HomeMy WebLinkAbout05062004 BSC Agenda Item 4 Page 1 of 1
Annette Arriaga
From: James L. Dougherty, Jr. [jdough8 @attglobal.net]
Sent: Tuesday, May 04, 2004 1:28 PM
To: Sallye Clark; Ron Wicker; Nes Tesno; Michael Ross; Kay Holloway; Dennis Mack
Cc: Annette Arriaga
Subject: BSC preparations
Hello, Dennis,
Here is a re-draft of the foundation section. The main changes are to reference the ASCE-TS
and adopt a performance standard for foundations (PVR of one inch or less). The ASCE-TS
document simply referred to the TxDOT standard for measuring PVR, but I could not find a
copy of it in any of the materials. Does anyone have it?
If Council adopts the PVR performance standard, will it cover all the other details the BSC
discussed (like boring spacing, depth of piers, PI, fill, compaction, finish grading, etc.)?
Note: I left in the exception for "old stock" housing, but limited to SFR use only.
Does the BSC also want to see a flood plain ordinance amendment (to require the use of the
HIGHER of the three water levels: old map, new map, or crown of street)?
/s/ Jim
James L. Dougherty, Jr.
Attorney at Law
5120 Bayard
Houston, Texas 77006
Phone: 713-880-3800
Fax: 713-880-1417
E-Mail: jim @jdlaw.prserv.net
CONFIDENTIALITY NOTICE: THIS MESSAGE (WHICH INCLUDES ANY ATTACHMENTS)
IS INTENDED ONLY FOR THE NAMED ADDRESSEE(S). IF YOU ARE NOT AN INTENDED
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5/4/2004
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.
is :: : •• : •. :: : : . • _
professional engineer("RPE"), and the work shall be:
illustrated in complete plans andmpvcifications signed and scaled by the RPE;
lr 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
• _ 4 S
inspected by an RPE who certifies proper construction,before work proceeds
furthei-
All foundations and structural framing for new buildings with a gross floor area 400
square feet or more must meet all the following criteria.
a. Engineering. Foundations and structural framing must be constructed in
accordance with complete plans and specifications prepared, signed and sealed by
a licensed professional engineer("LPE"). 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. Soils Report. The plans and specifications for each foundation must be based on a
written soils report. The report must meet criteria as to authorship, scope, content
and form specified by the building official. The building official may adopt
standard criteria published by the Structural Committee of the Foundation
Performance Association, Houston,Texas or the American Society of Civil
Engineers, Texas Section (ASCE-TS).
c. Foundation Performance Standard. Each foundation must be designed, installed
and constructed to achieve a potential vertical rise (PVR) of one inch or less.
PVR is determined in accordance with Texas Department of Transportation
Method 124-E, dry conditions, as publishing in_ and
dated . The BSC may issue a special exception for a foundation
that does not meet this criterion, but only upon a showing of engineering data and
opinion to demonstrate that the proposed foundation will minimize PVR to the
smallest level that is practicable.
d. Professional Observation & Certification. Each foundation and all structural
framing must be professionally observed and must be certified by an LPE before
work proceeds further.
(1) Observations must:
(i) be performed either by the certifying LPE or by a person under that
LPE's direct supervision and control whose professional
qualifications are approved by the LPE,
(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.
(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 LPE and reported to the building official and
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, and
(v) be signed and sealed by the certifying LPE.
Exception ("old stock"housing):The preceding criteria do not apply to a foundation for a
new building (or building addition)on a building site where: (i)there is an existing
principal building used for single-family residential purposes, at least part of which is at
least ears old and ii after com.letion of the new buildin•s and additions the total
framed area of all buildings on the site will not exceed %of the area of the building
site.
4. All concrete piers, footings and foundations must be cured for at least 72 hours before any
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.
3. Even if permitted by this code,copper tubing shall not be used for the yard service line.
4. Amend Section 311.2 to read in its entirety as follows: "Low pressure(not to exceed 0.5
PSI) gas piping shall withstand a pressure of at least 10 inches of mercury for a period of
time not less than 10 minutes without showing any drop in pressure, except that the
following shall apply in the case of new construction: The newly-constructed system
must withstand a pressure of at least 25 PSI for a period of not less than 10 minutes
without showing any drop in pressure as an initial pressure test,and the system must also
withstand a pressure as a final test. Higher pressure piping must withstand pressure of at
least 10 PSI,but never less than twice the maximum pressure to which the piping will be
subjected in operation, for a period of at least 10 minutes without showing a drop in
pressure,but the higher pressures required for new construction,above,shall be used to
test new construction in lieu of the 10-PSI level prescribed by this sentence."
5. There must be a permanently-installed stairway,either fixed or folding,to serve attic
space where appliances or equipment are located.
6. Even if permitted by this code,undiluted liquefied petroleum gas,or"LPG",shall not be
used at any fixed location in the City. Exception: This does not prohibit the use of such
gas in quantities of 10 gallons or less.
7. Each new or replaced gas meter shall be located on the same building site that it serves.
International Mechanical 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. Add to Section M306.3: "There must be a permanently-installed stairway, either fixed or
folding,to serve attic space where appliances or equipment are located."
3. Add to Section M603: "All return air ducts must be installed within 10 inches of the
finished floor in all new residential construction and wherever possible in existing
buildings."
