Advances in Unsaturated Geotechnics

Soil suction (i.e., negative pore-water pressure) measurements are essential to predicting and verifying the behavior of an unsaturated soil. Some difficulties were encountered with soil suction measurements made using earlier versions of thermal conductivity soil suction sensors. An improved thermal conductivity sensor has been developed at the University of Saskatchewan, Saskatoon, Canada, The durability and the accuracy of the new sensor has been significantly improved by using a specially designed ceramic and improved electronics.

The study of matrix and total suction versus depth is important in the analysis and design of slabs on grade such as light structure building slabs and highway pavements. This paper quantifies the seasonal moisture profiles with depth in two typical expansive Oklahoma clays. These soils can be characterized as being medium to highly plastic, blocky structured, with a medium stiff consistency, shallow in depth and residual. Following a detailed field investigation and laboratory soil analysis, the project involves some variation in typical access tube installation technique and moisture monitoring with a capacitance probe. Adjustments to the access tube installation appear to work well. The paper presents characteristic curve equations for different soil horizons which are used along with the monitored moisture profiles to produce suction versus depth changes with time in the field. To date, some change in suction within the active zone has been recorded.

In situ tests, mainly the Standard Penetration Test, SPT, are commonly used in Brazilian foundation practice. This paper presents the estimating of ultimate bearing capacity of a 14.0m deep instrumented precast concrete pile with 0.18m diameter, that was driven in an experimental research site at the University of Campinas — Unicamp. The soil of the site is characterized as residual of Diabase. The soil is unsaturated up to 18.0m depth and its geotechnical characterization obtained by laboratory and field tests is presented here. The skin friction and point resistance were obtained by an instrumented load test. The load test results were compared with results obtained by methods to predict the bearing capacity of the foundation element using the Standard Penetration Test with torque measurement, SPT-T, Cone Penetration Test, CPT, and Flat Dilatometer Test, DMT, results.

This paper examines cylindrical cavity expansion in unsaturated soil and its application to the interpretation of pressuremeter test results. An analytical solution for cylindrical cavity expansion in unsaturated soil is presented and used to examine the influence of unsaturated soil characteristics on the pressuremeter response. The pressuremeter response is characterized by the limit pressure and pressuremeter modulus. Effects of matric suction and initial degree of saturation are examined in light of unsaturated soil mechanics theories. Application of the unsaturated theory to the design of foundations is discussed in comparison to existing methods of pressuremeter interpretation. A methodology for predicting pressuremeter response under different moisture conditions is presented along with an example.

In this study, tests were conducted to investigate the use of ultrasonic methods to determine compaction characteristics of clayey soils. In particular, through-transmission test method was used to determine P-wave velocities in compacted clayey soils. Effects of soil type and compaction conditions on velocity were investigated. Tests were conducted on three soils that had low, medium, and high plasticity. Soils were prepared at water contents ranging from dry to wet of optimum using standard and modified compaction efforts. It was observed that velocity increased with increasing compactive effort and decreasing plasticity and clay content. Moreover, the variation of ultrasonic velocity with water content was similar to the variation of dry density with water content. Shapes of the water content versus velocity plots correlated well with the shapes of compaction plots. Access to two opposite surfaces of samples was required to perform through-transmission measurements. Additional tests were conducted to determine the feasibility of using surface-transmission with access required only to top surface of soils to measure velocity. Velocities that were similar to through-transmission velocities were obtained with surface measurements using correction factors. Through-transmission can be used in the laboratory; whereas surface-transmission can potentially be used in the field to determine ultrasonic velocity of compacted clayey soils. This method shows promise for determination of compaction properties of clayey soils in the field.

The grain-size distribution is commonly used for soil classification; however, there is also potential to use the grain-size distribution as a basis for estimating unsaturated soil behavior. Mathematically representing the grain-size distribution provides several benefits to soil mechanics. An example is the estimation of the soil-water characteristic curve from the grain-size distribution. Much emphasis has recently been placed on the estimation of unsaturated soil-property functions from the soil-water characteristic curve. Several methods have been proposed that use the grain-size distribution as the basic information for the estimation of the soil-water characteristic curve. Two mathematical forms are presented to represent grain-size distribution curves; namely, a unimodal form and a bimodal form. The equations presented in this paper can provide a close representation of a wide variety of grain-size distribution for different soil types.

