Note: reference is made by several documents in this page to programs developed by the U.S. Army Corps of Engineers. These cannot be downloaded from this site; however, they are available here.
Also, we have additional information on pavements, especially airfield pavements, both concrete and asphalt.
Geotechnical Aspects of Pavements
Barry R. Christopher, Ph.D., P.E., Charles Schwartz, Ph.D., P.E. and Richard Boudreau, P.E.
FHWA NHI-05-037
May 2006
This is the Reference Manual and Participants Workbook for the FHWA NHI’s Course No. 132040 – Geotechnical Aspects of Pavements. The manual covers the latest methods and procedures to address the geotechnical issues in pavement design, construction and performance for new construction, reconstruction, and rehabilitation projects. The manual includes details on geotechnical exploration and characterization of in place and constructed subgrades as well as unbound base/sub-base materials. The influence and sensitivity of geotechnical inputs are reviewed with respect to the requirements in past and current AASHTO design guidelines and the mechanistic-empirical design approach developed under NCHRP 1-37A, including the three levels of design input quality. Design details for drainage features and base/sub-base material requirements are covered along with the evaluation and selection of appropriate re-mediation measures for unsuitable subgrades. Geotechnical aspects in relation to construction, construction specifications, monitoring, and performance measurements are discussed.
Analysis of One-Dimensional Consolidation
Roy E. Olson
1980
The term consolidation is used to describe a process whereby fluid is forced out of the void spaces in a soil to allow the soil to decrease in volume. The term is also used in a more general way to include swelling as well as compression. The term one-dimensional consolidation refers to a consolidation process in which both fluid flow and deformation occur along a single axis. In field problems, this axis is vertical. Because soils are not infinitely permeable, time is needed for escape of pore fluid; thus, consolidation is a time-dependent process.
Nearly all practical analyses are performed using what we will term the “classical method”. When this method is applied, the total compression is first calculated using a suitable one-dimensional stress-strain curve for the soil, and then the time rate of compression is calculated using Terzaghi’s theory. This theory, and a number of simple extensions of it, is reviewed in this report.
The report goes on to describe a program FD31 which implements this theory and solves the equations using finite-difference methods.
Bearing Capacity Determination by Limit Analysis
W.F. Chen and H.L. Davidson
Fritz Engineering Laboratory Report No. 355.15
Lehigh University
January 1972
The upper bound technique of limit analysis is used to develop approximate solutions for the bearing capacity of cohesive soils with weight. Solutions are presented for smooth and rough and surface and subsurface footings. Soil is treated as a perfectly plastic medium with the associated flow rule after Drucker. The limit analysis solutions for smooth, surface footings are shown to compare favorably with slip-line solutions. Meyerhof’s solutions and the limit analysis solutions for rough, subsurface footings are shown to agree remarkably well.
Bearing Capacity of Soils
We also feature the AXILTR program, which allows estimation of vertical displacements of deep foundations (principally drilled shafts.)
Engineer Manual EM 1110-1-1905
30 October 1992
This manual presents guidelines for calculation of the bearing capacity of soil under shallow and deep foundations supporting various types of structures and embankments. This information is generally applicable to foundation investigation and design conducted by Corps of Engineer agencies.
Principles for evaluating bearing capacity presented in this manual are applicable to numerous types of structures such as buildings and houses, towers and storage tanks, fills, embankments and dams. These guidelines may be helpful in determining soils that will lead to bearing capacity failure or excessive settlements for given foundations and loads.
Bearing capacity is the ability of soil to safely carry the pressure placed on the soil from any engineered structure without undergoing a shear failure with accompanying large settlements. Applying a bearing pressure which is safe with respect to failure does not ensure that settlement of the foundation will be within acceptable limits. Therefore, settlement analysis should generally be performed since most structures are sensitive to excessive settlement.
Compaction and Settlement of Existing Embankments
Robert L. Parsons, Ph.D., P.E., Derek H. Foster, and Stephen A. Cross, Ph.D., P.E., University of Kansas
KTRAN: KU-00-8
December 2001
Unanticipated settlement of compacted earth fill has been a continuing problem for embankments managed by KDOT. This report contains the results of an investigation of current compaction specifications, with particular emphasis on the Type B compaction specification that relies on visual verification of compaction by sheepsfoot roller walkout.