4. Delete: Appendix MB(Recommended Permit Fee Schedule).
International Plumbing 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 P103,P106 and P109 and Appendices PA(Plumbing Permit Fee
Schedule)and PG(Vacuum Drainage System).
3. Add at the beginning of Section 303.1: "Even if permitted by this code(IPC), ,none of
the following is allowed for use in the City: Acrylonitrile-Butadiene-Styrene(ABS)pipe
or fittings,polyethylene pipe or fittings,Type M copper, lead-based pipe,aluminum
DWV pipe or components,or air admittance valves."
4. Even if permitted by this code(IPC),PVC and CPVC type water pipe and fittings are not
allowed for use in the City. Exception: PVC water pipe may be used where permitted by
this code(IPC),but only if: (i)it is installed underground and(ii)all joints are primed and
glued as required by the manufacturer's recommendations(and the primer must be purple
or another distinctive color,except on above-ground pool piping).
5. Even if permitted by this code(IPC),wet venting shall not be allowed except when
authorized by the BSC,as a special exception for hardship and unusual cases.
6. Amend Section 1101.2 to read in its entirety as follows: "The provisions of this chapter
are applicable to interior leaders, building storm drains, building storm sewers, exterior
conductors, downspouts,roof gutters and other storm drainage fixtures and facilities."
7. Maximum water meter size,unless an RPE can clearly and convincingly demonstrate the
need for a larger meter in a particular case, is: 3/4ths-inch for an irrigation system, or 1-
inch for a single-family dwelling.
International Residential 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. This code, in lieu of the other"International Codes," applies to all residential structures in
the City. "Residential"means having the character of a detached one-family or two-
family dwelling that is not more than three stories high with separate means of egress,
including the accessory structures of the dwelling. This code does not apply to: (i)any
dwelling that has a common means of egress, such as a common hallway, or(ii) any
dwelling or structure that has the character of a facility used for accommodation of
transient guests or a structure in which medical, rehabilitative, or assisted living services
are provided in connection with the occupancy of the structure.
3. All amendments and deletions to the other"International Codes"adopted by this
Schedule are also carried forward and adopted as amendments and deletions from the
International Residential Code.
4. Delete: Appendices RAF (Radon Control Methods),RAI(Private Sewage Disposal), and
RAE(Manufactured Housing Used as Dwellings).
5. This code does not apply to installation and maintenance of electrical wiring and related
components. See National Electrical Code,below.
(BOCA)National Building Code, 1996 Ed., Building Officials & Code Administrators
International,Inc.
Only Sections 3108 (Radio And Television Towers)and 3109 (Radio And Television
Antennas),together with any necessary definitions or interpretative aids, are adopted. See
Subchapter G of Chapter 6 of this Code.
National Electrical Code, as it existed on May 1,2001, National Fire Protection Association,
("NEC").
1. The administrative officer is the building official. All hearings, variances etc. are handled
by the BSC.
2. See Chapter 8 of this Code for various provisions which override or supplement the NEC.
Standard Housing Code, 1997 Ed., Southern Building Code Congress International, Inc.
1. The administrative officer is the building official. All hearings, variances etc. are handled
by the BSC.
SEMINAR
SOILS AND MATERIALS TESTING SEMINAR
FOR DESIGN AND CONSTRUCTION OF
RESIDENTIAL STRUCTURES
BY
DAVID A. EASTWOOD, P.E.
o Study of Area Faulting and Subsidence
o City Requirements for Geotechnical Exploration
o Field Studies
Site Conditions
Drilling Equipment
Sampling Methods
Groundwater Sampling
o Laboratory Testing
o Analysis of Data
o Expansive Soils
o Foundation Types, Design and Construction
Drilled Footings
Conventionally Reinforced Slab
Post-Tensioned Slab
o Quality Control
Soil Testing
Drilled Footing Inspection
Slab Inspection
Concrete Testing
o Foundation Failures and Repair Techniques
o Soil Stabilization
Lime Stabilization
Cement Stabilization
Administrative Form No. 127 Rev.07-02 File:wp8.0\administ\seminar.rs2
' GEOTECH ENGINEERING AND TESTING
Amendment
to set higher floor heights
(new flood maps)
5-6-04
Amend Section 6.401 of the Code of Ordinances, as follows:
Sec. 6.401. Standard codes and floodplain regulations.
(a) Standard Codes. The standard codes (including new"International Codes")
described in the Standard Codes Schedule are adopted,ordained and incorporated into this
chapter by reference,but subject to the exceptions and amendments stated in Standard Codes
Schedule. Exceptions: The International Plumbing Code, International Fuel Gas Code,
International Fire Code and National Electrical Code are adopted as part of other chapters of this
Code(e.g., Chapter 8, 9 or 17).
(b) Floodplain Regulations:Minimum Floor Elevation. The standard state-promulgated
flood damage prevention(floodplain)regulations are adopted, ordained and incorporated into
this chapter by reference, as set out in Appendix A of City Ordinance No. 1647,passed and
approved June 12, 2000 (and such regulations are also adopted and promulgated as rules under
Subchapter I, Chapter 16 of the Texas Water Code). Notwithstanding such regulations,the
minimum elevation of the lowest floor(including basement)of any new construction or
substantial improvement of any structure shall be inches above the highest of the following:.
