils with low water content also increase the tendency for water to be suctioned into the clay. Environmental factors other than climatic conditions can also affect expansive soils. Water extraction by tree roots or other vegetation, a process known as transpiration, can cause soil shrinking. Swelling can be a result of water infiltration into the soil from lawn irrigation systems, broken water pipes, flooded and leaking utility trenches, bad drainage, or leaking swimming pools, or it can be a result of slow moisture replenishment and equalization after the removal of a tree. Other less common phenomena may also occur. The combined effect and variability of all of these possibilities make it very difficult to predict expansive soil ground movements. Foundation movements are considered problematic only if they result in negative phenomena that detrimentally affect the performance or appearance of the building. The negative phenomena is considered to be structural if the load carrying capacity of the superstructure or foundation elements are affected, or is considered to be cosmetic if only the appearance of the exterior cladding or interior wall, floor, or ceiling finishes are affected. Negative phenomena may also affect the serviceability the building, such as the opening or closing of doors, or as may occur to the building utilities or other services, such as piping or ductwork. Negative phenomena due to foundation movement typically occur because of differential movements between various parts of the building. Differential movements often lead to high internal stresses in building components that manifest into distress in the form of cracks, splitting, bending, buckling, or separations in the exterior cladding systems such as brick, cement-board panels, or in the interior finishes such as gypsum drywall panels, wood paneling, and flooring. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 7 of 42 3M GENERAL DESIGN CONSIDERATIONS Aside from supporting the building loads, the goal of the structural foundation design in expansive soil areas should be to economically minimize the detrimental effects of foundation movement. This can be done by either isolating elements of the foundation system from potential soil movements or by using design methods and details that help to control the effects of the movement of the soil. Movements of expansive clay soils are generally restricted to an upper zone of soils known as the active zone. The lower boundary of this zone is commonly defined as the line of zero movement. The depth of the active zone usually varies from site to site. In the Houston area, this depth typically ranges from about 8 to 12 feet, but in some areas, it can be as much as 15 to 20 feet. The depth of the active zone is an important design parameter used in the engineering design of foundations on expansive soils, particularly when planning to use deep foundations. Another general consideration is the effect of the magnitude of surcharge pressure on the degree of swell that can occur. Lightly loaded foundation components, such as concrete flatwork, pavements, and building slab-on-grade floors, are much more impacted by expansive soil moisture changes than are heavily loaded foundation components such as heavily loaded bearing walls. Heavy loads reduce the amount of swell than can occur. Another goal of foundation design should be to maintain uniformity of bearing pressures as much as possible in order to reduce differential settlements. Various foundation system design options are available that meet these goals to varying degrees. Many options are also available in the design and selection of the components that make up these foundation systems; however, the choice shall be based upon an engineered geotechnical investigation. Different options are also available in the design of the site around the foundation and the selection of landscaping components. Advantages and disadvantages of these options are discussed in the following sections. 4.0 FOUNDATION SYSTEM DESIGN OPTIONS This section discusses the various types of foundation systems that are commonly used for residential and other low-rise buildings in the Houston area where expansive soils occur. In this document, the foundation system is considered to include the structural floor framing system at or near grade level and all other structural components underneath the building. The building superstructure consists of all structural elements above the grade level floor. The foundation systems are subdivided into two groups: deep support systems and shallow support systems. Each of these systems has an associated level of risk of damage that may occur to the building superstructure and architectural components due to differential foundation movements. Each of these systems also has an associated relative cost of construction. When comparing the various foundation systems, the level of risk is typically li I! I 1 FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 8 of 42 found to be inversely proportional to the level of cost. Higher risks are often accepted due to economic considerations. For example, shallow support systems have a relatively higher level of risk than deep support systems, but are often selected due to economics and affordability. Because risk of damage and economic considerations are involved, building owners and/or developers need to be involved in the selection process of the foundation system.To assist in this selection, the foundation systems are generally listed in the order of increasing levels of associated risk and decreasing levels of construction cost. 4.1 DEEP SUPPORT SYSTEMS Deep support foundation systems are defined as foundations having deep foundation components such as drilled piers or piles that extend below the moisture active zone of the soils and function to limit the vertical movements of the building by providing vertical support in a more stable lower level of soil that is not susceptible to movements caused by moisture fluctuations. 4.1.1 Isolated Structural Systems with Deep Foundations Isolated structural systems are characterized as having a superstructure and a grade level structural floor system that are designed to be physically isolated from the effects of vertical movements of expansive soils. This is accomplished by providing sufficient space between the bottom of the floor system components and the top of the soil that will allow the underlying expansive soil to heave into the space or sink without causing movement of the floor system or imparting forces on the foundation components. The structural floor system usually consists of a reinforced concrete slab and series of grade beams, but other types of materials and framing systems may be used.A crawl space may be created by constructing the floor system a few feet above the ground, or a smaller space may be created by using a void forming system. 4.1.1.1 Structural Slab with Void Space and Deep Foundations Structure slab Void boxes r-7551- T A: till: ( iacl( In.�tttl 01 — bGail] UIIIL,r�-vi �.. iomputLlll SOIL Deep ounditiou . Figure 4.1.1.1 Structural Slab with Void Space and Deep Foundations FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 9 of 42 This foundation system typically consists of a structural reinforced concrete slab with cardboard carton forms that create a void space that separates the slab from the surface soils. The depth of the void forms ranges from four to eight inches and depends on the expansiveness of the soils. The more expansive the soil (i.e. the higher the plasticity index), the deeper the cardboard carton forms needed. The slab is called a"structural slab" because it spans between reinforced concrete grade beams that are supported entirely by deep foundations, similar to an elevated parking garage. Because of the relatively small void space that is used with this system, the bottom portion of the grade beams are normally cast directly on the soil, even though they are designed to span between the deep foundations. In the past, void cartons were commonly used beneath grade beams with this system. The slabs typically range in thickness from four to eight inches. The reinforcement may consist of a single or double mat of rebar. The structural slab is designed in accordance with the American Concrete Institute (ACI) publication,Building Code Requirements for Structural Concrete, ACI 318. Void forms serve as formwork for the placement of concrete by acting as a temporary platform that supports the weight of the wet concrete until the concrete sets. Void forms typically are made of corrugated paper arranged in an open cell configuration. The exterior surface is wax impregnated to temporarily resist moisture during concrete placement. The forms are specifically designed to gradually absorb ground moisture, lose strength, disintegrate over time, and leaving a void between the expansive soils and the concrete slab. If the soil below the concrete heaves, it can expand into the space created by the void form without lifting the foundation. j FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 10 of 42 TABLE 4.1.1.1 STRUCTURAL SLAB WITH VOID SPACE AND DEEP FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Reduces vertical movements of 1. Usually results in higher 1. Slab is constructed about 4 to 8 inches slab-at-grade due to expansive construction cost. above grade using void carton forms. soils provided a sufficient void is 2. May require additional engineering 2. Slab is designed to span between maintained under slab and design effort than a slab-on-grade, grade beams.Grade beams are supporting deep foundations are and may result in higher engineering designed to span between deep founded sufficiently below active fees. foundations. zone. 2. Usually outperforms any other 3. Extra time required to construct 3. Slab is more heavily reinforced than type of foundation system. structurally isolated floor may non-structural slab. lengthen overall construction 4. Vapor retarders such as polyethylene 3. Reduces,but does not eliminate, schedule. sheathing should not be placed below need for a foundation 4. Improper carton form installation carton forms.Vapor retarders should maintenance program. may result in void that is insufficient be placed above carton forms in order 4. More rigid than a timber framed to provide for anticipated soil to allow moisture to degrade void floor with crawl space and deep expansion. boxes. foundations,resulting in less 5. Termites may be attracted to moist 5. Usually constructed with no carton differential movement of cardboard of carton forms. forms below grade beams due to superstructure. potential water infiltration into void 6. Grade beams that are in contact with 5. Allows a void under and down shafts of deep foundations. approximately 80-90%of soil may heave due to swelling of foundation when void cartons are expansive soils. 