Report of Design Sub-Committee of the Residential Foundation Committee Texas Board of Professional Engineers

Revised: August 25, 2000

Note: Some of the mathematical symbols used in this report have been changed from the original format for Internet purposes)

1.0 INTRODUCTION

Approximately 1/2 to 1 billion dollars is spent per year in the state for construction of residential foundations. Their function is to properly support a residential structure without permitting unusual distress or unsafe conditions to occur. The vast majority of foundations constructed in Texas consist of shallow, stiffened and reinforced slab on ground foundations. Many of these foundations have to be placed on expansive clays or fills, which must be stable to adequately support the foundation. Other types of foundations include suspended pier and beam, pier and beam constructed on grade, and strip or spread footing type foundations.

The situation of residential foundations which are slab on ground in contact with expansive clay soils creates one of the most complex design problems to be encountered in structural engineering. The problem is a soil structure interaction problem with the added complexity of the soil support conditions changing in response to environmental factors. In this context of technical complexity it is easy for inadequate design procedures or no design procedures to be utilized. The results of poor design and construction of a foundation are sometimes not noticed for several years after the construction is completed.

Because engineered foundations are not required by typical building codes for residential structures, many houses are constructed each year without benefit of engineering input. When such un regulated structures are placed on problem sites considerable financial loss can occur including over design, and distress of structures. This report addresses the three problems of lack of engineering requirements, inadequate engineering and poor construction phase practices with regard to residential foundations.

It is not the intent of this report to provide a prescriptive process for design of residential foundations, but to present a methodology which will produce a professional service. Other methods which produce comparable results could be used.

2.0 RECOMMENDED ACTIONS OF CITY AND COUNTY BUILDING OFFICIALS

It is recommended that the Board of Registration encourage local and state legislation promoting the inclusion in local building codes of a requirement for engineered foundations for residences on sites where any of the following conditions exist: the weighted PI using a weight of 3 for upper 5 feet, 2 for middle 5 feet and 1 for lower 5 feet or the PI of the upper three feet, which ever is greater, has a value greater than 20, the site settlement potential exceeds one inch under the expected structure loads, the structure will be structurally supported over man made fill, the PVR is greater than 1 inch, or sites with geologic hazards. If any of the above conditions exist, the local officials should require an "engineered foundation" as defined below.

3.0 MODEL STANDARD OF PRACTICE

3.1 Definition of "Engineered Foundation"
An engineered foundation is defined as one for which design is based on adequate site specific geotechnical information embodied in a report and prepared by a geotechnical engineer; the design of the foundation is performed by a foundation or structural engineer; and construction phase activities are observed with proper documentation.

3.2 Design Professional's Roles and Responsibilities
The geotechnical investigation and report shall be conducted under the supervision of and properly sealed by a geotechnical engineer. The design of the foundation shall be performed by a foundation or structural engineer and sealed by that person. The geotechnical and structural engineering may be performed by the same individual, provided that individual is sufficiently qualified in both disciplines.
The foundation design engineer will be the foundation engineer of record and this individual shall be responsible for performing the construction phase observations personally, by staff members under his direct supervision, or by outside agencies who are under his direct control. Quality control testing of construction materials as required by the foundation plans, may be provided by an independent testing laboratory, employed by others, provided reports are provided to the foundation engineer of record. The foundation engineer of record shall issue a compliance letter at the conclusion of construction activities stating that, to the best of his knowledge, the foundation was constructed in substantial accordance with the plans and specifications and any authorized modifications. Any modifications, additions, or alterations to the construction of the foundation shall be done by proper change order or modification authorized by the foundation engineer of record.

3.3 Geotechnical lnvestigation

3.31 Information to be Assembled by Geotechnical Engineer

Prior to laying out the investigative program, the geotechnical engineer will obtain the following, when available: a plat or map of the subdivision, local topography maps, aerial photographs, the existing grades and proposed final grading plan, geological outcrop maps, fault maps (if appropriate), soil conservation service report, and general information concerning the characteristics of the structures to be built.