(i) the base flood level at the location of the structure, as determined under the
floodplain regulations adopted by this subsection,
(ii) the base flood elevation(sometimes called"100-year"floodplain elevation) for
the location of the structure, as indicated in that certain map entitled"Flood
Hazard Recovery Data, Brays Bayou Watershed"published by Harris County
Flood Control District and dated March 2004, or
(iii) the crown of the street at the point closest to the center of the front street line of
the building site where the structure is located.
RECOMMENDED SCOPE OF GEOTECHNICAL ENGINEERING SERVICES FOR THE
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\westuniversity
• FPA Peer Review- Doc. No. FPA-SC-01-X Page 1 of 2
Annette Arriaga
From: Michael Skoller[michael @structural.nu]
Sent: Friday, April 02, 2004 6:47 AM
To: aarriaga @westu.org
Subject: FPA Peer Review- Doc. No. FPA-SC-01-X
This paper is a draft paper going into peer review. Below is the peer review cover letter.
Please call me with any questions.
Thank you,
Michael Skoller
713-956-2094
Dear Foundation Performance Association Member,
You are receiving this email because you are a current member of the FPA. As a member, we invite you
to participate in the peer review of the following document:
Doc.No. FPA-SC-01-X, "Foundation Design Options for Residential and Other Low-Rise
Buildings on Expansive Soils" The subcommittee (which is part of the Structural Committee) formed
over four years ago to develop this comprehensive document. The purpose of this document is to
educate the public on the advantages and disadvantages of the various foundation design options
available in the industry.
This peer review procedure is being conducted in accordance with the FPA peer review procedure at:
http://www.foundationperformance.org/peer_review.html, which states:
"3. The document shall be circulated by the chair of the committee or its subcommittee with
instructions for the FPA members to respond within 45 days (or longer if the committee allows such)
and to respond with written specific comments (i.e., "strike that, add this") or with a written explanation
of why a major portion of the document shall be re-written or omitted."
If you would like to submit comments on this document, you may do so to the undersigned by no later
than 15 May 2004.
This document is being sent to you in Adobe Acrobat and MS Word formats. If you cannot see the
attachments, try hitting "forward" to see if they appear in the new window.
If your comments are extensive, please consider making electronic comments in the Word file. This can
be done by just typing your additions and deletions when you open the file.
The structural committee plans to present this document in its final form at the FPA meeting on 16 Jun
01. See: http://www.foundationperformance.org/upcomingevents.html
For more information on the structural committee, to review our other projects, or to see how you may
join the committee, please see: http://www.foundationperformance.org/structural.html
4/5/2004
FPA Peer Review- Doc. No. FPA-SC-01-X Page 2 of 2
Thank you for your participation,
Ron Kelm, P.E., Structural Committee Chair, Committees Chair
Forensic Engineers Inc.
9930 Shadow Wood Dr. Houston, Texas 77080-7110 USA
T: 713-468-8100 F: 713-468-8184 E: ronkelm @forensiceng.com
4/5/2004
~
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 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
www.foundationperformance.orq
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
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 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
f
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.
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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.
4
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3.0 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
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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
Structural slab Void boxes
TA
Non-struefral li11_,
'`Grade
In situ or beam
oi het-\l=1SC
competent soil Deep
jundation
Figure 4.1.1.1 Structural Slab with Void Space
and Deep Foundations
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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.
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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.
6. Grade beams that are in contact with potential water infiltration into void
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.
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4.1.1.2 Structural Floor with Crawl Space and Deep Foundations
Block Wall Structural floor
or isolated
support 1
with beams
�*► Termite shield -
Crawl space
Grade beam__.-_..__
In situ or
ti
d
f
Deep foundation otherwise
competent soil
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.
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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 framing,steel framing,precast
program. 5. Less rigid than a stiffened slab, concrete planks,or precast double
which may allow more differential
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.
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4.1.2 Stiffened Structural Slab with Deep Foundations
Stiffened
Structural slab spans to grade beams grade
beams
Non-structural fill
}
In situ or
Deep )�}
foundation!
competent soll
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.
*Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3.
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4.1.3 Stiffened Non-Structural Slab with Deep Foundations
Stiffened
Soil snppol tLd slab
grade
beams
C'oropacicii` �
:auto'al fill
In Situ ot-
1)c0p * otherwise
foundation competent soil
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 4.2.1
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4.1.4 Non-Stiffened Slab-on-Grade with Deep Foundations
Grade beams below heavy
walls and columns only
NNA
ampdctLd
tl actor al lily
In situ or other
competent fill
Deep ��
ftmndation °
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.
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4.2.1 Grade-Supported Stiffened Structural Slab
Grade beams are supported direedy by the underlying soils
,,,,,Stiftelted structural slab
I-di, need not be
� --
Lump rtcd a, ;
j
_ `b Inchcs —..
In situ or otherwise competent soil
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 I. 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 supported 5. Grade beams may be wider or more
foundations. pp closely spaced than other grade-
systems.
4. Faster to construct than slabs with supported slabs.
deep foundations. 5. Does not prevent foundations from
tilting.
*Compared to other foundation systems as described in Sections 4.1.1 to 4.2.3.
1
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4.2.2 Grade-Supported Stiffened Non-Structural Slab
Stiffened
grade
Grade suported slab beams
r
chip acted
1 ,,ir uci ur ti ( 1l_
L
In situ or otherwise competent soil
Figure 4.2.2 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.