The most widely-used constitutive relations for unsaturated soils use matric soil suction as a state variable. The soil-water characteristic curve (SWCC), the relationship between soil suction and some measure of the water content, can be measured or predicted based on soil index properties such as the grain-size distribution (GSD) function. Estimation based on index properties is highly desirable due to its simplicity and low cost and would be the path of choice to the SWCC, provided the accuracy of the estimate were adequate. Whether measured or estimated, there is variability and uncertainty associated with the SWCC that in turn will directly impact any model for the unsaturated soil behavior that makes use of this relationship. The variability in the SWCC associated with direct suction measurements as well as the variability associated with the prediction of the SWCC based on GSD was investigated. Three different soils that cover a typical range of soils encountered in practice were used in this study. The investigated sources of variability related to direct suction measurements included the equations used to fit the suction data, the different methods available to measure suction, the operator, the number of data points used to define the SWCC, and the range of suction covered in the measurements. The variability observed in the SWCC when different predictive algorithms are used is also reported. Finally, a new model for predicting the SWCC based on soil index properties is presented, using a database of 190 soils. Results showed that the operator as well as the range of suction covered in the measurements produced significant variability in the measured SWCC. Surprisingly, the variability in the SWCC as predicted by soil index properties and/or GSD-based algorithms was found to be as small or smaller than that associated with the operator (person(s) measuring the SWCC).

The proliferation of the use of the soil water characteristic curve (SWCC) to obtain the unsaturated soil parameters is unavoidable and in some cases desired. However, the complex interrelation between soil water content and soil water suction makes the adequate indirect determination of the unsaturated soil parameters a difficult task. There are many aspect to be considered when using SWCC for predicting mechanical or hydraulic behaviour of the soil. The paper describes a study involving a residual soil of Gneiss, compacted using three different compaction energies. Soil water characteristic curves (SWCC) are obtained with volume measurement during the drying process and the soil suction was obtained using the filter paper method. The filter paper was placed in direct contact with the soil sample. Discussions on the relationship between the compaction energy, SWCC shape and the drying behaviour of the soil samples are given.

Environmental hazards are created by both on and offshore oil spills which contaminate soil. Oil contamination may also change the physical properties of soil. This paper summarizes the results relating to compaction, hydraulic conductivity, and shear strength of laboratory tests conducted with oil-contaminated sand. Only one type of sand was used for the tests primarily at two relative densities of compaction; however three different oils were used as contaminants to evaluate the effect of their kinematic viscosity on the geotechnical parameters studied. Oil contamination increased the compactibility of sand whereas it reduced the hydraulic conductivity and shear strength.

Geotextiles are increasingly being used in a wide variety of applications, such as reinforcement, separation and drainage. These geotextiles are often in contact with unsaturated soils; however the hydrologic behavior of geosynthetic materials in unsaturated environments is most often ignored because the basic characterization of these materials does not exist. Test methods for measuring water retention data and transmissivity under suction have been developed for nonwoven geotextiles. These measurements complement simple contact angle and capillary rise measurements to characterize nonwoven geotextiles under unsaturated conditions. Results indicate that the tested nonwoven geotextiles exhibit properties and behavior similar to coarse, uniform soils. The unsaturated behavior has been found to be hysteretic; that is the behavior depends on whether the geotextile is being dried or wetted. Penetration of adjacent soil into the geotextile has also been found to affect its behavior. Soil column tests that include nonwoven geotextiles confirm that they will significantly affect unsaturated flow in soils.

A fully coupled analysis procedure for dynamic analyses of unsaturated porous media is presented. The analysis procedure is based on the finite element implementation of the governing equations derived utilizing the Theory of Mixtures with Interfaces (TMI). TMI explicitly considers the interfaces existing between various bulk phases in a multiphase porous media such as the contractile skin between the pore water and the pore air in an unsaturated soil. The resulting computer code, U_DYSAC2, is used to predict the behavior of an unsaturated kaolin embankment subjected to base shaking and the results are compared with the saturated soil behavior. Unsaturated soil embankment is shown to behave more rigidly than the saturated embankment resulting in lower displacements and higher accelerations. U_DYSAC2 also predicts increase in pore water and pore gas pressures during shaking. The results presented show that it is possible to economically design an unsaturated soil structure for seismic loads using a fully coupled analysis.