3.32 Minimum Field Investigation Program

The geotechnical engineer will lay out the proposed investigative program including the location and depths of borings, sampling procedures and laboratory testing required. A minimum investigative 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. As a minimum standard for believed uniform subsurface conditions borings shall be placed at a 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 one boring or more placed as determined jointly by the geotechnical engineer in consultation with the foundation engineer. Borings should be a minimum of 15 feet in depth unless confirmed bedrock is encountered at less depth. In certain circumstances, some borings should be placed near trees to obtain depths of probable root penetration. Borings shall extend through any fill or potentially compressible materials even if greater depths are required. All borings shall be sampled by either augured samples or semi disturbed samples at a minimum interval of one per two feet of boring in the upper 10 feet and at 5 foot intervals below that. Borings shall either be sampled and logged in the field by a geotechnically trained professional or all borings shall be sampled as described such that a geotechnical engineer may examine and confirm the driller's logs in the laboratory.

Investigative borings 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 resort to drilled borings.

Presence of roots shall be logged by depth in all borings.

Intermittent or aquifer water levels shall be noted in borings that indicate groundwater and if material to needs of the project. Observations shall be based on a minimum of one hour observation of the bore holes or pits.

During the field investigations, sketches shall be made of the apparent extent of fill, seepage areas, major vegetation and approximate slopes. The presence of fence lines, old roads or trails or other manmade constructions should also be noted in addition to the borings.

If appropriate, fault studies shall be performed.

3.33 Minimum Laboratory Testing Program

Sufficient laboratory testing shall be performed to identify all significant strata found in the borings across the site. Testing need not be done in every boring provided sufficient correlation can be obtained between borings by a qualified geotechnical professional. Characterization of each significant stratum shall include the moisture content profiles, Atterberg limits, laboratory or field penetrometer estimates of cohesion and the Unified System classification. Additional testing required for each significant stratum will include hydrometer testing to determine the percent fine clay (2 micron size) and the #200 sieve size percentage.

At least one volume change/pressure relationship test is recommended to be performed for each significant clay stratum if deemed necessary by the geotechnical engineer. On sites with more than seven feet of expansive clay soils, defined as having plasticity indices greater than 20, it is recommended that at least 1 out of every 10 borings have sufficient samples obtained to provide an in situ moisture content test and soil suction test at two foot intervals to the entire depth of the boring.

All laboratory testing shall be generally in accordance with ASTM Standards.

3.34 Site Characterization

The geotechnical engineer shall characterize the site for design purposes. If shallow foundations are recommended, report for each lot in the project the Soil Support parameters (including em and Ym for edge and center lift modes) as found in Appendix "A", bearing capacity and the presence and condition of fill. In addition, the geotechnical report shall indicate the approximate slopes of each lot and discuss whether downhill creep or other instability may be present. Vegetation shall be noted as well as other visible or known features such as seeps, manmade improvements, fence lines or other linear features and their impact on design. If the site is subject to settlement, an estimate shall be made of the probable settlement based on the proposed structure loading and soil properties. In lieu of the above recommendations, alternate recommendations may be provided by the geotechnical engineer if so requested by the foundation engineer.

The Soil Support parameters shall be developed by calculation using formulations compatible with the principles of unsaturated soil mechanics. Refer to Appendix A for an acceptable technique. These procedures are applicable to all types of shallow foundations on expansive soils, whether reinforced with re bars or post tensioning.

If pier and beam foundations are to be recommended, the bearing capacity and establishment depth of piers shall be calculated and values of skin friction or alternate reinforcing steel shall be recommended to be used in uplift and down drag calculations as well as bearing analysis of the piers shall be calculated. The piers shall be anchored below the depth where t vertical movement is calculated to be zero or this depth may be based on extensive local experience based on measurements. This depth will be greater within the root zones of major vegetation.

Lateral pressures to be applied in design of retaining type structures shall be determined by the geotechnical engineer.

3.35 Geotechnical Report

Geotechnical reports shall contain, as a minimum, an introduction of the project, the investigative procedures, the laboratory testing procedures utilized, the results of laboratory testing, the geologic conditions, slopes, logs of borings and plans showing boring location, a plan showing areas of probable fill in existence at the time of the investigation and, as appropriate, other features such as seeps, vegetation or lineations and man made structures.