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4.2.3 Grade-Supported Non-Stiffened Slab of Uniform Thickness
Mat or slab of unitbrin thickness
K ompn.toti stluctm.ai fill
In situ or otherwise competent 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
gg g soil.
beams. potentially has more vertical
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.
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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
Competed
j
stlueturai fill
In situ or otherwise competent sail
Variable depth
foundations
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.
{9i
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4.3.2 Mixed Depth System for Building Additions with Deep Foundations
Slab on compacted
structural fill or
structurally
isolated slab
Addition Existing structure
I ��' Structural dowels
Deep foundation
In situ 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
Existlilg structure / Addition
■
L , _I L. situ ° ther ise onlpetent soil
Slab on compacted
SYSTEM NOT RECOMMENDED structural till or
structurally isolated slab
Figure 4.3.3 Mixed Depth System for Building
Addition with Shallow Foundations
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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
Zone of
active Depth of
moisture
suction
Constant •
'—t
45"tllintmUt l
moisture
zone
Figure 5.1.1 Drilled and Underreamed Concrete
Piers
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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.
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5.1.2 Drilled Straight-Shaft Concrete Piers
Zone of
of
active constant
rrlolSCtire suction
Pier
Constant �..
I I moisture
zone
i
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.
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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
piles in very stiff sandy soils. 6. Requires waiting until concrete
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
_ )J
Zone of active
moisture —�
Depth or/
constant
Constant suction
moistUI C
zone
Figure 5.1.3 Auger-Cast Concrete Piles
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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
I. 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
moisture
l
Depth of
constant
Constant suction
moisture
/one
Displacement 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.
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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,
2. Only one trade typically involved during installation can be a problem. swamps,marshes,or other soft soil
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.t5 Helical Piers
Zone of
Depth of
Zone
ac tire constant
moisture suction
licliy
c. Constant
moisture -
zone —�
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
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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 corrosion may limit life expectancy.
soils.
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.
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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.
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TABLE 5.2.1 POST-TENSIONED REINFORCING
ADVANTAGES* DISADVANTAGES* COMMENTS
I. 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 yin 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 ofPost-
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 uncracked slabs are lightly reinforced with
section,therefore should require 4. Post-tension design utilizing deep average concrete compression levels
less concrete. beams is less effective than ranging only between 50 psi and 100
deformed bar reinforcing because of
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.
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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
reinforcing. 3. Slower to construct than post-
tensioned 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).
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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). 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 3. Application is generally limited to foundations using only welded wire
reinforcing,which can result in fabric.
slabs only.
reducing required cross-sectional
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
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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.
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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.I 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.
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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.
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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.
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
1. 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
I. More effective in retarding lateral 1. Higher cost. I. 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.
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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
1. 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. if used around perimeter of equipment.
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.
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 38 of 42
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
I. 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 2. 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.
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.
1
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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
foundation movements,not stop it. minimize root damage.
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.
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.
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.
• FINAL DRAFT DOCUMENT
(Developed at the 01-Feb-02 Meeting held in San Antonio,Texas)
American Society of Civil Engineers
Texas Section
Recommended Practice for the
Design of Residential Foundations
Table of Contents
Section 1. Introduction 1
Section 2. Definition of"Engineered Foundation" 2
Section 3. Design Professionals' Roles and Responsibilities 3
3.1 Geotechnical Services 3
3.2 Design Services 3
3.3 Construction Phase Services 3
Section 4. Geotechnical Investigation 4
4.1 Minimum Field Investigation Program 4
4.2 Minimum Laboratory Testing Program 4
4.3 Geotechnical Report 5
Section 5. Design of Foundations 8
5.1 Design Information 8
5.2 Design Procedures for Slab on Ground 8
5.3 Design Procedures for Structurally Suspended Foundations 9
5.4 Design Procedures for Footing Supported Foundations 10
5.5 Minimum Plan and Specification Information 10
Section 6. Construction Phase Observation 11
6.1 Responsibility for Observations 11
6.2 Minimum Program of Observation and Testing 11
6.3 Compliance Letter 11
APPENDIX A 12
APPENDIX B 13
Section B.1 FILL 13
B.1.1 Engineered Fill 13
B.1.2Forming Fill 13
B.1.3 Uncontrolled Fill 13
Section B.2 Building on Non-Engineered (Forming Or Uncontrolled) Fill 14
1
Section 1. Introduction
The function of a residential foundation is to support the structure. The majority of foundations
constructed in Texas consist of shallow, stiffened and reinforced slab-on-ground foundations. Many are
placed on expansive clays and/or fills. Foundations placed on expansive clays and/or fills have an
increased potential for movement and resulting distress.
National building codes have general guidelines which may not be sufficient for the soil conditions and
construction methods in the State of Texas. The purpose of this document is to present recommended
practice for the design of residential foundations to augment current building codes to help reduce
foundation related problems. Where the recommendations in this document vary from published
methods or codes, the differences represent the experience and judgment of the majority of the
committee members.
On sites having expansive clay, fill, and/or other adverse conditions, residential foundations shall be
designed by licensed engineers utilizing the provisions of this document. Expansive clay is defined as
soil having a weighted plasticity index greater than 15 as defined by Building Research Advisory Board
(BRAB) or a maximum potential swell greater than 1 percent. We propose that local and state
governing bodies adopt this recommended practice.