Earthquake-induced ground deformations resulting from contractive volumetric strains in unsaturated compacted soil have been observed in numerous earthquakes. This process, termed seismic-compression, is investigated using a robust cyclic simple shear device. Testing is performed on re-compacted specimens using soils retrieved from three sites near Santa Clarita, California which experienced varying levels of ground deformation during the 1994 Northridge earthquake. A key objective of the testing is to evaluate the adequacy of existing design procedures that are based on simple shear testing of clean, uniform sands for soils containing significant fines. A total of seven soils were tested: a clean uniform sand and six different soils from the three field sites possessing varying levels of fines content and plasticity. The soils containing fines were compacted to a range of water contents and dry densities to evaluate the dependence of volume change susceptibility on compaction conditions. Both formation water content and dry density were found to significantly affect the magnitude of volumetric strains induced by cyclic loading. At the same relative compaction and induced shear strain, soils containing fines generally underwent smaller contractive volumetric strains than clean sands. Soil specimens compacted wet of the line-of-optimums generally experienced less seismic-compression than soil specimens compacted on the dry side.

Several procedures have been proposed in the recent years to predict the shear strength of an unsaturated soil. The soil-water characteristic curve has been used as a tool either directly or indirectly in the prediction of the shear strength along with the saturated shear strength parameters in these procedures. This paper provides comparisons between the measured and predicted values of unsaturated shear strength using these procedures for three soils both for limited and large suction ranges. The three soils used in the study for comparisons have different gradation properties, percentages of clay and plasticity index, I_p values. The advantages and limitations associated with predicting the shear strength of unsaturated soils using the procedures is discussed in the paper.

Unsaturated soil condition which give rise to negative pore pressure is important in evaluating the stability of geotechnical structures such as slopes, retaining walls, excavations and footings. Its importance is more so in tropical countries, where rainfall and intense chemical weathering have resulted in the formation of such soils. In this study, the shear strength characteristics of granite residual soil were investigated using consolidated drained triaxial tests with special emphasis on the effects of the negative pore pressure, i.e. the matric suction. These tests involved shearing under either a constant net confining pressure and varying matric suctions or under a constant matric suction and varying net confining pressure. The influence of confining pressure and matric suction on shear strength was analyzed and a linear relationship was found to relate matric suction and shear strength.

Fabric plays a vital role in determining the engineering behaviour of compacted soils, particularly under unsaturated conditions. Clayey materials compacted dry of optimum moisture content develop an aggregated or "packet" fabric. The presence of aggregations causes the soil to behave in a coarser fashion than would be justified by the grading. This means that the friction angle φ^a (representing the strength component due to net stress) depends on the fabric of the soil. Dry of optimum moisure content, φ^a > φ' (the angle of friction in saturated conditions). Similarly, the packet fabric also affects the angle φ^b (representing the strength component due to matric suction). Dry of optimum moisture content, φ^b becomes significantly lower than φ' and at low water contents drops to zero. The normal practice for determining the shear strength properties of unsaturated soils (by multi-stage tests in which the suction is increased from an initial low suction state) will result in the packet fabric being destroyed in the first stage of shearing. Therefore, to properly study the shear strength behaviour of unsaturated compacted soils, tests should be performed on the soil in an "as compacted" condition.

Loess, a uniform-sized, wind-blown, cohesive soil can be problematic in geotechnical engineering applications. Natural deposits have high strength and exhibit stable vertical slopes; however, dramatic collapse can occur when loess becomes saturated. In remolded, compacted applications, the behavior of loess is less well known. Regionally (central USA), embankments of compacted loess can exhibit deformation and slope failure after years of service. The initial phase of a larger effort to characterize the strength behavior of compacted loess is reported herein. This phase is limited to assessing the short-term, undrained shear strength of compacted loess. Undrained strengths of compacted loess were measured to be up to 500 to 650 kPa. Higher compactive effort, dry of optimum water content, maximizes the undrained strength. Modified Proctor energy increased undrained shear strength by up to 2.5 to 3.0 times higher than loess compacted at the same water contents using standard energy. Loess compacted at or greater than optimum moisture content, as determined in the standard Proctor test, had similar undrained strength (100 to 200 kPa) regardless of the compactive effort. In order to maximize short term strength of compacted loess, use the greatest compactive effort and compact the soil well dry of optimum moisture content.