6. Installation of an expendable hard not used under grade beams. 7. Depending on slab elevation,may surface above carton forms such as allow water to collect below slab. Masonite sheeting will facilitate 6. No need for select structural fill. construction. Fill may be comprised of expansive or non-expansive soil. Fill need only be compacted to a density sufficient to support slab during setup. 7. Geotechnical design parameters are more accurately defined than for shallow support foundation systems. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. YI FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 11 of 42 4.1.1.2 Structural Floor with Crawl Space and Deep Foundations Block wall Structural floor or tsolatecl support with beams ``w k Termite shield' Crawl space Grade beam In situ or Deep to Ln1dz+1i011 whom Ise competent sail Figure 4.1,1.2 Structural Floor with Crawl Space and Deep Foundations This foundation system is similar to the previous system, except that the vertical space used to isolate the floor system is much larger, usually at least 2_feet, which is sufficient to allow access underneath the floor, hence the name "crawl space". The structural floor system can be constructed utilizing any of the following common structural components: (a)wood subfloor and joists supported by wood, steel, or concrete beams; (b) concrete floor slab and joists supported by concrete beams; or (c) steel deck and open web bar joists or cold-formed sections supported by steel or concrete beams. Other combinations of these floor-framing components are possible, and other materials may be used such as precast concrete planks or T-sections. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 12 of 42 TABLE 4.1.1.2 STRUCTURAL FLOOR WITH CRAWL SPACE AND DEEP FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Reduces vertical movement of 1. Usually results in highest 1. Ground floor is typically constructed slab-at-grade due to expansive construction cost. 30 to 42 inches above grade,but can soils,provided sufficient crawl 2. Requires more extensive design easily be greater. space is maintained under slab effort,and will result in higher 2. Floor beams typically consist of steel, and supporting deep foundations engineering fees. concrete,or wood beams spanning are founded sufficiently below between piers over a 12-30-inch high active zone. 3. Takes longer to construct because it crawl space. 2. Usually outperforms any other is labor intensive. type of foundation system. 4. Void below floor can collect water if 3. Also known as Post-and-Beam, nearby grade or other surrounding Block-and-Beam,or Pier-and-Beam. 3. Reduces,but does not eliminate, sites are at a higher elevation. 4. Flooring typically consists of wood need for foundation maintenance program. 5. Less rigid than a stiffened slab, framing,steel framing,precast which may allow more differential concrete planks,or precast double 4. Void cartons,sometimes tees. problematic in their installation, movement of superstructure,causing are not required. more cosmetic distress. 5. Crawl space should be ventilated to 6. Crawl space may allow sufficient evaporate moisture,which 5. No need for select structural fill. oxygen for roots to grow,which accumulates due to natural soil 6. Accommodates certain may cause soil shrinkage. suction,drainage problems,and architectural styles with raised plumbing leaks. first floors. 7. Proper drainage must be provided in crawl space. 7. Exposed below-floor plumbing is 8. Exposed below-floor plumbing may accessible. freeze. 8. More suitable for flood-prone areas since ground floor is generally higher than for other foundation systems. 9. Floor is easier to level than a slab- on-grade or structural slab with void space. 10.Helps to preserve nearby existing trees by allowing oxygen to root zones. 11.Allows a void under approximately 95%of foundation when void cartons are not used under grade beams,or nearly 100%when all beams are raised completely above grade. 12.Reduces settlement from soil shrinkage. 13.Geotechnical design parameters are more accurately defined than for grade-supported systems. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 13 of 42 4.1.2 Stiffened Structural Slab with Deep Foundations Stiffened Structural slab spates to grade beams grade �``„.. 1 beams �4 , on stttsciura( till 1 In Situ 01 I uh + ntherlrisc foundation 1011 S011 L Figure 4.1.2 Stiffened Structural Slab with Deep Foundations The stiffened structural slab with deep foundations is the same as the structural slab with void space and deep foundations with the following exception: the slab is placed, without a void, over the expansive soils and new fill, and the foundation must be designed to accommodate the pressures from the swelling soils. The foundation is designed as a"stiffened" slab. The grade beams form a grid-like or"waffle"pattern in order to increase the foundation stiffness and reduce the potential bending deflections due to upward movement of the foundation. Using continuous footings in a grid-like fashion helps to reduce differential deflections. The deep foundations are used to minimize downward movement, or settlement, caused by shrinking soils. The stiffened structural slab with deep foundations should be designed to resist heave in accordance with the BRAB 33 (Building Research Advisory Board), Wire Reinforcement Institute (WRI)publication,Design of Slab-on-Ground Foundations;the ACI publication,Design of Slabs on Grade, ACI 360R, or the Post-Tensioning Institute (PTI) publication,Design and Construction of Post-Tensioned Slabs-on-Ground. TABLE 4.1.2 STIFFENED STRUCTURAL SLAB WITH DEEP FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Eliminates need for removing 1. Does not limit heave that may occur. 1. Slab is designed to span between existing non-compacted fill. 2. Requires additional design effort and grade beams. Grade beams are 2. Fill may be comprised of higher design and construction cost. designed to span between deep expansive or non-expansive soil. foundations. Fill need only be compacted to a 2. Slab is typically 4 to 8 inches thick, density sufficient to support slab beam spacing is less,and slab is more during setup. heavily reinforced than for stiffened 3. Reduces settlement from soil slab on fill. shrinkage. 3. Stiffening grade beams should be continuous. 4. Slab is more heavily reinforced than non-structural slab. 'k Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 14 of 42 4.1.3 Stiffened Non-Structural Slab with Deep Foundations Stiffened Soil supported slab grade beams ..-, ,t`:omp ict<d trtictuta( Jill - � 10 situ 01 peep other∎N isc fOu tid 111011 coalpctent soil 1 Figure 4.1.3 Stiffened Non-Structural Slab on Fill with Deep Foundations This type of foundation system is a stiffened concrete slab that may bear on non-expansive select structural fill,with the stiffening grade beams spanning to deep foundations. Select structural fill can be defined as sandy clays with a plasticity index between 10 and 20, and a liquid limit less than 40. The fill acts as a buffer zone between the expansive soils and the slab, reducing the potential differential movement of the foundation.The foundation is designed as a ribbed mat that is"stiffened"with relatively deep and closely spaced grade beams. The grade beams are laid out in a grid-like or"waffle"pattern and are designed with sufficient stiffness to reduce the bending deflection caused by shrinking or swelling soils. See Section 4.1.2 for additional design information. TABLE 4.1.3 STIFFENED NON-STRUCTURAL SLAB WITH DEEP FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Usually less expensive than 1. Due to potential uplift forces,grade 1. Stiffening grade beams should be structurally isolated systems with beams may be deeper than those of a continuous across slab. deep foundations. structurally isolated system. 2. Select structural fill may be used to 2. Provides high stiffness without 2. Fill,if used,has to be field-verified reduce potential vertical rise. adding much concrete or for conformance to geotechnical reinforcement. specifications. 3. Settlement from soil shrinkage is usually less than that of shallow supported foundations. 4. Slab thickness and reinforcing is usually less than that of structurally isolated systems. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 15 of 42 4.1.4 Non-Stiffened Slab-on-Grade with Deep Foundations Grade beams below heavy Lwalls and columns only r� - ('nmopa.teP �tnictuird1 Iill 1 In silo or otherwise competent till Deep t(.iondiitiOn Figure 4.1.4 Non-Stiffened Slab-on-Grade with Deep Foundations This system consists of a slab-on-grade with grade beams under load bearing walls supported by deep foundations. The foundation will move with the underlying soils. The foundation has little resistance to soil movement with this system. Perimeter grade beams are typically provided with this system to support the exterior wall system and to prevent undermining of the slab by erosion. They may also function as a root retarder or vertical moisture retarder. Interior grade beams are also usually provided under all interior load-bearing walls and shear walls. Interior columns are typically supported directly by deep foundations. TABLE 4.1.4 NON-STIFFENED SLAB-ON-GRADE WITH DEEP FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Comparatively easy and quick to 1. Does not limit amount of vertical 1. Flat slab rests directly on underlying construct. movement that may occur. soil. 2. Typically has fewer grade beams 2. Does not limit amount of vertical 2. Warehouses are often constructed than stiffened slab foundation differential displacement that may using this method where interior slab systems. occur except for relatively small and movements can be tolerated. 3. Construction joints and isolation thick slabs. 3. Select structural fill may be used to joints may be used with this 3. Lack of grade beams makes it reduce potential vertical movements. system to allow separate concrete difficult to jack against if 4. Fill,if used,has to be field verified placements. underpinning is later required. for conformance to geotechnical 4. Slab may not provide sufficient specifications. stiffness for jacking if future underpinning is required. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. 4.2 SHALLOW SUPPORT SYSTEMS Shallow support foundation systems are defined as foundations having shallow foundation components that do not extend below the moisture active zone of the soils and are subject to vertical movements due to volumetric changes of the expansive soils. I $ 1 FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 16 of 42 4.2.1 Grade-Supported Stiffened Structural Slab Grade beams are supported directly by the underlying soils „Stiffened structural slab I ill, need not he LonI octcd In situ or t)ther'.wt,,c competent cut Figure 4.2.1 Grade-Supported Stiffened Structural Slab This foundation system is similar to that discussed in Section 4.1.2, except that the grade beams are supported directly by the underlying soils instead of spanning to deep foundations. The key advantage of this system over that discussed in Section 4.2.2, is that as long as the grade beams penetrate a minimum of six inches into the competent natural soils or properly compacted fill. Fill placed between the grade beams is only required to be compacted enough to support the concrete during placement. TABLE 4.2.1 GRADE-SUPPORTED STIFFENED STRUCTURAL SLAB ADVANTAGES* DISADVANTAGES* COMMENTS 1. Compaction of new fill below 1. May experience more vertical 1. Also referred to as a ribbed mat or slab not as critical,and eliminates movement than stiffened slabs on "super slab". need for removing existing non- deep foundations. 2. Grade beams must be supported by compacted fill. 2. More expensive than slab-on-grade competent soils. 2. Usually performs better than other and non-structural systems due to 3. Slab is designed to structurally span grade-supported slabs. more concrete and reinforcement. between grade beams. 3. If fill below slab is loosely 3. Requires more design effort than 4. Slab is typically 4 to 6 inches thick, compacted,potential vertical rise non-structural slab systems. depending on beam spacing. may be reduced as compared to 4. Experiences more vertical other grade-supported movement than dee ply su orted 5. Grade beams may be wider or more foundations. pp closely spaced than other grade- systems. supported slabs. 4. Faster to construct than slabs with 5. Does not prevent foundations from deep foundations. tilting. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 17 of 42 4.2.2 Grade-Supported Stiffened Non-Structural Slab Stiffened grade Grade su.ported stab beams ,z eclat,t1 till =�' x s in situ or othcivvi c coutpetcut soil Figure 4.21 Grade-Supported Stiffened Non- Structural Slab This foundation system is similar to that discussed in Section 4.1.2, except that the grade beams are supported directly by the underlying soils instead of spanning to deep foundations. It is also similar to Section 4.2.1 except that the entire stiffened slab is supported by the surface soils that are susceptible to the seasonal moisture fluctuations and movement. The foundation is designed utilizing continuous stiffening beams that form a grid like pattern. Grade-supported stiffened slabs should be designed in accordance with the WRI publication, Design of Slab-on-Ground Foundations, the ACI publication,Design of Slabs on Grade, ACI 360R, or the PTI publication,Design and Construction of Post-Tensioned Slabs-on-Ground. TABLE 4.2.2 GRADE-SUPPORTED STIFFENED NON-STRUCTURAL SLAB ADVANTAGES* DISADVANTAGES* COMMENTS 1. Most economical system used 1. May experience more vertical 1. Stiffened slabs are sometimes called where expansive soils are present. movement than stiffened slabs "waffle"or"floating"foundations. 2. Faster to construct than slabs with supported on deep foundations. 2. Grade beams must be supported by deep foundations. 2. Does not prevent foundations from competent soils. tilting. 3. Most commonly used foundation system in Houston area. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 18 of 42 4.2.3 Grade-Supported Non-Stiffened Slab of Uniform Thickness Mat or slab of uniform thickness r C nip,lcted strnenn ll till In situ or otlLcrl\,iSc ctottipLftnt soil Figure 4.2.3 Grade-Supported Non-Stiffened Slab of Uniform Thickness This system consists of a concrete slab-on-grade of uniform thickness with no deep support foundation components. The slab may be supported on in situ soils or compacted fill. This foundation system should be designed by the PTI method or other methods to resist the potential bending moments induced by the differential deflections of the slab when subject to expansive soil movements. TABLE 4.2.3 GRADE-SUPPORTED NON-STIFFENED SLAB OF UNIFORM THICKNESS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Faster to construct than stiffened 1. May experience more vertical 1. Also called a"California Slab". slabs and deeply supported movement than stiffened slabs on 2. Behaves similar to a mat foundation. foundations. deep foundations. 2. Eliminates digging of grade 2. For same amount of concrete, 3. Flat slab rests directly on underlying beams. potentially has more vertical soil. 3. Lack of grade beams makes it differential displacement that may 4. May include a perimeter grade beam easier to jack against if occur except for relatively small and as a root retarder or to prevent underpinning is later required. thick slabs. erosion. 3. Typically uses more concrete and 5. Typically reinforced with steel reinforcement than a stiffened conventional deformed bar reinforcing slab-on-grade(Example: 15"flat or post-tensioned cable. slab is required to provide 6. Suitable for deep sandy soil or equivalent stiffness of a 4"slab foundations having consistent subsoil stiffened with 12"wide x 24"deep formations with low propensity for grade beams spaced on 10'centers. volumetric movement. This dictates 1.9 times more concrete for a flat slab). 4. More easily allows roots to grow below foundation if there are no perimeter grade beams. 5. Subgrade and fill must be compacted if it does not meet minimum density requirements. 6. Does not prevent foundations from tilting. *Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Othe r Low-Rise Buildings on Expansive Soils s 1Apr' April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 19 of 42 4.3 MIXED DEPTH SYSTEMS Mixed depth systems are foundations that extend to different bearing depths. They are sometimes used due to concentrated loads. Although their use is discouraged for certain applications, mixed depth foundation systems are sometimes used. They may be used for new buildings with large plan areas located on a site with widely varying soil conditions, for new buildings on sites with a substantial amount of deep fill, for new buildings on a sloping hillside, for new buildings located next to a waterway or slopes greater than 5%, for existing buildings when adding a new addition, etc. When a new addition is added onto an existing building, consideration must be given to the depth of the new foundation system used for the new addition versus the depth of the existing foundation system used for the original structure. 4.3.1 Mixed Depth System for All-New Building Construction ufl p.Itttd ctut)i fait _f I In situ or otherwise competent sail 1 Variable depth foundation,-, Figure 4.3.1 Mixed Depth System for All New Building Construction Because of the increased possibility of differential movement, mixed depth systems are not often used for all-new construction except in areas of sloping grades and sloping strata. TABLE 4.3.1 MIXED DEPTH SYSTEM FOR ALL-NEW BUILDING CONSTRUCTION ADVANTAGES* DISADVANTAGES* COMMENTS 1. More economical than uniformly 1. More likely to experience 1. Pier depth,if included,may vary to deep foundation system. differential movement than follow bearing stratum or to address foundations of uniform depth. slope instability issues. 