- 1 -
Section 2. Definition of"Engineered Foundation"
An engineered foundation is defined as one for which design is based on three phases:
a. geotechnical engineering information
b. the design of the foundation is performed by a licensed engineer
c. construction is observed with written documentation
These phases are described herein.
- 2 -
Section 3. Design Professionals' Roles and Responsibilities
3.1 Geotechnical Services
The geotechnical investigation and report shall be conducted prior to foundation design under the
supervision of and sealed by a geotechnical engineer.
3.2 Design Services
The foundation design engineer shall prepare the plans and specifications for the foundation, and
shall be the engineer of record. The foundation shall be built in accordance with the design. The
engineer of record shall approve any design modifications. The geotechnical and foundation
design engineering may be performed by the same individual, provided that individual is
sufficiently qualified in both disciplines.
3.3 Construction Phase Services
The engineer of record shall specify on the plans that construction phase observations shall be
incorporated into the foundation construction. These activities shall be performed by: the engineer
of record or a qualified delegate. The qualified delegate may be a staff member under his/her
direct supervision, or outside agent approved by the engineer of record. The observation reports
shall be provided to the engineer of record. The engineer of record shall issue a compliance letter
as described in Section 6.3.
- 3 -
Section 4. Geotechnical Investigation
4.1 Minimum Field Investigation Program
The geotechnical engineer, in consultation with the engineer of record, if available, shall lay out the
proposed exploration program. . A minimum exploration program for subdivisions shall cover the
geographic and topographic limits of the subdivision, and shall examine believed differences in
geology in sufficient detail to provide information and guidance for secondary investigations, if
any. The geotechnical exploration program should take into account site conditions, such as
vegetation, depth of fill, drainage, seepage areas, slopes, fence lines, old roads or trails, man made
constructions and other conditions that may affect the foundation performance.
As a minimum for unknown but believed to be uniform subsurface conditions, borings shall be
placed at maximum 300 foot centers across a subdivision. Non-uniform subsurface conditions
may require additional borings. A single lot investigated in isolation shall have a minimum of two
borings. Borings shall be a minimum of 15 feet in depth unless confirmed rock stratum is
encountered at less depth. Borings shall extend through any known fill or potentially compressible
materials even if greater depths are required.
All borings shall be sampled at a minimum interval of one per two feet of boring in the upper 10
feet and at 5-foot intervals below that. In clayey soil conditions, relatively undisturbed tube
samples should be obtained. In granular soils, samples using Standard Penetration Tests should be
obtained. Borings shall be sampled and logged in the field by a geotechnically trained individual
and all borings shall be sampled such that a geotechnical engineer may examine and confirm the
driller's logs in the laboratory.
Exploration may either be by drill rig or by test pit provided the depth requirements are satisfied.
Sites, which are obviously rock with outcrops showing or easily discoverable by shallow test pits,
may be investigated and reported without resorting to drilled borings.
Field logs shall note inclusions, such as roots, organics, fill, calcareous nodules, gravel and man
made materials. If encountered, the depth to water shall be logged. Additional measurements
shall be taken at the direction of the geotechnical engineer.
4.2 Minimum Laboratory Testing Program
The geotechnical engineer, in consultation with the engineer of record, if available, shall develop
the laboratory-testing program. Sufficient laboratory testing shall be performed to identify
significant strata and soil properties found in the borings across the site. Such tests may include:
a. Dry Density
b. Moisture Content
c. Atterberg Limits
d. Pocket Penetrometer Estimates of Cohesive Strength
e. Torvane
f. Strength tests
g. Swell and/or Shrinkage Tests
h. Hydrometer Testing
i. Sieve Size Percentage
j. Soil Suction
k. Consolidation
- 4 -
All laboratory testing shall be performed in general accordance with the American Society for
Testing and Materials (ASTM) or other recognized standards.
4.3 Geotechnical Report
4.3.1 Report Contents
Geotechnical reports shall contain, as a minimum:
a. purpose and scope, authorization and limitations of services
b. project description,including design assumptions
c. investigative procedures
d. laboratory testing procedures
e. laboratory testing results
f. logs of borings and plan(s) showing boring locations
g. site characterization
h. foundation design information and recommendations
i. Professional Engineer's Seal
The following sections address site characterization and foundation design information and
recommendations.
4.3.2 Site Characterization
The geotechnical engineer shall characterize the site for design purposes. The report shall
comment on site conditions which may affect the foundation design, such as:
a. topography including drainage features and slopes
b. trees and other vegetation
c. seeps
d. stock tanks and other man made features
e. fence lines or other linear features
f. geologic conditions
g. surface faults,if applicable
h. subsurface water conditions
i. areas of fill detected at the time of the investigation
4.3.3 Foundation Design Information and Recommendations
Reports shall contain the applicable design information and recommendations requested by
the engineer of record for each lot in the project. If the engineer of record is not known at
the time of the geotechnical report, the following design information should be presented, if
applicable:
4.3.3.1 Soil movement potential as determined by the estimated depth of the active zone
in combination with at least two of the following methods (identify each method
used):
a. Potential Vertical Rise as determined by the Texas Department of
Transportation Method 124-E, dry conditions
b. Swell tests
c. Suction and hydrometer tests
d. Linear Shrinkage tests
- 5 -
e. Any other method which can be documented and defended as good
engineering practice in accordance with the principles of unsaturated soil
mechanics
4.3.3.2 Post-Tensioning Institute (PTI) parameters (using their most current design
manual and technical notes)including:
a. em and ym for edge lift and center lift modes (The em and ym in the PTI design
manual are based on average climate controlled soil movements and the
design recommendations should take into account the added effect of trees and
other environmental effects, as noted in the PTI design manual.); and
b. Bearing capacity of the soil.