This paper presents basic engineering properties of a gneissic residual soil from a natural slope in which a greater landslide occurred. Emphasis is given to the design strength parameters obtained from laboratory tests. A stability analysis is done taking into consideration unsaturated condition. The landslide causal factors are also presented and discussed. The study results have shown that considering a low value of initial soil matric suction in the field the strength parameters mobilized in the first time failure are characterized by the laboratory results (peak or deformation near to the critical state condition).

Arid regions are widely distributed around the world. Soil water fluxes are related to the actual evaporation that is closely related to ground surface temperature and relative humidity. A soil can be dried such that its water content is below residual conditions. Of particular interest in this paper is the shear strength of very dry soils. This paper reports results on the relationship between total suction and shear strength beyond the residual water content state. This study involved performing unconfined compression tests for compacted, unsaturated soils and measuring the soil-water characteristic curves. A silty soil and kaolin were used in this test program. Two types of compacted, unsaturated soils were subjected to total suctions as high as 124,000 kPa and the shear strengths were measured. The soil-water characteristic curves were measured over a wide range of suctions. The residual state for the silty soil is clearly defined. The residual state for the kaolin is difficult to define. The shear strength in the residual state appears to increase slightly. This paper concludes that the failure envelope is essentially horizontal in the residual states.

This paper presents the effect of using pozzolanic material as a new stabilizing agent on shear strength and swelling properties of unsaturated clayey soil. Three clayey soils were selected from the northern part of Jordan and used for this research. Different percentages of pozzolanic material by dry weight of the soils namely; 2.5%, 5.0%, 7.5% and 10.0% were added to the selected soils. Atterberg's limits tests, pr octor density test, direct shear test, unconfined Compression test, swelling pressure test and swell potential test were conducted on specimens treated with pozzolan. It was found that the increase in pozzolanic material will decrease the plasticity index and increase the unconfined compressive strength of the soils use. Furthermore, the addition of 10.0% of pozzolanic material by dry weight of the soil could eliminate both the swelling pressure and the swell potential of the used soils.

Pulverised coal bottom ash (PCBA) obtained as residue from the firing of coal was tested on cement stabilised tropical black clay classified as A-7-6 or CL in the AASHTO and Unified Soil Classification Systems, respectively. PCBA at stepped concentrations (5,10,15, 20,25 and 30%) were admixed with soil — cement mixtures containing 2,4,6 and 8% cement by weight of dry soil, respectively, in order to establish the soil improving potentials of the pulverized coal bottom ash. Unconfined compressive strength (UCS) and California bearing ratio (CBR) tests on the admixture stabilised tropical black clay specimens (i.e. soil — cement — PCBA) yielded peak values at 5% PCBA concentration, in all cases considered, which were markedly higher than values obtained for the natural soil and soil — PCBA combination.

Deformation of an excess spoil fill subjected to infiltration and groundwater seepage is simulated with a matric-stress critical state model. The model includes groundwater and elastoplastic soil constitutive relationships and is formulated in terms of fluid head and net stress plus matric stress, where matric stress is the stress generated by matric suction. Results of the simulation are presented in terms of the pore-water pressure distribution, phreatic surface, shear strain distribution, and settlement at selected points on the fill.

Isotropically consolidated-drained triaxial compression tests were performed on undisturbed structured/cemented silt to gain a better understanding of the shear behavior of and hyperbolic stress-strain parameters for this material. The results show that the hyperbolic stress-strain parameters for the structured/cemented silt are anisotropic and differ significantly from the hyperbolic parameters developed for silt reconstituted at the field total unit weight and water. As the confining pressure in a triaxial compression test approaches the preconsolidation pressure, the effects of the structure/cementation are reduced due to bond breakage, and the structured/cemented silt exhibits a stress-strain behavior similar to that of reconstituted silt.