2. Often used for perimeter and point loaded commercial buildings. *Compared to other foundation systems as described in Sections 4.3.1 to 4.3.3. i' il FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low•Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 20 of 42 4.3.2 Mixed Depth System for Building Additions with Deep Foundations Slab on compacted structural fill or structurally isolated slab Addition Exi tui±!,0 uc(Lire LJ Structural dowels "" D cp foundation In�itt1 or otherwise competent soil Figure 4.3.2 Mixed Depth System for Building Addition with Deep Foundations Sometimes building additions are designed with deeper foundations than the original building in order to reduce movement of the addition.This is because the foundation of the older portion of the building has stabilized. TABLE 4.3.2 MIXED DEPTH SYSTEM FOR BUILDING ADDITIONS WITH DEEP FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Addition is more stable than a 1. More expensive than using a 1. When used in conjunction with a new new grade-supported foundation shallow foundation system for new structurally isolated slab(i.e.isolated due to less seasonally active soils addition. from soil movement,and structurally at increased foundation depth. connected to existing building) minimizes risk of differential movement. *Compared to other foundation systems as described in Sections 4.3.1 to 4.3.3. 4.3.3 Mixed Depth System for Building Addition with Shallow Foundations structural dowels Existing structure / Addition -,In.situ olt,E therwi:,c uripc,tent soil Slab on compacted SYSTEM NOT RECOMMENDED structural fill or structurally isolated slab Figure 4,3.3 Mixed Depth System for Building Addition with Shallow Foundations FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 21 of 42 li Sometimes additions are built with shallower foundations than the original building in order to reduce the cost of construction. TABLE 4.3.3 MIXED DEPTH SYSTEM FOR BUILDING ADDITION WITH SHALLOW FOUNDATIONS ADVANTAGES* DISADVANTAGES* COMMENTS 1. More economical than uniformly 1. On expansive,compactable,or 1. Not recommended due to high deep foundation system. compressible soils,more shallowly probability of differential movement. supported addition is likely to move more than existing building. *Compared to other foundation systems as described in Sections 4.3.1 to 4.3.3. 5.0 FOUNDATION COMPONENT DESIGN OPTIONS This section covers the advantages and disadvantages of common component design options for the systems that were discussed in Section 4.0. Components are referenced to other components in the same category. 5.1 DEEP SUPPORT COMPONENTS This section discusses deep foundation support components that are commonly used in new construction for residential and other low-rise buildings. This includes drilled and underreamed piers, drilled straight-shaft concrete piers, auger-cast concrete piles, displacement piles, and helical piers. 5.1.1 Drilled and Underreamed Concrete Piers Zonc or I l L),.p1Jf of i constant ntoistuf suction / E (onsiant innumunl V illOI',tU L , zone Figure 5.1.1 Drilled and Underreamed Concrete Piers • FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low.Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 22 of 42 Drilled and underreamed piers are cast-in-place concrete foundation components with an enlarged bearing area extending downward to a soil stratum capable of supporting the loads. Drilled and underreamed concrete piers have also been referred to as drilled piers, drilled shafts, caissons, drilled caissons, belled caissons, belled piers, bell-bottom piers, foundation piers, bored piles, and or drilled-and-underreamed footings. The depth of the drilled pier should extend to a depth below the moisture active zone that is sufficient to anchor the pier against upward movements of swelling soils in the upper active zone. TABLE 5.1.1 DRILLED AND UNDERREAMED CONCRETE PIERS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Has a long track record, 1. Installation requires a minimum of 1. Many contractors falsely believe approaching a century of use. four different procedures:shaft underreams should be founded at a 2. Provides better lateral load drilling,underreaming,reinforcing certain color or stiffness clay rather resistance than other deep steel placement,and concrete than at depth shown on foundation foundations with smaller placement. engineering drawings. projected surface areas. 2. Requires removing excavated soils 2. Slump should be greater than 5 inches 3. Underreamed portion off-site. to prevent honeycombing. economically provides large end 3. Sloughing of soils at pier shaft and 3. Vertical reinforcement must be used bearing capacity. bell may create problems. to resist tensile forces due to friction 4. Most commonly used deep 4. Difficult to confirm integrity of on shaft from swelling soils. foundation component in Houston concrete placed under groundwater 4. May be constructed in areas with high area. or slurry conditions. groundwater table by using slurry 5. Easier to install than displacement 5. May be difficult for some displacement method. piles in very stiff sandy soils. contractors to install drilled piers 5. Can be installed through sandy layers below a depth of 15 feet because of by using retrievable casing. equipment limitations. 6. Requires waiting until concrete sufficiently cures before applying load. 7. Drilling drilled piers below soil active moisture zone in Houston area often results in encountering water or sands. *Compared to other deep supporting elements as described in Sections 5.1.2 to 5.1.5. 1 FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 23 of 42 5.1.2 Drilled Straight-Shaft Concrete Piers t ( 1 (,ti1 ut i I'' Zone c3) cunst,ant active I suction inc)Isturc II I 1 Piet ( on�talt inelimitie zone Figure 5,1,2 Drilled Straight-Shaft Concrete Piers Drilled piers are cast-in-place concrete foundation components extending downward to a soil stratum capable of supporting the loads. Drilled straight-shaft concrete piers are not underreamed. • FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 24 of 42 TABLE 5.1.2 DRILLED STRAIGHT-SHAFT CONCRETE PIERS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Quality control is simpler than for 1. Soil borings are required to be 1. Many contractors falsely believe drilled and underreamed piers. deeper than underreamed piers, underreams should be founded at a 2. Shafts are efficient at preventing which adds to cost. certain color or stiffness clay rather lateral movement.Shafts are 2. Geotechnical reports do not than at depth shown on foundation typically larger diameter than routinely provide shaft allowable engineering drawings. drilled and underreamed piers, skin friction capacity values. 2. Only recently used as an alternative to and provide better lateral load 3. Usually requires removing drilled and underreamed footings in resistance. excavated soils off-site. light foundation industry. 3. Typically installed deeper than 4. May require steel casing to drill 3. Slump should be greater than 5 inches underreamed piers due to easier through sandy soils. to prevent honeycombing. inspection. 4. Vertical reinforcement must be used 4. Has a long track record,more 5. May require slurry or concrete to be to resist tensile forces due to friction than a century of use. pumped to bottom of hole when on shaft from swelling soils. groundwater is encountered. 5. Easier to install than displacement 6. Requires waiting until concrete piles in very stiff sandy soils. sufficiently cures before applying load. 7. Sloughing of soils may create problems. 8. Difficult to confirm integrity of concrete placed under groundwater or slurry conditions. 9. May be difficult for some contractors to install drilled piers below a depth of 15 feet because of equipment limitations. 10.Drilling straight-shaft piers often results in encountering water or sands. *Compared to other deep supporting elements as described in Sections 5.1.1 and 5.1.3 to 5.1.5. 5.1.3 Auger-Cast Concrete Piles Zone (II'actilc 1 — moisture lkpthoe! constant Constant suction Imm.turC tone Figure 5.1.3 Auger-Cast Concrete Piles FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 25 of 42 Auger cast piles are installed by rotating a continuously-flight hollow shaft auger into the soil to a specified depth. Cement grout is pumped under pressure through the hollow shaft as the auger is slowly withdrawn. TABLE 5.1.3 AUGER-CAST CONCRETE PILES ADVANTAGES* DISADVANTAGES* COMMENTS 1. Drilling through water or sands is 1. Reinforcing cage must be installed 1. Commonly utilized in situations not a problem. after auger is removed,which limits drilling through collapsing soils and 2. Can be installed in low headroom depth of reinforcing cage that can be emerging free water. applications. installed and may result in inadequate concrete cover due to 3. Can be easily installed at angles cage misalignment. other than vertical. 2. If singly reinforced,auger-cast piles do not provide significant bending resistance. 3. Higher mobilization costs than for other systems. 4. Fewer contractors are available that offer this system,making construction pricing less competitive. *Compared to other deep supporting elements as described in Sections 5.1.1,5.1.2,5.1.4 and 5.1.5. 5.1.4 Displacement Piles Zone of active L_, ia tut-Us-tuns /- Depth of con,toot constant suction inoIsture LOOC Displaccnicnt piles Figure 5.1.4 Displacement Piles For the purpose of this document, displacement piles are defined as relatively long slender members driven, vibrated, or pressed into the soil while displacing soil at the pile tip. i R FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 E Issued for Comments Foundation Performance Association-Structural Committee Page 26 of 42 TABLE 5.1.4 DISPLACEMENT PILES ADVANTAGES* DISADVANTAGES* COMMENTS 1. No excavated soils to remove. 