c. If suction values are used to determine the depth and value of suction
equilibrium or evaluate special conditions such as trees, the values shall be
determined using laboratory suction tests. ym determination shall be based on
suction profile change and laboratory determined values of suction-
compression index.
d. em and ym shall be reported for design conditions for suction profile varying
from equilibrium, and for probable extreme suction conditions.
4.3.3.3 Wire Reinforcing Institute parameters including:
a. Climatic Rating(CO of the site
b. Weighted Plasticity Index
c. Slope Correction Coefficient(Cs)
d. Consolidation Correction Coefficient (Co)
4.3.3.4 BRAB design information including:
a. Climatic Rating(CW)of the site
b. Weighted Plasticity Index
c. Bearing capacity of the soil
4.3.3.5 Deep Foundation (pier/pile) design information including:
a. Bearing capacity and skin friction along the pier length
b. Pier types and depths, and bearing strata
c. Uplift pressures on the pier and estimated depth of active zone (pier depth
must be below the active zone and provide proper anchorage to resist the
uplift pressures)
d. Down drag effects on the piers
4.3.3.6 Shallow foundations (including post and beam footings) design parameters.
a. Bearing capacity and footing depth
b. Minimum bearing dimension
4.3.3.7 Soil treatment method(s) to reduce the soil movement potential and the
corresponding reduction in predicted movement.
4.3.3.8 Lateral pressures on any retaining structures or on piers undergoing lateral forces.
4.3.3.9 Trees and other site environment concerns that may affect the foundation design.
Information useful for design and construction of residential foundations is
presented in Appendix A.
4.3.3.10 Moisture control procedures to help reduce soil movement.
4.3.3.11 Surface drainage recommendations to help reduce soil movement.
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- 4.3.3.12 Potential for load induced settlement.
4.3.3.13 On sloping sites, recommend whether a slope stability analysis is required due to
possible downhill creep or other instability that may be present.
4.3.3.14 The presence and methods of dealing with existing and proposed fill. Fill criteria
useful for design and construction of residential foundations is presented in
Appendix B.
4.3.3.15 Geotechnical considerations related to construction.
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Section 5. Design of Foundations
5.1 Design Information
The foundation design engineer shall obtain sufficient information for the design of the foundation.
This may include:
a. information gathered by a site visit
b. the subdivision plan, site plan or plat
c. the topography of the area including original and proposed final grades
d. the geotechnical report
e. special requirements of the project
f. the project budget
g. the architectural elevations and floor plans and sufficient additional architectural information to
determine the magnitude, construction materials and location of structural loads on the
foundation
h. exposed or architectural concrete schedule,if applicable
5.2 Design Procedures for Slab on Ground
5.2.1 The foundation engineer shall utilize one of the following methods, with the modifications
presented in this section:
a. PTI
b. WRI
c. BRAB
d. Finite Element
e. other methods which can be documented and defended as good engineering practice
5.2.2 Slab-on-ground foundations with piers shall be designed as stiffened soil supported slabs
for heave conditions and as structurally suspended foundations with the beams and slabs
spanning between piers for shrinkage and settlement conditions. Piers shall not be attached
to the slabs or grade beams unless the connections and foundation systems are designed to
account for the uplift forces.
5.2.3 Input variables for residential slab-on-ground foundations shall be as follows:
5.2.3.1 PTI:
a. Use the current design manual and technical notes, and the following design
provisions.
b. Provide minimum residual average prestress of 100 psi.
c. Maintain the calculated prestress eccentricity within 5.0 inches.
d. If the computed concrete tensile stress at service loads, after accounting for
prestress losses, exceeds 44r c, provide bonded additional reinforcement at
both the top and bottom of the beam, each equal to 0.0033 times the gross
beam section. The transformed area of steel may be used to determine a new
stiffness value for the beam.
e. The em and ym in the PTI design manual are based on average climate
controlled soil movements and the design analysis should take into account
the added effect of trees and other environmental effects, as noted in the PTI
design manual.
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5.2.3.2 WRI:
a. Use the current design manual and technical notes, and the following design
provisions.
b. Regardless of the actual beam length, the analysis length should be limited to
a maximum of 50 ft; and
c. The minimum design length (La) shall be increased by a factor of 1.5 with a
minimum increased length of 6 ft.
5.2.3.3 BRAB:
a. Use the current design manual and technical notes, and the following design
provisions.
b. Regardless of the actual beam length, the analysis length should be limited to
a maximum of 50 ft; and
c. Use a maximum long term creep factor as provided in ACI 318, Section
9.5.2.5.