A servo-controlled true triaxial (cubical) testing device was modified to test 10-cm cubical specimens of unsaturated soil under varying matric suction conditions. Recompacted silty sand specimens were tested following a multi-stage testing scheme by controlling the major, intermediate, and minor principal stresses. Matric suction states in the specimens were induced and maintained constant during testing by using the axis-translation technique. True triaxial testing data from a series of drained, stress/suction-controlled triaxial compression, triaxial extension, and simple shear tests, were used to evaluate the nature of principal strain response of unsaturated soils under varying matric suction conditions, and for shear stress paths not achievable in a conventional cylindrical apparatus. Normalized test data of octahedral stress versus principal strain, on the basis of matric suction level, indicate consistent stress-strain behavioral patterns. The paper presents these test results and proposes behavioral trends for unsaturated soils as a function of matric suction.

Preconstruction treatment or the selection and design of a foundation system both rely on accurate estimates of the potential heave of the supporting expansive soil. Swell percentage of expansive soils is commonly obtained from oedometer swell tests under fully lateral restraint conditions. In situ heave, however, is not a one-dimensional volume change but more likely a three-dimensional process. Consequently, surface heave predicted from results of oedometer tests is usually conservative and largely differs from heave actually observed in the field. To account for the discrepancy between oedometer and in situ boundary conditions, some researchers have suggested a lateral restraint factor to be applied to heave predictions evaluated based on parameters obtained from oedometer swell tests. This paper presents a laboratory testing procedure by which the vertical and the volumetric swells of compacted expansive soil samples are concurrently measured using a hydraulic triaxial stress path apparatus. A series of triaxial swell tests is conducted and the results are compared with the volume change measured in specimens tested under one-dimensional loading conditions in the oedometer. The percentages of vertical swell obtained from the triaxial swell tests are found to be considerably lower than the corresponding values measured in the oedometer tests. For the triaxial swell tests, the ratio of swell in the vertical direction to the volumetric swell, SR, is evaluated. The test results indicate that SR increases with increasing applied confining pressure, and for a particular swell test, the value of SR is not constant but rather changing with elapsed swelling time.

This study examines the extent to which mineralogy and swelling potential can be correlated in the expansive clays and shales along the Colorado Front Range Urban Corridor. 182 undisturbed samples were collected from 20 sites from Boulder to Pueblo. Sites were selected in Cretaceous shales, including the Pierre Shale, that have been uplifted into steeply dipping strata near the foothills of the Rocky Mountains, and that are well known to be hazardous to residential and light commercial developments in this region. For each sample, mineralogy was determined by x-ray diffraction and swelling potential was obtained from moisture content (w), suction (h), and clod volume (V) measurements in terms of the suction potential (dh/dw) and suction-compression index (dV/dh) parameters used in the classification scheme McKeen proposed in 1992. Swelling potentials were also obtained on more limited suites of samples with conventional and labor-intensive schemes including Seed and Chen's schemes based on tests for grain-size distribution and Atterberg limits, and with swell-consolidation measurements in response to saturation, consolidation, and rebound in an oedometer. The results show the percent total smectite provides a useful index of the swelling potential concept defined by Seed and correlates reasonably well with the swelling potential indices developed by Seed, Chen, and McKeen. However, the percent total smectite does not correlate well with conventional swell-consolidation test indices. The causes for this lack of correlation appear to be placement and environmental factors such as the initial moisture content, stress history, and surcharge loading that are beyond the scope of Seed's swell potential concept and Seed, Chen, and McKeen's swelling potential indices.

In this paper, thermocouple psychrometric technique is used to measure soil suction and reliability of swell percentage prediction method based on soil suction data is evaluated. Kaolinite-bentonite clay mixtures were prepared to obtain expansive soils in a wide range of plasticity indices. Suction measurements using thermocouple psychrometer technique and free swell tests in oedometers were made on statically compacted specimens. Soil suction-water content relationships were evaluated. The results indicated a linear relationship between log suction and swell percentage measured in oedometer. It has been shown that the predicted swell percentages by using suction test results underestimate the ultimate swell percentages in oedometer free swell tests.