1. Vibrations and noise that occur 1. Typically used at shoreline locations, during installation can be a problem. swamps,marshes,or other soft soil 2. Only one trade typically involved during installation. 2. Difficult to install through stiff sand areas. 3. Easier to remove than drilled piers strata. if future demolition is required. 3. Because of relatively small 4. Can be installed through soft diameters,grouping or clustering soils and water-bearing strata. may be required,which can lead to other potential problems. *Compared to other deep supporting elements as described in Sections 5.1.1 to 5.1.3,and 5.1.5. 5.1.5 Helical Piers Depth of Zone of 1 constant inohture suction 1]clix Constant Iltt flirt ttlrC COCK Figure 5.1.5 Helical Piers Steel helical piers, also known as screw anchors or screw-in piles, have been used since the early 1950s as tie back anchors for retaining walls and as foundations for lighthouses, substations,towers, heavy equipment, and other similar applications. They are now gaining popularity for use in supporting heavier foundations such as residential and other low-rise buildings. The anchor consists of a plate or series of steel plates formed into the shape of a helix to create one pitch of a screw thread. The shape of the plate permits easy installation, which is accomplished by applying torque to the shaft of the anchor and screwing it into the ground using rotary motors. The anchors may be used to resist a tensile or compressive load, which is accomplished by means of bearing pressure resistance on the area of each helix, and not by skin friction along the shaft. The plate helices of helical pier foundations are attached to a central high-strength steel shaft that may be segmented to facilitate construction and to allow various combinations of the number and diameter of helices used. The pier is screwed into the soils until the applied torque readings indicate that the necessary load capacity has been achieved or until the desired depth below the moisture active zone of the expansive soils is obtained. In new construction, the pier shafts are typically anchored to the grade beams by using fabricated brackets that are tied to the grade beam reinforcing before placing the concrete, and bolted to the top of the pier shafts. The bracket consists of a flat horizontal plate FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low•Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 27 of 42 welded to a vertical square tube that slips over the shaft of the pier. The plate is embedded into the grade beam concrete. TABLE 5.1.5 HELICAL PIERS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Can be installed in low headroom 1. Fewer contractors are available that 1. Additional protective coatings(e.g., applications in limited access offer this system,and construction coal-tar epoxy)or cathodic protection areas. pricing is less competitive. may be used to control corrosion. 2. Shaft of helical pier has small 2. Usually requires on-site test pier to surface area,which limits amount verify installability and load bearing of uplift or drag-down frictional capacity. forces that may occur due to 3. Although steel is often galvanized, vertical movements of expansive soils. corrosion may limit life expectancy. 3. Only licensed contractors may be 4. In very soft soils with low lateral used to install helical pier restraint,external concrete jacket is foundations,which provides some required to prevent buckling of means of assuring that contractors small shaft under large loads. are tested and trained in all facets 5. Square shafts of helical piers disturb of helical pier construction. soil around shaft during installation 4. Does not require excavation of to some extent,and may result in soil for installation,thus gaps occurring between soil and providing minimal disturbance to shaft along full length of pier.This site. gap may become pathway for water to flow down around shaft and 5. Can be installed in less time than activate swelling of dry expansive drilled piers or augered piles. soils in non-active zone,and may 6. Can easily be installed at a batter also allow air and moisture to speed to resist lateral loads on up rate of steel corrosion. foundation. 6. Vertically installed helical piers 7. Shaft extensions can easily be provide little resistance to lateral added to install load-bearing forces because of their small shaft helices deep below soil moisture diameter. active zone. 8. Loads may be applied immediately after installation. 9. Can be installed in all types of local weather conditions. 10.Only one trade typically involved during installation. *Compared to other deep supporting elements as described in Sections 5.1.1 to 5.1.4. 5.2 SLAB AND GRADE BEAM REINFORCING Since concrete is weak in tension, concrete slabs and grade beams are almost always reinforced with some type of steel reinforcing. The most common design options include post-tensioned reinforcing, deformed bar reinforcing, welded wire fabric reinforcing, and fiber reinforcing. Under special circumstances, unreinforced plain concrete may also be used. Advantages and disadvantages of these types of reinforcement for slab-on-grade and grade beam applications are discussed below. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 28 of 42 5.2.1 Post-Tensioned Reinforcing Post-tensioned concrete is a type of prestressed concrete in which the cables are tensioned after partial curing of the concrete has occurred. Pretensioned prestressed concrete, in which the cables are tensioned before placement of concrete around them, is not commonly used for slabs and grade beams in residential and other low-rise buildings. On the other hand, post- tensioned reinforcing has become the norm for residential slabs in most Texas metropolitan areas. Post-tensioned reinforcing consists of high-strength steel wire strands, typically referred to as tendons or cables, which are encased in plastic sheathing or ducts. When also used near the bottom of grade beams, the tendons are usually located near the top of the grade beam at the ends of the span and draped into the bottom portion of the grade beam near mid-span. Tendons typically consist of 1/2-inch diameter high-strength seven-wire strands having a yield strength of 270 ksi. The tendons are elongated by hydraulic jacks and held in place at the edges of the foundation by wedge-type anchoring devices. The type of tendons typically used in residential slabs and grade beams are single-strand unbonded tendons, in which the prestressing steel is not actually bonded to the concrete that surrounds it except at the anchored ends. This is accomplished by coating the steel strands with corrosion-inhibiting grease and encasing them in extruded plastic protective sheathing that acts as a bond-breaker. The tendons are typically fully stressed and anchored 3 to 10 days after concrete placement. I li FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 29 of 42 TABLE 5.2.1 POST-TENSIONED REINFORCING ADVANTAGES* DISADVANTAGES* COMMENTS 1. Costs less than conventional steel 1. Requires specialized knowledge and 1. Normally used locally to reinforce rebar reinforcing. expertise to design,fabricate, stiffened slabs-on-grade but may be 2. Speeds up construction because assemble,and install. used for other configurations as well. there are fewer pieces to install 2. Geotechnical design parameters, 2. Post-tensioned foundations are than conventionally reinforced. such as ym and em,are not typically designed using Post 3. Reduces post construction cracks consistently defined among Tensioning Institute(PTI)publication in concrete slabs. geotechnical engineers. Design and Construction of Post- 3. Slab design can be compromised if Tensioned Slabs-on-Ground. 4. Can reduce required amount of cracks open before stressing and fill 3. Compared to other types of post- control joints in slab. with debris. tensioned construction,residential - 5. Slab is designed as an untracked 4. Post-tension design utilizing deep slabs are lightly reinforced with section,therefore should require beams is less effective than average concrete compression levels less concrete. deformed bar reinforcing because of ranging only between 50 psi and 100 greater eccentricity and associated psi' bending moments. 5. Additional operations such as stressing,cutting,and grouting are required after concrete placement. 6. If a tendon or anchorage fails or a blowout occurs,additional operations are required for repair. 7. Tendon end anchorages,which are highly stressed critical elements of system,are located at exterior face of foundation where exposed strand ends and anchors may be susceptible to corrosion. 8. Post-tensioned reinforced foundations are susceptible to potentially dangerous blowouts,in which sudden concrete bursting failure occurs during or after stressing. 9. Making penetrations into slab can be hazardous due to presence of tensioned cables. 10.Cannot prevent cracks prior to stressing caused by plastic shrinkage,plastic settlement,and crazing at slab surface. *Compared to other types of slab and grade beam reinforcing described in Sections 5.2.2 to 5.2.5. 5.2.2 Deformed Bar Reinforcing Deformed bar reinforcing, commonly call rebar, typically consists of ASTM 615 steel having a yield strength of either 40 or 60 ksi. Grade 40 rebar was more common in pre-1970 construction, and Grade 75 rebar is expected to become more common in the future. Deformed bar reinforcing is categorized as "passive"reinforcement since it does not carry any force until the concrete member deflects and cracks under applied loads. On the other hand, post-tensioned tendons are considered "active"reinforcing because they are prestressed and carry tensile force even when loads are not applied to the concrete member. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 30 of 42 TABLE 5.2.2 DEFORMED BAR REINFORCING ADVANTAGES* DISADVANTAGES* COMMENTS 1. Less technical installation 1. Requires specialized knowledge and 1. Some local building officials do not operations than for a post- expertise to design,fabricate, require deformed bar reinforced tensioned foundation system. assemble,and install. foundations to be engineered,even 2. Post-construction slab 2. Costs more than post-tensioned though design has similar difficulty as penetrations are less hazardous to reinforcing. does post-tensioned foundations. install than in slabs with post- 2. Deformed bar reinforced slab tensioned reinforcing. 3. Slower to construct than post- foundations are typically designed per tensioned reinforced slabs American Concrete Institute(ACI) 3. Field splices are easier to 4. Foundation performance is more publication ACI 360R,Design of implement. sensitive to correct placement of Slabs on Grade,charts from Portland slab reinforcement. Cement Association(PCA) publications Concrete Floors on Ground and Slab Thickness Design for Industrial Concrete Floors on Grade,Wire Reinforcement Institute (WRI)publication Design of Slabs on Grade,Building Research Advisory Board(BRAB)publication Criteria for Selection and Design of Residential Slabs-on-Grade,or finite element methodology. 3. Local building officials may not require construction certification by engineers of foundations using only deformed bar reinforcing. *Compared to other types of slab and grade beam reinforcing described in Sections 5.2.1 and 5.2.3 to 5.2.5. 5.2.3 Welded Wire Fabric Reinforcing Welded wire fabric concrete reinforcing consists of cold-drawn wire in orthogonal patterns, square or rectangular, that is welded at all intersections, and is typically used in slab construction. Welded wire fabric (WWF) is commonly called "wire mesh", but mesh is a much broader term that is not limited to concrete reinforcement. Welded wire fabric may be made of smooth wire (ASTM A185) or deformed wire (ASTM A497), and may be manufactured in sheets (usually wire sizes larger than W4) or rolls (usually wire sizes smaller than W1.4). 4� FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 31 of 42 TABLE 5.2.3 WELDED WIRE FABRIC REINFORCING ADVANTAGES* DISADVANTAGES* COMMENTS 1. Welded wire fabric rolls can be 1. Welded wire fabric is difficult to 1. Welded wire fabric reinforced manufactured in any lengths,up position and hold in place within residential foundations are typically to maximum weight per roll that thickness of slab. designed per Wire Reinforcement is convenient for handling(100- 2. Shipping restrictions as well as Institute(WRI)methodology. 200 ft). pp g manufacturing limitations may limit 2. Local building officials may not 2. Has greater yield strength than maximum sheet size. require engineering certification of conventional deformed bar foundations using only welded wire reinforcing,which can result in 3. Application is generally limited to fabric. reducing required cross-sectional slabs only. area of steel. 4. Heavy welded wire fabric 3. Development lengths are typically reinforcement may not be readily much smaller than for deformed available,and may require special bar reinforcing. order and/or long lead time. 4. Concrete shrinkage cracks can be 5. Slab crack performance is more kept smaller due to confinement sensitive to correct placement of offered by welded cross wires, reinforcement. and microcracking is better 6. Practice of placing wire mesh on distributed. subgrade and using hooks to lift it, 5. Labor costs to install welded wire as workers walk on mesh,invariably • fabric are less than to install results in large areas of mesh conventional deformed bar remaining at bottom of slab. reinforcing(through elimination of tying reinforcing rods and faster placement of large sheets). • *Compared to other types of slab and grade beam reinforcing described in Sections 5.2.1,5.2.2,5.2.4,and 5.2.5. 5.2.4 Fiber Reinforced Concrete Fiber reinforced concrete consists of synthetic or steel fibers that help to control plastic shrinkage cracking, plastic settlement cracks. Helps reduce bleeding and water migration to slab surface, which helps to control water-cement ratio and produce concrete with less permeability and improved toughness. Fiber reinforced concrete helps increase impact resistance and surface abrasion resistance of concrete. Fiber reinforced concrete is not a substitute for structural reinforcing per ACI 544R-88. Therefore, advantages and disadvantages are not given. 5.2.5 Unreinforced Concrete Unreinforced concrete, also known as "plain" concrete, is concrete without any reinforcing. Soil-supported concrete slabs may be designed as plain concrete, as well as continuously supported grade beams. Unreinforced concrete is not used for structural foundations on expansive soils. Not recommended for use in slabs subject to movement unless cracking is not objectionable. Therefore, advantages and disadvantages are not given. 5.3 VOID SYSTEMS UNDER GRADE BEAMS AND PIER CAPS Voids used by some engineers under grade beams and concrete caps for deep foundations are commonly created by using the same type of wax-impregnated corrugated cardboard forms described in Section 4.1.1.1. If the grade beams are constructed by using the trenching method FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 32 of 42 in which concrete is cast directly against the soil in the excavated trenches, self-disintegrating void forms must be used. An alternate method of grade beam construction is to form the sides of the concrete grade beams, particularly for foundation designs that use non-expansive backfill that elevate the slab above the surrounding grade for drainage purposes. In this case, it is possible to use removable forms to create the voids under the grade beams, and to install strip forms along the bottom edges of the grade beams to keep the soil from filling the voids when backfilling against the sides of the grade beams. 5.4 VAPOR RETARDERS A vapor retarder(sometimes called a vapor barrier) is sheeting material,usually polyethylene film, which is placed under a ground level concrete slab in order to reduce the transmission of water vapor from the soils below the foundation up through the concrete slab. Vapor retarders are commonly used where moisture can migrate from below the slab and cause damage to floor coverings, household goods, or stored materials. Vapor retarders should be overlapped at least 6" at the joints, and should be carefully fitted around pipes and other service penetrations through the slab. In typical southeast Texas area slab-on-grade construction,the vapor retarder is normally placed directly on top of the finish-graded in situ clay or sandy clay soil, or when fill is added, on top of the non-expansive select fill. If cardboard void cartons are used,the vapor retarder should be placed above the void forms in order to allow moisture to degrade the void boxes. For residential construction in areas having expansive soils, concrete is most commonly placed directly on the vapor retarder. At grade beam locations,the vapor retarder is normally draped down into the excavated trench and may be continuous around the exterior surface of the grade beam. 5.5 GRADE-BEAM-TO-PIER CONNECTIONS Traditionally, drilled pier shafts in new construction have been tied to the grade beams with hooked or long straight rebar anchorage to create a connection. The recent trend over the last decade in the Houston area residential construction market is to allow the grade beams to float on top of the deep foundation components with no vertical restraints. This eliminates stresses due to fixed pier-to-beam connections. 5.5.1 Grade-Beam-to-Pier Connections with No Restraints Grade-beam-to-pier connections with no restraints means that the foundation grade beams are cast atop the already cured drilled piers, which are flat and allow the grade beam to translate relative to the pier in all directions except vertical downward. �r I FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low•Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 33 of 42 TABLE 5.5.1 GRADE-BEAM-TO-PIER CONNECTIONS WITH No RESTRAINTS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Allows foundation to rise more 1. Foundation may move laterally due uniformly if underlying soils to swelling soils or sloping sites. swell. 2. After concrete is placed,cannot 2. Extended pier steel is not bent easily verify if reinforcing steel was during construction of grade installed in piers. beams 3. Pier steel does not interfere with trencher during grade beam excavation. 4. Easier to clean mud from top of pier caps. *Compared to other types of grade-beam-to-pier connections described in Sections 5.5.1 to 5.5.3. 5.5.2 Grade-Beam-to-Pier Connections with Horizontal-Only Restraints Grade-beam-to-pier connections with horizontal-only restraints means that the foundation grade beams have a positive connection to the piers in any lateral direction, but the grade beams are allowed to translate vertically upward, relative to the piers. TABLE 5.5.2 GRADE-BEAM-TO-PIER CONNECTIONS WITH HORIZONTAL-ONLY RESTRAINTS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Horizontal foundation movement 1. Requires additional labor and 1. Bond-breakers,such as sleeved is limited. material. deform bars,non-deformed bar 2. Allows foundation to rise more dowels,and shear keys may be used. uniformly if underlying soils 2. Used at sloping sites where lateral swell. resistance is required. *Compared to other types of grade-beam-to-pier connections described in Sections 5.5.1 to 5.5.3. 