5.2.3.4 Finite Element:
a. Use soil support parameters that can be documented and defended as good
engineering practice in accordance with the principles of unsaturated soil
mechanics;
b. Use a cracked moment of inertia for beams that exceed the cracking moment;
and
c. Use a maximum design deflection ratio of 1 / 360 (deflection ratio is defined
as the maximum deviation from a straight line between two points divided by
the distance between the two points).
5.2.4 Special Design Considerations
The structural engineer should consider the following (deviation shall be based on
generally accepted engineering practice):
5.2.4.1 Exterior corners may require special stiffening. This can be accomplished with
diagonal beams or parallel interior beams near the perimeter beams.
5.2.4.2 Provide continuous beams at reentrant corners.
5.2.4.3 Provide stiffening beams perpendicular to offsets in perimeter beams.
5.2.4.4 Provide interior beams at concentrated loads such as fireplaces, columns and
heavy interior line loads.
5.2.4.5 Sites with ym exceeding 1.0 inch shall have special design considerations such as
strengthened sections, revised footprint, site soil treatment, or structurally
suspended foundation if any of the following conditions is present:
a. a shape factor(SF) exceeding 20, (SF=perimeter squared divided by area)
b. extensions over 12 ft
c. concrete lengths exceeding 60 ft
5.3 Design Procedures for Structurally Suspended Foundations
5.3.1 Structurally suspended floors supported by deep foundations shall be designed in
accordance with applicable building codes.
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5.4 Design Procedures for Footing Supported Foundations
5.4.1 Design in accordance with applicable building codes.
5.4.2 Shallow individual or continuous footing foundations should not be used on expansive
soils, unless the superstructure is designed to account for the potential foundation
movement.
5.5 Minimum Plan and Specification Information
5.5.1 Plans shall be signed and sealed by the engineer of record, and be specific for each site or
lot location. Plans shall include the client's name and engineer's name, address and
telephone number; and the source and description of the geotechnical data.
5.5.2 The engineer's drawings shall contain as a minimum:
a. a plan view of the foundation locating all major structural components and
reinforcement
b. sufficient information to show details of beams, piers, retaining walls, drainage details,
etc., if such features are integral to the foundation
c. sufficient information for the proper construction and observation by field personnel
d. information or notes addressing minimum perimeter and lot drainage requirements
5.5.3 The engineer's specifications shall include as a minimum:
a. descriptions of the reinforcing or pre-stressing cables and hardware;
b. concrete specifications including compressive strengths;
c. site preparation requirements;
d. notes concerning nearby existing or future vegetation and the required design features to
accommodate these conditions; and
e. the schedule of required construction observations and testing.
5.5.4 The engineer's plan shall address site fill:
a. The plans shall address fill existing at the time of the design or to be placed during
construction of the foundation and shall require any fills which are to support the
bearing elements of the foundation to be tested and approved by a geotechnical engineer
assisted by a qualified laboratory (Bearing elements of a suitably designed slab-on-
ground foundation are defined as the bottoms of exterior or interior stiffener beams.)
b. The plan shall require that a geotechnical engineer issue a summary report describing
the methods, and results of investigation and testing that were used, and a statement that
the existing or placed fills are suitable for support of a shallow soil-supported slab-on-
ground, or that the foundation elements should penetrate the fill to undisturbed material.
See Appendix B for more detailed information on fills.
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Section 6. Construction Phase Observation
6.1 Responsibility for Observations
Construction phase observations and testing shall be performed in accordance with this document.
6.2 Minimum Program of Observation and Testing
At a minimum, foundations should be observed and tested as applicable to:
a. see that fill conditions are satisfied in accordance with the plans and specifications;
b. piers are observed for proper placement and depth;
c. observation of all foundation elements, including reinforcement, immediately before concrete
placement;
d. observation of concrete during placement; and
e. observation during stressing to document the elongation and stressing load of each tendon.
6.3 Compliance Letter
6.3.1 At the satisfactory accomplishment of all the requirements of the plans and specifications,
the engineer of record shall provide a letter to the client indicating, to the best of his
knowledge (which may be based on observation reports by a qualified delegate as defined
in Section 3.3), the construction of the foundation was in substantial conformance with:
a. the minimum standards of practice presented in this document; and
b. the engineer's plans and specifications including any modifications or alterations
authorized.
6.3.2 A non-compliance letter shall be issued if the construction of the foundation did not meet
the requirements of Section 6.3.1.
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•
APPENDIX A
IMPACT OF MOISTURE CHANGES ON
EXPANSIVE SOILS
Most problems resulting from expansive soils involve swelling or shrinking as evidenced by upward or
downward movement of the foundation producing distress to the structure. The difference between the
water content at the time of construction and the equilibrium water content is an important consideration.
Potential swell increases with lower initial moisture content. Moisture contents and shrink/swell
movements may vary seasonally even after equilibrium is reached.
Precipitation and evapotranspiration control soil moisture and groundwater levels. A slab will greatly
reduce the evapotranspiration rate beneath the slab and partially reduces the inflow due to precipitation
or irrigation because of groundwater's ability to migrate laterally. Therefore, soils beneath a slab are
normally wetter than soils at the same depth away from the slab. However, an unusually wet season may
result in wetter conditions away from the slab than under the slab. With time and normal precipitation
patterns, the soil moisture profile will return to its normal condition. Seasonal variations in soil moisture
away from the slab will generally occur fairly quickly. Seasonal variations in soil moisture beneath the
slab will be slower. In addition roots from trees and large vegetation will seasonally remove moisture
from nearby soils.