This investigation consisted of two parts: a field investigation of existing embankments with varying levels of performance and a telephone survey of other DOT’s to determine the status of compaction specifications. Eight embankments constructed between 1994 and 2000 were selected for undisturbed field sampling. Two borings were drilled in each embankment and Shelby tube samples were collected for testing at regular intervals. Samples of the cuttings were also collected for testing.
A telephone survey of all state DOT’s was conducted to assess current practice with regard to specifications for compaction of fills. Thirty-two states, including Kansas, responded to the survey. It was determined that a number of compaction and moisture specifications are currently in use, however there were common themes among the specifications.
Based on the results of this research it is recommended that KDOT specify a relative compaction standard and a moisture content range based on the optimum moisture content for compaction of embankments. It is recommended that the compaction standard be at least 95 percent of maximum density as determined by KT-12 (AASHTO T 99), or an equivalent relative compaction based on modified effort (AASHTO T 180). It is also recommended that the specified moisture range be centred about optimum or a point slightly above optimum, as the average moisture content of the existing embankments is slightly above optimum.
A Computer Program to Calculate the Magnitude of Settlement of a Multi-Layered Soil System (MAGSET-II)
Vincent Partyka, David M. Jubenville and Robert L. Schiffman
University of Colorado
GESA Report Number D-76-9
September 1976
This program utilizes Terzaghi’s one dimensional consolidation theory, simplified to apply to a two-dimensional condition, for estimating settlements in cohesive soils. The program applies a vertical stress influence factor, due to the loading, to the effective stress history. Some very complex loadings can be accounted for, such as: unloading due to excavation,temporary and/or permanent changes in water table, live loads applied to the structure, and loadings due to adjacent structures or construction. There are two built in methods to account for strain influence or allows the user to enter a set of influence factors. Also, for granular soils, three methods are available for estimating settlements. These are due to Meyerhof, d’Appolonia and Schmertmann.
Computer Programs for Settlement Analysis
Reed L Mosher and N. Radhakrishnan
U.S. Army Corps of Engineers
Instruction Report K-80-5
October 1980
This report documents and gives example runs of three computer programs for performing settlement analysis of foundations and embankments. The programs are I0016, MAGSETII and FD31.
Design and Construction of Mat Foundations
Lawrence D. Johnson
USAEWES, Geotechnical Laboratory
Miscellaneous Paper GL-89-27
November 1989
Mat foundations commonly support all types of structures. Flat mats from 2 to 8 ft in thickness often containing two-way steel reinforcement top and bottom usually support multi-story or heavy structures. Mats less than 1 ft thick often constructed with steel reinforced ribs or stiffening crossbeams usually support light one or two story structures. Many of these mats have been designed and constructed for supporting permanent military facilities, particularly in heaving/shrinking and compressible soil. Some of these mats have experienced significant differential movement leading to cracking in the stricture and have required costly remedial work. Attempts to reduce such maintenance expenses of some structures have lead to substantially increased design and construction costs for mat foundations.
This report provides information on serviceability of structures, guidelines for evaluation of soil, and some structure input parameters for design analysis and guidelines for design and construction of ribbed mat foundations in expansive soils. Methods have been developed for evaluation of effective soil elastic moduli and stiffness of structures. New concepts are proposed for determining some soil input parameters for design in expansive soils such as the depth of the active zone for heave and edge moisture variation distance. Several case history studies of ribbed and flat mat foundations have been investigated to assist determination of suitable procedures for calculating deformation behaviour of mat foundations.