Determination of the suction compression index, C_h, is necessary for predicting expansive soil heave based on soil suction. The CLOD Test is a convenient laboratory procedure for determining C_h, however it is not widely used. Rather, engineers often rely upon empirical correlations to find C_h. A laboratory investigation was conducted to verify previously established empirical relationships. The investigation involved CLOD tests on relatively undisturbed drive samples of clay and claystone bedrock from the Denver, Colorado area. Test results generally matched previous studies. Samples utilized in this investigation spanned a large range of suction and moisture characteristics. A new empirical relationship for determining C_h over a broader range of materials is presented. In an effort to expedite the CLOD test, studies were performed to ascertain the effect of various procedural simplifications on the test results. Information obtained by these studies is used to suggest a modified procedure for professional practitioners. Filter paper suction, one-dimensional swell, Atterberg Limits, and gradation tests were also performed on numerous select samples. Swell prediction based on soil suction and the CLOD test results compared well with direct laboratory swell measurements. The average C_h measured in the Denver area is shown to be a function of bedrock geology. A semi-empirical relationship is also presented between Atterberg limits, percent passing the No. 200 sieve and C_h.

A protocol has been developed for calculation of potential future heave of basement floors in residential structures in the Colorado Front Range area. Data from swell/consolidation tests commonly used in the area and soil suction tests are used to predict potential heave. The calculations are very dependent upon the anticipated depth of wetting. Pre-construction soil suction data were compared to post-construction suction data. The results indicate the assumptions of depth of wetting developed for the protocol are appropriate in most instances. Unusually deep wetting may occur in areas where geologic conditions allow water to penetrate more easily into soils and sedimentary bedrock.

Application of "effective" stress principles in the prediction of heave in unsaturated expansive clay requires an evaluation of the swell index and changes in the matric suction. The latter, in turn, requires knowledge of water balance principles. This paper presents a case study to illustrate the practical application of an effective stress approach to heave prediction. The purpose of this paper is to present application of a method to predict heave proposed by Fredlund and Rahardjo in typical engineering studies. A brief review of the method is provided, followed by areas of application. A case history with measured movement, swell pressures and matric suction is presented to illustrate the information required and its application in an engineering environment.

Adequate design of post-tensioned residential slab-on-ground foundations in expansive soils is dependent on understanding the geotechnical and climatic factors inherent to the procedure. A method for design of ground-supported slab foundations was developed by the Post-Tensioning Institute (PTI). The design stiffness factors (edge moisture variation distance, e_m, and differential movement, y_m) are derived in part based on the average Thornthwaite Climatic Index, I_m, for the design area. The purpose of this paper is to show, by case study, the importance of changes in water balance as measured by the I_m on expansive soil behavior and in particular to the PTI design stiffness parameters, e_m and y_m. Surveyed differential movements of post-tensioned foundations in two expansive clay formations in the North Texas region, where average I_m, not including the influence of irrigation, is 0, are provided. Data is presented to illustrate that use of the average I_m value results in slab foundations that are under-designed. Recalculation of the published average I_m value for the design area is recommended to account for changes in water balance caused by development, for example the increase in I_m from landscape watering or extreme dry periods. The case histories show that the PTI values for e_m and y_m, using the recalculated I_m are in closer agreement with observed differential movement. The studies also show that PTI y_m values derived using average Thornthwaite values can be in error by 100 percent or more.

Expansive soils are well known in Denver and Colorado. These soils are also found in 40 of the 50 United States and worldwide in all but the polar regions. Different remedies have been used to meet the expansive soils challenge. One material being used is the geomembrane. This geotextile is treated to make it relatively waterproof. Moisture change will cause the expansive soil to swell or shrink. Minimize the moisture change and the volumetric change will be minimized. The DuPont Company brought their geomembrane to the United States in the 1960's. One of the first geomembrane projects in this country took place in Colorado. The geomembrane was placed horizontally between sand layers. Initial reports were encouraging. The Texas Highway Department used the geomembrane on a San Antonio urban arterial street project in 1976. Testing took place prior to and following construction. The geomembrane was placed horizontally on the subgrade, base dumped and spread, watered, and compacted. Paving and testing followed. The next Texas highway geomembrane project was on Interstate Loop 410 in San Antonio. The fabric was placed 8 feet deep along the shoulders of both northbound lanes. The adjacent southbound lanes of the divided highway received no geomembrane. Post construction testing showed the geomembrane protected lanes provided the smoother ride and required less maintenance. Over twenty additional Texas highway projects used the geomembrane usually as a deep vertical moisture barrier and usually successfully. Wyoming DOT has used the geomembranes on over 50 projects across the state beginning in 1981 totaling more than 4,000,000 square yards at prices of $1.25 per square yard. Their early projects placed geomembrane horizontally. Later, and currently they are placing it horizontally and vertically, and are pleased with the results. Other states have used the geomembranes on their highways. Australia is using geomembranes on their highways. and around some buildings. They have developed a trenching boom capable of cutting 6 to 6-1/2 feet deep, only 3 inches wide, permitting the use of a flowable backfill of cement, sand, water, and fly ash. Several buildings sites in Texas have been similarly protected by using geomembranes. The fabric is placed horizontally and then vertically. The geomembranes have been used on railroads in France, China, and Russia. Israel and the United States have used them on canals and airport runways. Tests of geomembrane material confirm its decades long durability. The geomembrane costs have significantly decreased in Texas from early bids of $20 per lineal foot down to $6 per lineal foot. Geomembranes can be an effective remedy when dealing with expansive soils and their resultant damages.