Reports shall contain specific recommendations for the foundation engineer to use for the design of foundations for each lot on the project, including suitable foundation types and, ( as appropriate, the Soil Support parameters for each lot, shallow bearing values, deep foundation bearing and skin friction as well as depth of establishment, lateral pressures for use in designing retaining structures or deep grade beams, and other specific recommendations concerning site construction. Sufficient geotechnical data shall be included to permit the foundation engineer of record to adjust design inputs for specific needs of the design. In lieu of the above recommendations, alternate recommendations may be reported if requested by the foundation engineer. The report shall be prepared, signed and sealed by a geotechnical engineer.

3.36 Fill

The presence and methods of dealing with existing and proposed fill to be placed during construction shall be discussed by the geotechnical engineer in his report. Fill criteria useful for design and construction of residential foundations may be seen in Appendix B.

3.4 Design of Foundations

3.41 Information to be Assembled by Foundation Engineer

The foundation engineer shall assemble or be provided by his client the subdivision plan, the topography of the area including original and proposed final grades, the geotechnical report, the architectural floor plan of the structure and sufficient additional architectural information to determine the magnitude, construction materials and location of structural loads on the foundation. If exposed or architectural concrete is to be used in finished concrete surfaces, this information should be provided to the foundation engineer.

3.42 Design Procedures

The foundation engineer shall utilize a procedure that will provide designs that will meet minimum criteria of either the Standard Building Code 1997 Section 1804.3.3.3 or Uniform Building Code 1997 Section 1815, 1816, for the design of slab on ground foundations for potentially expansive clay sites. Geotechnical procedures shall be those of Section 3.

Foundations utilizing piers with suspended slab and beams shall be designed by building code procedures.

Soil supported slab on ground foundations with piers shall be designed as a stiffened slab on ground by above procedures and piers shall not be attached to the slab or grade beams.

Permissible design deflection ratios of residential foundations shall meet the following criteria:

Construction	Permissible Foundation Deflection Ratios
Masonry Walls	1 /800
Sheetrock Interior Walls Edge or Center Lift Mode	1/480
Full Span Roof Trusses Edge Lift Mode	1 /960

Deflection ratio is defined as (A)/L where (A) is the vertical movement at the center of a symmetrical bowl shaped depression or. mound and L is the distance from side to side of the considered depression or mound.

Foundations which will span between piers, or slab panels which span between stiffener beams or perimeter beams or other points of load transfer, shall be designed to meet the above deflection criteria assuming all dead loads and live loads.

3.43 Minimum Plan and Specification Information

The engineer's drawings shall show a plan view of the foundation locating all major structural components and reinforcement. Details shall be shown to indicate construction and dimensions of stiffener beams, piers, retaining walls, drainage details, etc.,if such features are integral to the foundation. Drawings shall contain sufficient information for the proper construction and observation by field personnel. Specifications shall include the reinforcing or pre stressing cables and hardware; concrete specifications including compressive strengths; notes concerning existing or proposed fill, nearby existing and known future vegetation and the required design features to accommodate these conditions; and the schedule of required inspections.

Minimum perimeter and lot drainage requirements shall be shown or noted on the plan.

Plans shall be specific for each site or lot location and shall include the client's name and engineer s name, address and telephone number and the geotechnical data and source used.

3.44 Foundation Engineer's Specification for Fill

The foundation engineer's 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. Such approval shall include a summary report by the geotechnical engineer of 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 also Appendix B for more detailed information on fills.

3.5 Construction Phase Observation

3.51 Responsibility for Observations

The foundation engineer of record will be responsible for performing observations of construction personally or having them performed by a staff member under his direct supervision or by an independent third party agency qualified to do such observation, which reports to the foundation engineer of record. In any event, the responsibility for issuing the final compliance report shall rest with the foundation engineer of record. Fills which are to support bearing elements of a foundation shall be tested and approved by a geotechnical engineer, assisted by a qualified laboratory.