Analysis of the performance of a large ribbed mat foundation supporting building 333, Red River Army Depot, proves the viability of selected instrumentation and methodology. The observed earth pressure distribution shows extremely large concentrations of soil pressure near the perimeter indicating rigid behaviour on an elastic soil or soil shear at the perimeter. The extended distribution of earth pressures from column loads indicates the effectiveness of stiffening beams in spreading applied loads. Evidence is presented indicating that concrete shrinkage and foundation distortions during construction may sometimes let stiffening beams of ribbed mats hang in the trenches without soil support, which may contribute to mat fractures when superstructure loads are applied. Observed strains in the concrete mat were generally consistent with observed deformation patterns.
A preliminary systematic damage record system was developed to catalogue most frequent damages, assist identification of causes of damage from foundation movements, and assist determination of requirements for maintenance and repair of military facilities. Recommendations are made for field surveys of detailed surface soil and foundation movement patterns and other work to investigate a new frequency spectrum approach and ground modification methods to improve understanding and performance of military facilities, improve design of foundations, and reduce maintenance and repair requirements.
EMBANK: A Microcomputer Program to Determine One-Dimensional Compression Settlement Due to Embankment Loads
The EMBANK computer program itself can be downloaded here.
Dr. Alfredo Urzua
FHWA-SA-92-045
May 1993
The objective of this report is to introduce a microcomputer program for computing one-dimensional compression vertical settlement due to embankment loads. The program follows the equations presented by Lambe & Whitman (1969), Ladd (1973), and Poulos & Davis (1974). For the case of a strip symmetrical vertical embankment loading, the program superimposes two vertical embankment loads. For the increment of vertical stresses at end of fill, the program internally superimpose a series of 10 rectangular loads to create the end-of-fill condition. The report presents the equations and analytical procedures utilized by the program and examples of the capabilities of the user-friendly data entry form. The computer program is coded in the Turbo Pascal 4.0 language and takes full advantage of the stand-alone, (single-user) characteristics of the IBM-PC through the use of “friendly” input menus and data-checking routines.
The code implements copyrighted portions of the microcomputer programs SAF-I and STRESS developed by PROTOTYPE Engineering, Inc., Winchester, MA, and uses the screen editor Turbo Magic From Sophisticated Software.
Expedient Road Construction Methods for Sand and Soft Soil Subgrades
Jeb S. Tingle
U.S. Army Engineer Research and Development Centre
October 2003
Recent military operations in the Balkans and Southwest Asia have demonstrated the need to develop solutions for rapidly constructing temporary roads in the theater of operations. Loose sands and very soft soils have historically provided obstacles to efficient military manoeuvre. The U.S. Army Engineer Research and Development Centre (ERDC) has conducted a series of research projects over the last seven years to evaluate commercial-off-the-shelf technology for constructing temporary roads. These projects included two problematic subgrade types, loose sand and soft soil. These projects have evaluated a variety of products including geosynthetics, lightweight fill, and matting. This paper describes various systems for rapidly constructing temporary roads over loose sand or soft subgrade soils. This paper summarizes results from numerous full-scale test sections designed to evaluate the performance of expedient road systems under realistic traffic conditions. Each system’s performance is summarized, and specific recommendations are provided for material use. A comparative analysis is included to present tradeoffs in performance, cost, weight, volume, and installation rate. This paper represents the state-of-the-art in temporary road construction for military applications.
Expedient Road Construction Over Sands Using Lightweight Mats
Steve L. Webster, Jeb S. Tingle
U.S. Army Corps of Engineers
Waterways Experiment Station
Technical Report GL-98-10
June 1998
The field experiment evaluating the lightweight mats presented in this report was conducted in the Hangar 4 Test Facility during the period June through September 1997 by the U.S. Army Engineer Waterways Experiment Station Vicksburg, MS. Traffic was applied to the lightweight mats using a 5-ton military truck loaded to a gross vehicle weight of 41,600 lb. The field traffic experiment was performed to evaluate the potential of each mat as an expedient road surfacing when placed over sand subgrades and trafficked with wheeled military vehicles. A summary of each material investigated and its performance is presented in this report An analysis of the field data was conducted to determine the potential of these expedient surfacings under actual loading conditions.