5.5.3 Grade-Beam-to-Pier Connections with Horizontal and Vertical Restraints Grade-beam-to-pier connections with horizontal and vertical restraints means that the foundation grade beams are connected to the top of the piers in such a way that there can be no relative translation in any direction. TABLE 5.5.3 GRADE-BEAM-TO-PIER CONNECTIONS WITH HORIZONTAL AND VERTICAL RESTRAINTS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Can provide uplift resistance from 1. Impedes or prevents jacking of a 1. Connection typically is an extension swelling soils if piers are foundation that must be lifted.Pier of pier shaft vertical deformed adequately anchored. reinforcing must severe before reinforcement. 2. Horizontal foundation movement lifting. 2. Necessary at sloping sites where is limited. 2. Does not allow grade beams to lateral resistance is required. freely lift off piers if upper strata heave occurs.This can cause distress in slab because it is more flexible than grade beams. 3. Beams,beam-to-pier connections, and slab must be designed for uplift forces due to swelling soils. *Compared to other types of grade-beam-to-pier connections described in Sections 5.5.1 to 5.5.3. i! FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 34 of 42 6.0 FOUNDATION SITE DESIGN OPTIONS The various types of mitigation options to limit the damaging effects of soil swelling due to improper drainage and soil shrinkage due to transpiration of trees and shrubbery that are discussed in this guide may be categorized into two basic groups: (1)moisture control systems, and (2) vegetation control systems. 6.1 MOISTURE CONTROL SYSTEMS Moisture control systems mitigate damage by controlling the amount of water and moisture that enter into the site soils. This includes methods to control stormwater runoff and methods of providing irrigation to site vegetation. 6.1.1 Site Drainage Systems Three methods of controlling site drainage include site grading, French drains, and area drains. These systems reduce vertical movements of building foundations by moderating the effects of seasonal moisture changes. 6.1.1.1 Site Grading Site grading causes the excess water to flow away from the foundation via surface sloping and drainage swells. TABLE 6.1.1 SITE GRADING ADVANTAGES* DISADVANTAGES* COMMENTS 1. Fill materials are readily 1. Improper grading may cause water 1. Materials should consist primarily of available. to shed to adjacent properties. clay.Do not use bank sand or clayey 2. Less maintenance required 2. Fill materials require proper sand or silts. afterwards. compaction and material. 3. Inadequate drainage is easier to detect. 4. Most economical. *Compared to other types of site drainage systems described in Sections 6.1.1. 6.1.1.2 French Drains French drains are subsurface drainage systems that are used around the perimeter of a foundation to remove free water in the subsoil. Is 3s FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 35 of 42 TABLE 6.1.1.2 SUBSURFACE DRAINAGE SYSTEMS(FRENCH DRAINS) ADVANTAGES* DISADVANTAGES* COMMENTS 1. Helps reduce moisture infiltration 1. Not very effective in expansive clay 1. Normally used only when site is too from underground water sources. soils because soil suction is high flat to accommodate proper grade 2. Suitable when site grading is not when soil permeability is very low. slopes. an available option. 2. Can cause erosion of surrounding 2. Usually consists of a 4"or larger PVC soil,causing settlement of perforated pipe,covered with sand foundation if too close. and gravel and sloped to a positive 3. Requires some maintenance but is outlet. difficult to monitor. 3. Also utilized in removing moisture 4. French drain fabric membranes may behind retaining and basement walls. tear or be punctured. 5. If clogged,allows water to collect adjacent to foundation and may cause soil heaving. 6. If used in conjunction with a vertical moisture retarder on one side, sufficient geotechnical or geophysical testing is required to determine natural flow direction and source of groundwater. 7. Most expensive system. *Compared to other types of drainage systems described in Sections 6.1.1. 6.1.1.3 Area Drains Area drains(catch basins) are surface collection systems used around the perimeter of a foundation to remove surface water by gravity flow or mechanical lifting. . TABLE 6.1.1.3 AREA DRAINS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Sumps can be utilized to 1. A clogged drain may cause localized 1. Commonly used in back yards when discharge water in low areas flooding. site is too flat to accommodate proper 2. Suitable when site grading is not 2. If used in conjunction with a vertical grade slopes. an available option. moisture retarder on one side, 2. Usually consists of a 4"or larger PVC 3. Water from downspouts can be sufficient geotechnical or non-perforated pipe. geophysical testing is required to discharged into area drainage 3. When used sump failure may cause system. determine to natural flow direction flooding and may require periodic and source of surface water. maintenance. 3. A leaking pipe may cause soils to 4. May be used in conjunction with site erode or swell. grading. *Compared to other types of drainage systems described in Sections 6.1.1. 6.1.2 Moisture Retarder Systems Moisture retarder systems are used to reduce moisture transfer to the soils underneath foundations. Such systems include horizontal moisture retarders and vertical moisture retarders. These systems help moderate effects of seasonal changes on foundation movements. 1? FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 36 of 42 6.1.2.1 Horizontal Moisture Retarders Horizontal moisture retarders usually consist of materials of low permeability. These systems extend outward around the edges of the foundation. Sidewalks, driveways, or parking lots may be multifunctional, also serving as moisture retarders. TABLE 6.1.2.1 HORIZONTAL MOISTURE RETARDERS ADVANTAGES* DISADVANTAGES* COMMENTS I. Readily inspected and maintained 1. Requires larger area to be effective. 1. Usually concrete or asphalt pavement. 2. Requires less slope than green 2. Heaving of retarder may occur due 2. Horizontal moisture retarders slow space(e.g.,only 1/8"/ft.)to to soil hydration. root growth by reducing oxygen achieve positive drainage away 3. Fabric membrane retarders more transmission to roots. from foundation. prone to tear or puncture. 3. Doubles as root retarder. 3. Can double as pavement for 4. Unsecured rigid retarders may parking or sidewalk. "walk-away"from building and allow water infiltration through gap created at edge of building if seal is not maintained. 5. Owner or future owner may remove it,not realizing it serves a design purpose,and inadvertently eliminate benefit it provides. 6. Only retards surface moisture. 7. May not be aesthetically acceptable. *Compared to other types of moisture retarder systems described in Sections 6.1.2. 6.1.2.2 Vertical Moisture Retarders Vertical moisture retarders usually consist of materials of low permeability that extend downward from grade level around the perimeter of the foundation. TABLE 6.1.2.2 VERTICAL MOISTURE RETARDERS ADVANTAGES* DISADVANTAGES* COMMENTS 1. More effective in retarding lateral 1. Higher cost. 1. Usually consists of concrete,steel, moisture migration. 2. Requires severing tree roots and polyethylene or fabric sheets,or 2. Controls vertical movements may compromise tree health. bentonite clay. better. 3. Is difficult to inspect and to know 3. Also functions as a root retarder. when to repair or replace. 4. Fabric membrane retarders more prone to tear or puncture. 5. May retain moisture from under slab leaks and exacerbate heaving. *Compared to other types of moisture retarder systems described in Sections 6.1.2. 6.1.3 Watering Systems Watering systems are usually used to induce moisture into the soils and to water vegetation around the foundation, thereby attempting to provide a constant and uniform moisture condition. During droughts, water may be rationed, preventing use of these systems. A soil moisture sensor with automatic controls is recommended with these watering systems. FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 37 of 42 6.1.3.1 Sprinkler Systems An irrigation system consists of below grade piping and above grade sprinkler heads. TABLE 6.1.3.1 SPRINKLER SYSTEMS ADVANTAGES* DISADVANTAGES* COMMENTS L May attract tree roots away from 1. If not properly monitored,may 1. Only enough irrigation should be foundations if system is properly result in excess watering,resulting applied to sustain vegetation,so that drained,zoned,and set. in heave or loss of soil bearing. there is no ponding or algae buildup. 2. Provides moisture during dry 2. Leaks may not be detected thereby periods,thereby reducing causing localized foundation movement due to soil shrinkage. movement. 3. Can provide more uniform 3. Requires more maintenance than moisture content to site. other watering systems. 4. Overspray onto superstructures may occur. 5. Results in more waste of water by runoff and evaporation. *Compared to other types of watering systems described in Sections 6.1.3. 6.1.3.2 Soaker Hose Systems Soaker hoses are permeable water conduits resembling garden hoses normally used to water localized areas. TABLE 6.1.3.2 SOAKER HOSE SYSTEMS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Properly maintained soaker hoses 1. Hoses may be subject to premature 1. Normally limited to garden and apply water more slowly to soil deterioration. foundation applications. than sprinkler systems. 2. Sensitive to damage from freezing. 2. Can be buried to reduce evaporation 2. May be used to provide moisture 3. May attract roots toward foundation and avoid damage from lawn to vegetation. equipment. if used around perimeter of 3. Easiest to install. foundation. 