Quest for a solution to construct stable civil engineering structure including light structure on expansive soil deposits started for the author in the year 1953, when he was told that the buildings he was supervising will heave and crack. Thus, a programme of acquisition of knowledge and planning a long-term four-stage research commenced. The first and second stage research were directed towards understanding swelling pressure and heave phenomenon, through field simulated large scale and small scale experiments, which lead to identifying role played by Coulumbian cohesion bonds, structure of clay minerals and Newtonian weight, in causing, swelling pressure, heave and self equilibrating phenomenon/ no volume change zone. This helped in putting forward fundamental relations between alteration in cohesion and change in stress-deformation and strength behaviour of saturated expansive soil system. This also led to discovery of CNS technology and its possible application to engineering problems. In the third and fourth stage research prototype experiments were performed with CNS technology in MRBC to evolve material and construction specifications for field application. In 1986 a review of performance of CNS technology was conducted, which resulted in putting forwards simplified guidelines by CWC. The last performance review was conducted on 3^rd and 4^th of September 1999 to assess long-term performance. Out come of this endeavor covering entire spectrum from concept to construction to long-term performance is briefly described in this paper.

Soil samples treated in the field (Crowley, Texas) with a hydrogen ion exchange chemical (HIExC) solution over two years ago, were analyzed in the laboratory using different techniques, such as X-ray diffraction, zeta potential, transmission electron microscopy/ energy dispersive X-ray analysis and environmental SEM to better understand the mechanism of soil stabilization by the HIExC solution. A limited number of engineering properties such as swell-shrink, strength, and permeability were also tested. Untreated soil samples from the same location were included as a "control," to help delineate the intricate changes in the soil structure and engineering properties that occur as a result of the amendment with HIExC solution. The principal conclusion of this study is that a change in the shrink-swell behavior of smectite in soils treated with HIExC occurs, which is distinctly beneficial from the standpoint of its stabilization.

The purpose of this paper is to show that the soil-water characteristic and unsaturated hydraulic conductivity functions derived for soils with continuous air channels, in which the air is assumed to exist at constant atmospheric pressure, should not be used for simulating infiltration process. Ongoing experimental work is oriented towards investigating the influence of the entrapped air on the hydraulic constitutive relations of a compacted fine-grained soil imbibing liquid. Testing is being conducted in two phases to determine (1) soil-water characteristic function and (2) unsaturated hydraulic conductivity function for systems involving discontinuous air phase. Hydraulic constitutive functions for soils with discontinuous air phase are proposed using the obtained experimental results.

The movement of moisture into and through near-surface soils is of major importance in numerous areas of geotechnical and geoenvironmental engineering, impacting soil strength, contaminant transport, efficiency of waste containment systems, and slope stability. Numerical models are often used to simulate moisture movement and behavior of soil systems due to the cost, complexity, and time required to undertake field studies. However, these simulations often ignore or grossly simplify near-surface processes such as precipitation, snowmelt, run-off, and evapotranspiration. It is not uncommon to see complex systems simulated using a one-dimensional model in which the surface moisture flux for the site is represented by a steady-state infiltration rate that is based on the annual average precipitation. In this paper we examine the impact of incorporating non-steady state models for near surface processes such as evapotranspiration, precipitation, and surface runoff on simulations of soil moisture movement in capillary barrier systems. Simulations with and without transient near surface processes have been conducted for a variety of sites with different climates. These sites include a semi-arid climate, a site where the bulk of the precipitation occurs during the winter months, and a wet, humid climate where rainfall exceeds 2 meters per year. The results demonstrate that simplification or omission of near-surface processes can lead to erroneous results.