Implementation Guidance for Using Spread Footings on Soils to Support Highway Bridges
FHWA-RC-14-001
February 2014
Recent FHWA national surveys revealed that State Departments of Transportation (DOTs) have safely and economically constructed highway bridges supported on spread footings bearing on competent and improved natural soils as well as engineered granular and MSE fills, and that many State DOTs may be missing an opportunity to save time and costs by not doing so more often. The goal of this report is to promote the consideration and use of spread footings on soils when appropriate to support highway bridges. Initially, perceived obstacles in using spread footings are identified. Then, the report presents recommendations to address these obstacles and a guidance to help State DOTs implementing these recommendations that are centred around:
- Deployment of AASHTO and FHWA technical resources;
- highlighting practices of State DOTs that actively use spread footings, especially for selection of spread footings;
- a performance review of bridges constructed with spread footings bearing on soils; and
- LRFD implementation for spread footings.
State DOTs’ concerns with using spread footings on engineered and MSE fills and with integral abutments are addressed. Consideration of load tests on spread footings, instrumentation programs for bridges with spread footings, and deployment of adequate subsurface investigation and construction programs are recommended. The main concern of State DOTs that do not consider spread footings is excessive settlement of bridges. This report advances a rational procedure for settlement analysis of bridges supported on spread footings bearing on soils. This procedure and the results of the national surveys demonstrate that bridges with spread footings on soil can perform very well with respect to settlement. Development of LRFD design bearing resistances for footings on various types of soils is discussed. Based on the previous recommendations, the report finally provides a technical resource for State DOTs to develop LRFD guidance that would allow selection of spread footings in design when appropriate, and development of accurate and economical design methods for spread footings.
Large-Scale Load Tests and Data Base of Spread Footings on Sand
FHWA-RD-97-068
November 1997
Spread footings are most often less expensive than deep foundations. In an effort to improve the reliability of spread footings, this research project was undertaken. The results consist of
- A user friendly microcomputer data base of spread footings, case histories and load tests.
- The performance of five large scale square footings in sand.
- An evaluation of the current accuracy of settlement and bearing capacity prediction methods.
- Observations on the scale effect, the zone of influence, the creep settlement, and soil heterogeneity.
- A new and simple method to predict the complete load settlement curve for a footing as well as several correlations.
- Evaluation of the WAR test, a dynamic test for spread footings.
Performance of Highway Bridge Abutments Supported by Spread Footings on Compacted Fill
Albert F. Di Millio
FHWA-RD-81-184
October 1982
A visual inspection was made of the structural condition of 148 highway bridges supported by spread footings on compacted fill throughout the State of Washington. The approach pavements and other bridge appurtenances were also inspected for damage or distress that could be attributed to the use of spread footings on compacted fill. This review, in conjunction with detailed investigations of the foundation movement of 28 selected bridges, was used to evaluate the performance of spread footings on compacted fills. It was concluded that spread footings can provide a satisfactory alternative to piles especially when high embankments of good quality borrow materials are constructed over satisfactory foundation soils. None of the bridges investigated displayed any safety problems serious functional distress. All bridges were in good condition and many were found t o be in very good condition. In addition to the performance evaluation, cost effectiveness analyses and tolerable movement correlation studies were made to further substantiate the feasibility of using spread footings in lieu of expensive deep foundation systems. Cost analyses showed spread footings were 50-65 percent cheaper than the alternate choice of pile foundations. Foundation movement studies showed that these bridges have easily tolerated differential settlements of 1-3 inches (25-75 mm) without serious distress.
Proposed Design Criteria for Shallow Bridge Foundations
Felix Y. Yokel
Federal Highway Administration
NISTIR 90-4248
February 1990
Criteria for the design of spread footings for highway bridges are proposed. The criteria address working load as well as load and resistance factor design (LRFD) procedures. Importance factors to be used in conjunction with the LRFD design format are proposed. The importance factors increase the design loads as the span length increases and also otherwise account for the severity of the consequences of a structural or foundation failure. Further data on LRFD design will be available from studies presently in progress. Available information on tolerances of highway bridges and other structures to foundation displacements are reviewed. On the basis of this information, allowable foundation-displacement limits are proposed. Unconditionally allowable foundation displacements will not affect the strength and serviceability of bridges and therefore do not require structural design modifications. These allowable displacements can be doubled if it either can be demonstrated, using criteria proposed in this report, that the strength and serviceability of the bridge is comparable to that required in accordance with applicable American Association of State Highway and Transportation Officials (AASHTO) specifications, or if the design of the bridge is modified so that the predicted foundation displacements can be accommodated.