4. Hoses may not disburse water uniformly in long runs. *Compared to other types of watering systems described in Sections 6.1.3. 6.1.3.3 Under-Slab Watering Systems Under-slab watering systems are installed under slabs to provide moisture directly below the foundation. These systems typically consist of a network of piping, wells, and moisture sensors, which are intended to function together to maintain a uniform level of moisture in the soil beneath the structure. G: Y FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 gl Issued for Comments Foundation Performance Association-Structural Committee Page 38 of 42 gEg' TABLE 6.1.3.3 UNDER-SLAB WATERING SYSTEMS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Low to medium cost although 1. Requires strict monitoring and 1. Low cost foundation watering systems high-cost systems are also maintenance. that do not include a continual soil ' available. 2. May take a long time to stabilize moisture content monitoring system 2. Minimizes evaporation. vertical building movements. that is connected to moisture release valves is considered an unacceptable 3. Difficult to install underneath system. existing buildings.High-cost system requires cutting holes through slab- at-grade to install system. 4. Soil irregularities and discontinuities may limit effectiveness of system. 5. Dramatic subsidence could occur if system is disabled. 6. Attracts roots toward foundation, which may increase dependency on this system. 7. Monitoring program should be included that entails regular geotechnical testing and foundation level distortion surveys. 8. During droughts,water may be rationed,preventing its use. 9. Moisture sensors are subject to frequent replacement and performance may be unreliable. 10.Desired moisture content under slab is difficult to determine. *Compared to other types of watering systems described in Sections 6.1.3. 6.1.3.4 Drip Watering Systems Drip irrigation slowly applies water to soil under low pressure through emitters, bubblers, or spray heads placed at each plant. TABLE 6.1.3.4 DRIP WATERING SYSTEMS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Offers increased watering 1. Requires strict monitoring and 1. Drip irrigation slowly applies water to efficiency and plant performance maintenance. soil under low pressure through when compared to sprinkler Not permanent. emitters,bubblers,or spray heads irrigation. placed at each plant. 2. Can be installed without 3. Frost sensitive. 2. Normally limited to garden and excavation. 4. May attract roots toward foundation foundation applications. if irrigation is excessive near 3. A moisture meter is recommended building. with this type of system. 5. System may not disburse water uniformly. *Compared to other types of watering systems described in Sections 6.1.3. I 1 I FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 39 of 42 6.2 VEGETATION CONTROL SYSTEMS Vegetation control systems mitigate damage by providing some control over the growth of roots that may penetrate into unwanted areas and cause shrinkage of foundation soils by means of water withdrawal through the transpiration process. 6.2.1 Root Retarder Systems Root retarder systems are typically physical or chemically induced barriers that limit the growth direction of the roots of trees, shrubbery, and other large plants. 6.2.1.1 Vertical Root Retarders Vertical root retarders are vertical barriers that are installed in the ground adjacent to the perimeter of a foundation or around a tree or other large plant. I; FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low•Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 40 of 42 TABLE 6.2.1.1 VERTICAL ROOT RETARDERS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Minimal disturbance of 1. Requires severing tree roots, 1. May be an impervious type(e.g., landscaping. necessitating tree pruning or other plastic,concrete or sheet metal piling) 2. Impervious type root retarder also treatment when using near existing or a biocide type. doubles as moisture retarder, trees. 2. Vertical root retarders slow root which can prevent excess 2. May compromise tree health. growth below a foundation by forcing moisture resulting from drainage 3. May compromise tree stability. roots to grow to a greater depth. problems from reaching 3. Should be professional installed to foundation. 4. Will only help to control vertical minimize root damage. foundation movements,not stop it. 5. Limited warranty and limited period 4. Non-impermeable retarders require of effectiveness(for biocide more maintenance. systems). 6. Requires special details for penetration of building utility lines. 7. Very difficult to inspect and to know when to repair or replace. 8. Sometimes difficult or impractical to install below deepest lateral tree roots. 9. Usually aesthetically required to be installed completely below grade, which may allow roots to grow over retarder. 10.Impervious type of root retarder also acts as a moisture retarder,which may interrupt existing natural below-grade moisture movement due to soil suction or gradients. 11.Extensive geotechnical and geophysical testing may be required to ensure that installation of impervious retarders is not detrimental to foundation. 12.For cases where retarder is installed adjacent to structure,foundation support may be compromised in order to install retarder deep enough to be useful. *Compared to other types of vegetation control systems described in Sections 6.2.1. l . FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low.Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 41 of 42 6.2.1.2 Horizontal Root Retarders Horizontal root retarders are horizontal barriers that are installed on top of the ground adjacent to the perimeter of a foundation or around a tree or other large plant. TABLE 6.2.1.2 HORIZONTAL ROOT RETARDERS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Doubles as pavement for parking 1. Not as architecturally pleasing, 1. Normally concrete pavement. or sidewalks. especially for residences. Horizontal root retarders slow root 2. Its presence prevents planting of 2. Needs steel reinforcement and growth by minimizing oxygen flow to trees near foundation. expansion joints to prevent cracking roots. 3. Doubles as a horizontal moisture due to ground movement. 2. May not be effective in preventing retarder. 3. Expansion joints and cracks must be root growth. 4. Quick to install. sealed to retard oxygen transfer to soil. 5. Relative low cost. 4. Joint seals require maintenance, which is easily forgotten by Owner. 5. Owner or future owner may remove it,not realizing it serves a design purpose. 6. Tree roots may lift and break retarder causing vertical offsets. *Compared to other types of vegetation control systems described in Sections 6.2.1. 6.2.2 Root Watering Wells Root watering wells are installed near trees to provide moisture below grade. These systems typically consist of a drilled hole filled with coarse material. Piping may be inserted in the holes in order to maintain a clear path for water access. TABLE 6.2.2 ROOT WATERING WELLS ADVANTAGES* DISADVANTAGES* COMMENTS 1. Minimal excavated material. 1. Effectiveness of reducing moisture 1. If used,root watering wells should be 2. Minimal disturbance of withdrawal from under building is installed on side of tree opposite landscaping. questionable. foundation. 3. Can be beneficial to health of 2. Beneficial effects may take long trees by helping them establish time to materialize. roots at a greater depth. 3. Require maintenance. 4. May help keep new tree roots 4. Require ongoing operating costs and from growing under buildings water usage. provided wells are installed away 5. May hit utility line or tree root when from foundation. drilling. 5. Less likely to damage existing 6. Movement of water through tree roots during installation unfractured clays is extremely slow. process. 7. Chlorinated water directly applied to deep roots may be detrimental to tree health. Compared to other types of vegetation control systems described in Sections 6.2.1. I , FPA-SC-01-X(DRAFT) Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils 1 April 2004 Issued for Comments Foundation Performance Association-Structural Committee Page 42 of 42 6.3 TREE AND PLANT SELECTION When doing the initial site landscaping design, the proper selection of site vegetation with regard to tree and plant moisture requirements can directly affect future foundation performance. Vegetation selection may also be a deciding factor in the selection of other moisture and vegetation control system design options. TABLE 6.3 TREE AND PLANT SELECTION ADVANTAGES* DISADVANTAGES* COMMENTS 1. Proper tree and plant selection 1. Limits available vegetation for 1. An example of site vegetation control may also be aesthetically landscaping design. is Xeriscape landscaping,defined as pleasing. 2. Owner or future owner may remove quality landscaping that conserves 2. Proper tree and plant selection trees and shrubbery with low-water- water and protects environment,by may increase property value. requirements,not realizing they using plants and trees with low-water serve a design purpose. requirements. 2. Selecting plants and trees with low- water requirements can reduce potential for problems caused by vegetation water demands. 3. Using vegetation with low water requirements means less run-off of irrigation water that may carry polluting fertilizers and pesticides to nearby streams or lakes,and less permeation of irrigation water into ground that may leach nutrients deep into soil away from vegetation and increase chances of polluting groundwater. *Compared to other types of vegetation control systems described in Sections 6.2.