3.52 Minimum Program of Observation

At a minimum, foundations should be inspected as applicable to see that fill conditions are satisfied in, accordance with the plans and specifications; piers are observed for proper placement and depth, concrete and reinforcing installation; observation of all foundation elements immediately before concrete placement; observation of concrete placement; and a stressing observation noting and comparing the elongation of each cable to the calculated elongation and the stressing load applied to each cable.

3.53 Compliance Letter

At the satisfactory accomplishment of all the requirements of the plans and specifications, the foundation engineer of record shall provide a letter to the client indicating the geotechnical data and source used in the design; the design procedures used; the construction observation performed and results; and, as appropriate, a statement concerning the methodology of dealing with fill, either by satisfactory compaction compliance or by penetration; and the methodology of dealing with slopes, vegetation and drainage. The letter shall conclude with a statement that, in his opinion, the construction of the foundation was in substantial conformance with the engineer's plans and specifications including any modifications or alterations authorized.

4.0 REFERENCES

"Design and Construction of Post Tensioned Slabs on Ground", 2nd Edition, (PTI Manual) Post Tensioninq institute, Phoenix, 1996

American Society for Testing and Materials, West Consholhocken, Pennsylvania.

Standard for Residential Foundations on Expansive Clay (Draft), American Society of Civil Engineers, Reston, Virginia, 1999

"Prediction of Movement in Expansive Clay", Vertical and Horizontal Deformations of Foundations and Embankments, Geotechnical Special Publication No. 40, Lytton, R.L., Yeung, AT., and F- Iio, G.Y., ed., ASCE, New York, New York, Vol. 2, 1827 1845, 1994.

Residential Foundation New Design Subcommittee Members

Robert E Bigman, P.E.
Buchanan/Soil Mechanics, Inc.
Bryan/Texas

Robert C. Davis. P.E
Trinity Engineering Testing Corp.
Austin, Texas

David A. Eastwood. P.E.
Geotech Engineering & Testing Inc.
Houston, Texas

Richard Hale, P.E.
Perry Homes
Houston, Texas

Jim D. Mallory, P.E.
Safety Management Associates, Inc.
Hewitt, Texas

David Messersmith, AIA
Lubbock, Texas

Douglas S. Porter, P.E.
Round Rock, Texas

H. Platt Thompson, P.E.
Thompson Professional Group, Inc.
Houston, Texas

Kirby T. Meyer, P.E., Chairman
MLAW Consultants & Engineers
Austin, Texas

 

APPENDIX A

Procedures for Determining Soil Support Parameters For
Shallow Foundations on Expansive Soil Sites
(Revised 8 25 00 errata)

General
These support parameters arc equally applicable to all stiffened, reinforced slab foundations, whether reinforced with conventional re bar steel or post tensioning, constructed on expansive soil sites. Other procedures may be used, provided comparable foundation designs are achieved.

Expansive Soil Sites
Sites for which this procedure is applicable should meet either of the following criteria:

If neither of these criteria is met, the foundation should be designed as a non stiffened slab foundation, such as the B.R.A.B. #33 or Post Tensioning Institute (1996) Type 11 foundations, and the remaining procedures of this appendix arc not applicable.

Edge Moisture Variation Distance (em)
The edge moisture variation distance is the distance beneath the edge of a shallow foundation within which moisture will change due to wetting or drying influences around the perimeter. In an edge lift case, the moisture in the soil is higher at the edges than in the center. The center lift case is one in which the moisture is higher in the center than the edges. The major factor in determining the penetration distance is the unsaturated diffusion coefficient, a. This, in turn, depends on the level of suction, the permeability, and the cracks in the soil.. For the same diffusion coefficient, the em value will be larger for the center lift case in which moisture is withdrawn from moist soil under the center of the foundation. The ell, value will be smaller for the edge lift case in which moisture is drawn beneath the building into drier soil. Roots, layers, fractures or joints in the soil will increase the diffusion coefficient and the em value for both the edge lift and center lift conditions.