Wetting of expansive soils beneath slabs can occur as a result of lateral migration or seepage of water
from the outside. It can be aggravated by ponded water resulting from poor drainage around the slab or
landscape watering. Leaking utility lines and excessive watering of soil adjacent to the structure can also
result in foundation heave.
Foundations can experience downward movement as the result of the drying influence of nearby trees.
As trees and large bushes grow, they withdraw greater amounts of water from the soil causing downward
foundation movement. The area near trees removed shortly before construction may be drier and subject
to localized heave.
Some construction and maintenance issues include the following:
a. In general, set top of concrete at least eight inches above final adjacent soil grade for damp
proofing.
b. Provide adequate drainage away from the foundation (minimum ten percent slope in the first
five feet). The bottom of any drainage swale should not be located within four feet of the
foundation. Pervious planting beds should slope away from the foundation at least two inches
per foot. Planting bed edging shall allow water to drain out of the beds.
c. Gutters or extended roof eaves are recommended, especially under all roof valleys. All
extended eaves or gutter down spouts should extend at least two feet away from the foundation
and past any adjacent planting beds.
d. Avoid placement of trees and large vegetation near foundations (taking into account the water
demands of specific trees and vegetation).
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APPENDIX B
IMPACT OF FILL ON FOUNDATIONS
Section B.1 FILL
Fill is frequently a factor in residential foundation construction. Fill may be placed on a site at various
times. If the fill has been placed prior to the geotechnical investigation, the geotechnical engineer should
note fill in the report. Fill may exist between borings or be undetected during the geotechnical
investigation for a variety of reasons. The investigation becomes more accurate if the borings are more
closely spaced. Occasionally, fill is placed after the geotechnical investigation is completed, and it may
not be detected until foundation excavation is started.
If uncontrolled fill (see discussion below) is discovered later in the construction process, for instance, by
the Inspector after the slab is completely set up and awaiting concrete, great expense may be incurred by
having to remove reinforcing and forms to provide penetration through the fill. Therefore, it is
important to identify such materials and develop a strategy for dealing with them early on in the
construction process. Fill can generally be divided into three types: engineered fill, forming fill, and
uncontrolled fill. These three types of fill are discussed below.
B.1.1 Engineered Fill
Engineered fill is that which has been designed by an engineer to act as a structural element of a
constructed work and has been placed under engineering inspection, usually with density testing.
Engineered fill may be of at least two types. One type is "embankment fill," which is composed
of the material randomly found on the site, or imported to no particular specification, other than
that it be free of debris and trash. Embankment fill can be used for a number of situations if
properly placed and compacted. "Select fill" is the second type of engineered fill. The term
"select" simply means that the material meets some specification as to gradation and P.I., and
possibly some other material specifications. Normally, it is placed under controlled compaction
with engineer inspection. Examples of select fill could be crushed limestone, specified sand, or
crusher fines which meet the gradation requirements. Select underslab fill is frequently used
under shallow foundations for purposes of providing additional support and stiffness to the
foundation, and replacing a thickness of expansive soil. Engineered fill should meet
specifications prepared by a qualified engineer for a specific project, and includes requirements
for placement, geometry,material, compaction and quality control.
B.1.2 Forming Fill
Forming fill is that which is typically used under residential foundation slabs and is variously
known as sandy loam, river loam or fill dirt. Forming fill is normally not expected to be heavily
compacted, and no wise designer will rely on this material for support. The only requirements
are that this material be non-expansive, clean, and that it works easily and stands when cut. If
forming fill happened to be properly compacted and inspected in accordance with an engineering
specification it could be engineered fill.
B.1.3 Uncontrolled Fill
Uncontrolled fill is fill that has been determined to be unsuitable (or has not been proven
suitable) to support a slab-on-ground foundation. Any fill that has not been approved by a
qualified geotechnical engineer in writing shall be considered uncontrolled fill. Uncontrolled fill
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may contain undesirable materials and/or has not been placed under compaction control. Some
problems resulting from uncontrolled fill include gradual settlement, sudden collapse, attraction
of wood ants and termites, corrosion of metallic plumbing pipes, and in some rare cases, site
contamination with toxic or hazardous wastes.
Section B.2 Building on Non-Engineered (Forming Or Uncontrolled) Fill
Foundations shall not be supported by non-engineered fill. To establish soil supported foundations on
non-engineered fill, the typical grid beam stiffened slab foundation is required to penetrate the non-
engineered fill with the perimeter and interior beam bottoms forming footings. Penetration will take the
load supporting elements of the foundation below the unreliable fill. Penetration could be accomplished
by deepened beams, spread footings or piers depending on the depth and the economics of the situation.
Generally, piers are most cost effective once the fill to be penetrated exceeds about three feet, but this
depends on the foundation engineer's judgment and local practice. Floor systems shall be designed to
span between structurally supported foundation elements.
Pre-existing fill may be classified as engineered fill after investigation by the geotechnical engineer. The
approval may depend on the fill thickness, existence of trash and debris, the age of the fill, and the
results of testing and proof rolling. The geotechnical engineer must be able to expressly state after
investigation that the fill is capable of supporting a residential slab-on-ground foundation.
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