Selection of Spread Footings on Soils to Support Highway Bridge Structures
Naresh C. Samtani, PE, PhD, Edward A Nowatzki, PE, PhD and Dennis R. Mertz, PE, PhD
FHWA-RC/TD-10-001
February 2010
Revised December 2017
The FHWA believes that spread footings on soils are underutilized because designers encounter one or more of the following obstacles: (a) limited knowledge of AASHTO/FHWA technical references that pertain to spread footings on soils to support bridges; (b) limited knowledge of adequate performance data for spread footings; (c) unrealistic tolerable settlement criteria; (d) overestimation of loads used to calculate settlement; and (e) the use of conservative settlement prediction methods. These obstacles have resulted in institutional biases and overly conservative and excessively costly institutional processes that lead to the unnecessary use of costlier deep foundation systems. The primary goal of this report is to promote the use of spread footings bearing on competent natural soils, improved soils, and engineered fill materials as a routine alternative to deep foundations for support of bridges by addressing the factors identified above. Documented performance data is used to make this case.
The report also presents powerful concepts such as construction-point analysis and angular distortions to demonstrate the efficacy of using spread footings. Implementation of these concepts requires only that conventional computations be taken one step further without any requirement for advanced computational skills. The report presents sources of information that agencies and designers can use as references in their project applications. The report contains comprehensive appendices that treat in detail many of the topics discussed in the report. For example, one such appendix provides an introduction to Load and Resistance Factor Design (LRFD) that permits a rational approach to the consideration of spread footings on soils as a feasible alternative to deep foundations.
Settlement Analysis
Bonus: We also have the program VDISPL, a Windows implementation of the vertical displacement program shown in the manual.
EM 1110-1-1904
U.S. Army Corps of Engineers
30 September 1990
This manual presents guidelines for calculation of vertical displacements and settlement of soil under shallow foundations (mats and footings) supporting various types of structures and under embankments. Soil is a nonhomogeneous porous material consisting of three phases: solids, fluid (normally water), and air. Soil deformation may occur by change in stress, water content, soil mass, or temperature. Vertical displacements and settlement caused by change in stress and water content are described in this manual.
Settlement of Shallow Footings on Sand: Report and User’s Guide for Computer Program CSANDSET
Virginia R. Knowles
US Army Corps of Engineers
Technical Report ITL-91-1
June 1991
The CSANDSET computer program calculates the settlement of shallow footings on sand from 15 different methods. This report provides a user’s guide for this program and a theoretical section discussing aspects of sand settlement computations. Factors presented include the SPT blow count and its correction, groundwater and embedment corrections, and elastic and empirical types of settlement methods. Each of the 15 settlement procedures is presented with a brief background summary and all related equations. Examples of hand calculated settlement by all the methods are shown in Appendix A and used for program verification.
Simplified Computation of Vertical Pressures in Elastic Foundations
Nathan M. Newmark
University of Illinois Engineering Experiment Station Circular No. 24
September 1935
The general procedure for computing pressure due to a given load is to divide the loaded area into elements sufficiently small to permit the assumption that the load on the element of area is concentrated at a point. Then by use of Boussinesq’s formula for the vertical stress due to a concentrated load, the total stresses are computed as
the sum of the individual stresses due to the separate concentrations. This process becomes rather tedious and involves considerable time, but cannot be avoided when the loads are irregular.
However, it is possible to integrate Boussinesq’s formula to obtain the stress distribution due to a load uniformly distributed over a rectangle. The result is fairly simple, but more important, can readily be tabulated in such a way that the stress due to loads distributed over any combination of rectangular areas can quickly and easily
be determined.