The flow regime in a concrete foundation (stem wall and slab) resting on an unsaturated soil was analyzed to determine where and how water moves into and through the foundation in response to water pressures (or moisture suctions) acting at the boundaries. Flow nets were constructed using an iterative, finite difference technique. Steady state conditions and isotropic hydraulic conductivities were assumed in the analyses. Various cases were examined including effects of moisture barriers beneath the foundation and different degrees (or rates) of evaporation at foundation boundaries. An increasingly greater proportion of the total, advective water flow through the foundation occurred into and out of the sides of the stem wall as either 1) upward, water flow into the slab was prevented because of the presence of a moisture barrier beneath the slab, or if 2) a higher amount of evaporation occurred at the outside surface of the stem wall/grade beam compared to the upper surface of the slab. In every case water entering the concrete foundation from irrigated soil adjacent the foundation moved upward and into (and in some cases out of) the grade/beam stem wall. In no case did any of this irrigation water enter the slab. These illustrations and finding are significant with regard to the problem of possible sulfate damage to concrete foundations. The sulfates may occur naturally in the native soil or be introduced in the irrigation water or as sulfates added to the soil from agricultural amendments, e.g. gypsum or sulfur. The illustrations are consistent with the nature of minor damages (e.g., surface flaking and spalling) observed in concrete foundations next to irrigated plantings where sulfates are present in the soil or irrigation water.

Tests were conducted to determine the variation in volumetric water content and pore water suction for a variety of compacted clay soils used in the construction of landfill liners. The fit of the experimental data to an existing parametric model was investigated for two different fitting techniques. The first technique involves the use of the retention curve computer program (RETC) developed for the U.S. EPA. The second technique employs the Solver subroutine included in Microsoft Excel. The parametric models resulting from either technique correlated well to the experimental data. However, the individual curve fit parameters varied significantly. The effect of these variations on the unsaturated behavior of compacted clay liners was evaluated using the Hydrologic Evaluation of Landfill Performance (HELP) model. The curve fit parameters resulting from both the RETC and the Solver techniques were used as input to the HELP routine for simulation of variably saturated flow through a cover liner. There were no significant differences in the volume of leakage or rate of leakage predicted using the input from the two curve-fitting techniques. However, there was significant soil-dependent variation in the HELP output. Examination of the HELP output provides information regarding variation in the volumetric water content of the cover liner soil. This information can be used to predict pore water suction variations and susceptibility to desiccation cracking.

The impermeabilization of final cover layer of any municipal solid waste landfill (MSWL) is considered an important matter in reducing rain water infiltration as well as gas flow generated by waste organic matter decomposition process. Landfill gas (LFG) liberation may cause a number of environmental impacts, basically due to the presence of methane and carbon dioxide in its composition. An extensive program of laboratory and field tests were carried out in order to investigate the properties of the final cover layer at two municipal solid waste facilities in Recife-PE, Brazil. This paper presents experimental techniques developed to study gas flow through unsaturated soils. These studies included the use of a laboratory permeameter as well as the development of a simple device for field tests. The results allowed to determine the air permeability parameters and estimate of LFG flow to the atmosphere.

This paper describes the development of a two-dimensional soil-structure interaction model based on the finite element method for calculating the structural design parameters (i.e., moments, shears, and deflections) in a raft foundation (stiffened or constant thickness) resting on expansive soil. The model is capable of estimating the distorted mound shape by considering the soil suction distribution differences throughout the supporting soil mass and the associated volume changes (shrink/heave) with respect to time under a set of different edge conditions. The model requires only a statement of the initial soil suction conditions in the supporting expansive soil mass and the changes in the boundary conditions to predict the response of the raft foundation to these boundary condition changes. The model has been shown to yield results that agree well with reported field measurements of surface volume changes (shrink/heave).