Using representative values based on laboratory test results in each significant layer, the following parameters are required to determine edge moisture variation distance, em, and differential movement, Ym:

Calculate Yh
Yh, = Yo (%fc / 100)
Terms: Yo is the change of soil volume for a change in suction for 100% fine clay content. Yh is the correction of Y for the actual % of fine clay (%fc).

where Y is determined using the following steps:

Step 1 Determine Mineral Classification Zone 1, 11, 111 or IV from the chart below. If your data does not fall within one of the four zones, use the nearest zone.

Step 2 Proceed to the chart corresponding to the zone determined in Step 1 to determine Yo.

Differential Soil Movement (Ym)
Differential soil movement of the soil should be estimated using the change in soil surface elevation at two locations separated by a distance eon within which the differential movement will occur. An initial and a final suction profile should be used at each of the two locations to determine differential movement. For general analysis, the initial suction profile should be the same at both locations.

The final suction profile at each location should be determined from controlling suction conditions at the surface. A one dimensional finite clement analysis of the layered profile with measured or estimated suction profile envelope may be used to yield estimates of movement for the purpose of design and analysis. A two dimensional analysis maybe necessary to study the effects of trees, edge barriers, flower beds, lawn watering or pipe leaks.

In absence of local observations, controlling soil suction values at the ground surface are recommended as follows:

Controlling soil suction values below the soil surface occur at depths that are remote from the surface (Zm) and are as follows:

A typical vertical suction profile is computed by using the principles of steady state unsaturated flow which links the controlling suction values at the soil surface to the controlling suction below the surface. The principles of steady state unsaturated flow may be found in Lytton (1994).

Measures for estimating differential soil movements should be computer methods used to generate the design values of Ym for the edge lift and center lift conditions.

In the absence of computer methods, the following table may be used to estimate approximate design values of Ym. This method should only be attempted if atypical soil suction profile is present at the beginning and ending conditions. Otherwise, this procedure may not be conservative.

Notes: The positive sign indicates heave and the negative sign indicates shrinkage.

If suction profiles include the effects of flower beds, poor drainage or tree root zones, this approximate procedure is not valid

The estimated value of Ym can be determined from
Ym Yh mod x (Stress Change Factor from above table)

The modified value of Yh, Yh mod, is the averaged volume change coefficient of the soil supporting the slab. This should be calculated as a weighted average of the Yh values in each of the layers of soil to a depth of a minimum of nine feet. Depths greater than nine feet may be used if justified by geotechnical analysis. The yh values in the upper one third have a weight of three, in the next third a weight of two, and in the bottom third a weight of one. The sum of the products of layer thickness (feet), yh and weight for all layers should be used to obtain the weighted average as the modified value Yh mod., which is to be used to estimate the Ym value for edge lift (positive value) and center lift (negative value).

Barriers

Vertical moisture barriers may be used to reduce the soil support parameters (em and Ym) provided the barriers are properly designed to virtually stop moisture migration to or from the under slab area on a permanent basis.

The effect of a barrier on em and Ym may be estimated by the principles of unsaturated soil mechanics, most easily by the use of a two dimensional moisture flow analysis computer program. A vertical barrier should extend at least 2.5 ft below adjacent ground surface to be considered as having any effect.

APPENDIX B
Fill Guidelines

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

Uncontrolled 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.

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. When designing a foundation for this type of fill, the beam bottoms must penetrate it completely and slab panels should be designed to span between beams if more than 48" will exist below the slab panels.

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 will be considered uncontrolled fill. Uncontrolled fill 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.

BUILDING ON FILL

To establish soil supported foundations on fill, the typical grid beam, stiffened slab foundation is required to penetrate through forming fill or uncontrolled 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 done 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 3 feet, but this depends on the foundation engineer's judgment and local practice.

Pre-existing, uncontrolled fill can be approved through proper investigation by the foundation engineer or a geotechnicall engineer. The approval may depend on whether or not the fill is fairly shallow, free of trash, the age of the fill, and the results of testing and proof rolling. These procedures should be performed under the observation and approval of the engineer, who must be able to expressly state after his investigation that the fill is capable of supporting a residential slab on ground foundation.

The Company | Services | Project Types | Publications Employment | About Houston | Contact Us