By use of the formulas derived, a table has been computed giving the pressure in terms of the intensity of load at a point a unit depth below the corner of a rectangular area uniformly loaded. Various examples illustrating the use of t he table are given herein.
Shallow Foundations
FHWA-SA-02-054
September 2002
This document is FHWA’s primary reference of recommended design and procurement procedures for shallow foundations. The Circular presents state-of-the-practice guidance on the design of shallow foundation support of highway bridges. The information is intended to be practical in nature, and to especially encourage the cost-effective use of shallow foundations bearing on structural fills. To the greatest extent possible, the document coalesces the research, development and application of shallow foundation support for transportation structures over the last several decades.
Detailed design examples are provided for shallow foundations in several bridge support applications according to both Service Load Design (Appendix B) and Load and Resistance Factor Design (Appendix C) methodologies. Guidance is also provided for shallow foundation applications for minor structures and buildings associated with transportation projects.
Stress Distribution Within and Under Long Elastic Embankments
W.H. Perloff, G.Y. Baladi and M.E. Harr
Joint Highway Research Project #14
June 1967
The distribution of stresses within and under long elastic embankments continuous with the underlying material is presented. The magnitude and distribution of stress in the foundation material in the vicinity of the embankment is significantly different from that predicted by the usual assumption of stress proportional to embankment height applied normal to the foundation. Influence charts for a variety of embankment shapes are given.
User’s Guide: Computer Program for Bearing Capacity Analyses of Shallow Foundations (CBEAR)
Reed L. Mosher and Michael E. Pace
U.S. Army Corps of Engineers
Instruction Report K-82-7
June 1982
This user’s guide documents a computer program called CBEAR that can be used for analysis of the bearing capacity of shallow strip, rectangular, square or circular foundations on one- or two-layer soil systems. The bearing capacity can be computed considering the effects of:
- Embedment of the foundation;
- Inclination of the foundation base;
- Inclined loads;
- A sloping soil surface
- Eccentric loads in three dimensions;
- Submerged soil;
- Surcharge.
It contains a detailed description of the analysis procedures employed in the program, including extensive discussion of Vesic’s and Meyerhof’s methods for determining the bearing capacity of shallow foundations. It also included worked examples of problems the program was designed to solve.
User’s Guide: Computer Program for Determining Induced Stresses and Consolidation Settlements (CSETT)
Alexis E. Templeton
U.S. Army Corps of Engineers
Instruction Report K-84-7
August 1984
This report documents CSETT, a computer program designed to compute consolidation settlement of compressible soils resulting from simple and complex loading conditions. The program provides ultimate settlement and time-rate of consolidation for the total soil mass specified and for the individual compressible soil layers within the soil mass. Additionally, it provides the in situ overburden pressures and the induced stresses.
Induced stresses are determined by integration of either Boussinesq or Westergaard point load equations over general shaped loaded regions. Loaded regions may consist of simple or complex geometric shapes for singular or multiple loads entered as two-dimensional pressure profiles, two-dimensional soil embankment profiles, or three-dimensional polygons.
Settlement computations are based on strain versus effective stress or void ratio versus effective stress relationships. The rate of consolidation is determined using Terzaghi’s one-dimensional consolidation theory. The program provides for analyses of multiple soil layers and a variety of drainage conditions.
Data can be input interactively or by entering a predefined data file. Data files are created using command words that identify corresponding data items. Interactive input can be accomplished by a question-and-answer session or by using command words. Output consists of a total settlement, settlement of individual layers, and degree of consolidation for each time and location specified by the user.
Utilizing Pile Integrity Test in testing shallow foundation cracking
Abdalla Eliwa Abdalla Elmesallmay
Zagazig University
2001
Pile integrity test is non-destructive test that can assess the integrity and soundness of the reinforced concrete pile body. Non-destructive testing is superior over any other test for the fact that the structural element body is kept intact as before the testing. Pile integrity test idea starts with inducing an internal body wave and tracing the wave propagation along the pile body in an effort to pin-point the zone(s) of defects. More progress in the test allowed for identifying the type of defect. The idea of using wave propagation to assess the pile body integrity is further extended to identify the defects and/or cracks in shallow foundations.
The research objectives are employing the pile integrity test theory in detecting damages within shallow foundations, extending the test to theoretically locate cracks, and damaged zones within shallow foundations, interpreting pile integrity test signals and graphs in locating the cracks and damaged zones in footings and proposing procedure for correcting the readings of the pile integrity test to simulate the damage detection in reinforced concrete footings. The analysis is performed using the finite element software ADINA in modeling the stress wave propagation in piles as prismatic members first. Extending the numerical approach to circular footings, is a further step from the long prismatic member (pile) and the final intended shape of circular surface footings. Compiling a performance database and categorize waves based on their footing material properties and dimensions. Studying the wave propagation in piles and extending the study to footings of different shapes under cyclic force with sinusoidal function, and impact function. Studying the wave propagation in both intact and defected piles and footings. Providing a perspective for a compatible wave analysis of both intact and cracked piles, and extending the case for circular shallow footings in both cases of intact and defected footing bodies.
Vertical Stresses Beneath Embankment and Footing Loadings
U.S. Army Corps of Engineers
741-GI-F5010
March 1968
The program computes the vertical stresses induced in a semi-infinite mass by a group of uniformly loaded rectangular areas. Under one program option the vertical foundation stresses caused by an embankment loading are approximated by assuming that the embankment is composed of a series of uniformly loaded rectangular areas lying on the top of one another. The program can handle up to 100 footing loads. The vertical stresses may be calculated by either Boussinesq or Westergaard solutions for vertical stresses.
Wave Analysis of Mat Foundations to Resist Differential Soil Movements
Lawrence D. Johnson
USAE Waterways Experiment Station, Geotechnical Laboratory
Miscellaneous Paper GL-91-7
March 1991
Soil differential movement patterns are responsible for considerable damages to all types of structures and pavements such as military facilities, commercial buildings, housing, streets, highways, parking lots, and airfields. These differential movements occur from non-uniform soil volume changes, particularly in expansive clays and collapsible silty soils, attributed to a variety of mechanisms. Among the most important mechanisms leading to volume changes are those mechanisms that change the soil water content and stress. These mechanisms include desiccation from heavy vegetation, increased soil permeability from fissures, perimeter wetting and drying from climate changes, leakage from underground utilities, and many others.
This work develops a model for representing non-uniform volume changes by wave patterns of soil distortion underlying the foundation or at the ground surface. Soil-foundation distortion is measured in terms of angular distortion β which may be calculated as 4A/l where A is the amplitude and l is the wavelength of the wave pattern. The wave pattern model leads to simple methodology for rating the performance of structures subject to differential soil movement and can provide a tool to assist the design of foundations that reduce soil movement patterns. The model shows that some wavelengths are potentially more destructive than others depending on the magnitude of soil heave and the ability of the structure to tolerate soil-foundation distortion. This model may also be applicable to the analysis of pavements.
The model provides simple equations for calculating a relative thickness performance rating parameter Drel and an equivalent thickness Dc, of mat foundations required to reduce a given soil distortion. Elevation or level profile measurements of the first floors of a variety of structures subject to different degrees of soil distortion confirm that the maximum relative thickness Drelm. in an elevation profile is a consistent performance rating index, although only total deformation profiles are available at this time. Drelm is a function of the most damaging distortion and can be used to indicate the location of the most damaging distortion in a soil profile. The wave index WI, a root mean squared summation of amplitudes in an elevation profile, was nearly as consistent an indicator for rating performance. These results appear to verify Dc as a potential design tool for foundations. Distortions leading to Drelm > 40 ft correlate with differential soil movements > 5 in. and crack widths > 0.5 in. and prevent economical design of mat foundations.
Field test sections are required to measure soil distortions for different climates and wetting conditions and for characterizing soil-foundation displacement patterns to be used for improving foundation design and construction methodology. Results of field studies will be used to confirm Drelm as a performance rating index, to confirm and calibrate Dc as a design tool, and to determine optimum soil improvement methods in preparation for foundation construction.
vulcanhammer.net 