Shear Strength of Compacted Soil under Infiltration Condition

Landslides in residual soil slopes are commonly induced by rainfall infiltration. These residual soils are typically in an unsaturated state with negative pore-water pressures or matric suctions since the groundwater tables in steep slopes are often deep. The net normal and shear stresses of the soil remain essentially constant during rainwater infiltration into the slope. Failure of the slope during rainfall can be primarily associated with the decrease in the matric suction of the soil. The objective of the study was to investigate the strength and deformation characteristics of a residual soil of the Bukit Timah Granitic Formation during infiltration that leads to slope failure. There were two modified direct shear apparatuses used. One apparatus was used for the determination of shear strength under controlled suction conditions while the other apparatus was used for shearing-infiltration tests. The shearing-infiltration test results were compared with the shear strength values obtained from the shearing tests under constant suction. The shearing-infiltration test results indicate a close relationship between the decreasing matric suction and the increasing displacement rate of the soil specimen. At the initial part of the infiltration process, there is a rapid reduction in matric suction that is accompanied by little movement in the soil. When failure of the soil is imminent, the soil movement will accelerate.     

Shear Strength and Pore-Water Pressure Characteristics during Constant Water Content Triaxial Tests

Shear strength parameters used in geotechnical design are obtained mainly from the consolidated drained (CD) or consolidated undrained (CU) triaxial tests. However in many field situations, soils are compacted for construction purposes and may not follow the stress paths in CD or CU triaxial tests. In these cases, the excess pore-air pressure during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time. Under this condition, it can be considered that the air phase is drained and the water phase is undrained. This condition can be simulated in a constant water content (CW) triaxial test. The purpose of this paper is to present the characteristics of the shear strength, volume change, and pore-water pressure of a compacted silt during shearing under the constant water content condition. A series of CW triaxial tests was carried out on statically compacted silt specimens. The experimental results showed that initial matric suction and net confining stress play an important role in affecting the characteristics of the shear strength, pore-water pressure, and volume change of a compacted soil during shearing under the constant water content condition. The failure envelope of the compacted silt exhibited nonlinearity with respect to matric suction.     

Laboratory Study of Loose Saturated and Unsaturated Decomposed Granitic Soil

Weathered soils are used extensively as fill materials in slope construction in tropical and subtropical cities such as Hong Kong. The mechanical behavior of loose decomposed fill materials, particularly in the unsaturated state, has not often been investigated and is not yet fully understood. The objective of this study was to understand the mechanical behavior of loose unsaturated decomposed granitic soil and to study the effects of the stress state, the stress path and the soil suction on the stress–strain relationship, shear strength, volume change, and dilatancy via three series of stress path triaxial tests on both saturated and unsaturated specimens. It was found that loose and saturated decomposed granitic soil behaves like clean sands during undrained shearing. Strain-softening behavior is observed in loose saturated specimens. In unsaturated specimens sheared at a constant water content, a hardening stress–strain relationship and volumetric contractions are observed in the considered range of net mean stresses. The suction of the soil contributed little to the apparent cohesion. The angle of friction appeared to be independent of the suction. In unsaturated specimens subjected to continuous wetting (suction reduction) at a constant deviator stress, the volumetric behavior changed from dilative to contractive with increasing net mean stress and the specimen failed at a degree of saturation far below full saturation. It was revealed that the dilatancy of the unsaturated soil depends on the suction, the state, and the stress path.     

Is Matric Suction a Stress Variable?

The writer attempts to clarify and address two fundamental questions regarding the appropriate use of matric suction in unsaturated soil mechanics: Is matric suction a stress variable? and Is matric suction a stress state variable? These questions are examined by employing the universally accepted mechanical equilibrium principle, the concept of representative elementary volume (REV) for air-water-solid multiphase porous media, and physical and logical reasoning. It is clarified that matric suction is not a stress variable at a typical air-water-solid REV level, and it can be considered as a stress state variable. However, when it is considered as a stress state variable, there is an interdependency or coupling between matric suction and the net normal stress if both of them are concurrently used to describe the state of stress in unsaturated soils. It is illustrated that the answers to these questions bear important implications for the conceptualization, theorization, and application of unsaturated soil mechanics.     

Suction Stress Characteristic Curve for Unsaturated Soil

The concept of the suction stress characteristic curve (SSCC) for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: (1) active forces transmitted through the soil grains; (2) active forces at or near interparticle contacts; and (3) passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.     

Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus (Go) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil type on the Go-SWCC relationship was investigated. Finally, a relationship describing the small-strain modulus behavior of unsaturated compacted soils is proposed.     

Equivalent Stress Equation for Unsaturated Soils. II: Solid-Porous Model

Based on the study of the equilibrium of the particles of a soil showing a bimodal structure and subject to certain suction, it was possible to establish an analytical expression for the value of Bishop’s parameter χ (see the companion paper). This parameter can be written as a function of the saturated fraction and the degree of saturation of the unsaturated fraction of the soil. However, the determination of these last two parameters cannot be made from current experimental procedures. Therefore, a solid-porous model simulating the structure of the soil is proposed herein and used to determine these parameters. The data required for the solid-porous model are obtained from the grain and pore size distributions, void ratios, and secondary soil–water retention curves of the soil. The plots of the deviator stress versus equivalent stress shows a unique failure line for a series of tests performed at different confining net stresses and suctions, confirming that the proposed equivalent stress equation is adequate to predict the shear strength of unsaturated soils. It also results in different strengths for wetting and drying, as the experimental evidence suggests.     

Unsaturated Infinite Slope Stability Considering Surface Flux Conditions

A slope stability model is derived for an infinite slope subjected to unsaturated infiltration flow above a phreatic surface. Closed form steady state solutions are derived for the matric suction and degree of saturation profiles. Soil unit weight, consistent with the degree of saturation profile, is also directly calculated and introduced into the analyzes, resulting in closed-form solutions for typical soil parameters and an infinite series solution for arbitrary soil parameters. The solutions are coupled with the infinite slope stability equations to establish a fully realized safety factor function. In general, consideration of soil suction results in higher factor of safety. The increase in shear strength due to the inclusion of soil suction is analogous to making an addition to the cohesion, which, of course, increases the factor of safety against sliding. However, for cohesive soils, the results show lower safety factors for slip surfaces approaching the phreatic surface compared to those produced by common safety factor calculations. The lower factor of safety is due to the increased soil unit weight considered in the matric suction model but not usually accounted for in practice wherein the soil is treated as dry above the phreatic surface. The developed model is verified with a published case study, correctly predicting stability under dry conditions and correctly predicting failure for a particular storm.     

Equivalent Stress Equation for Unsaturated Soils. I: Equivalent Stress

In 1959 Bishop stated his effective stress equation for unsaturated soils. However, the difficulties in estimating the value of its main parameter χ, made this equation useless and it was abandoned for some time. Only recently, it has been recognized that the use of Bishop’s stress equation can lead to simpler and more realistic constitutive models for unsaturated soils. However, up to now the most successful equations to quantify the value of parameter χ are empirical and not satisfactory for most soils. Based on the analysis of the equilibrium of the solid particles of a soil showing a bimodal structure and subject to certain suction, it was possible to establish an analytical expression for Bishop’s parameter χ. The resulting stress has been called equivalent stress (in contrast with effective stress) and can be used to predict the shear strength of unsaturated soils. The equivalent stress is written as a function of the net stress and suction and requires two parameters: the saturated fraction and the degree of saturation of the unsaturated fraction of the soil. This equivalent stress clarifies some features of the strength of unsaturated soils that up to now had no apparent explanation. However, the determination of its two parameters cannot be made from current experimental procedures. A method for the determination of these parameters and a comparison between experimental and theoretical results for the shear strength of unsaturated soils are presented in a companion paper.     

Engineering Properties of Lunar Soil Simulant JSC-1A

This study was carried out to assess the tensile and shear strength in lunar soil, and to examine the variation as a function of density and confinement. Geotechnical engineering properties of the lunar soil simulant designated Johnson Space Center Number One-A lunar soil simulant (JSC-1A) have been investigated experimentally. To better understand these soil properties, a variety of conventional and unconventional experiments were conducted on JSC-1A to determine its grain-size distribution, cohesion, friction angle, dilatancy angle, tensile strength, and appropriate low strain elastic constants. These experiments were conducted on JSC-1A at a variety of densities prepared through tamping densification to quantify the response of the soil over a range of conditions. To simulate lunar conditions, the samples were prepared at medium to very high relative densities. Grain-size distribution, shear strength, tensile strength, dilatancy angles, and elasticity modulus of the JSC-1A were compared with lunar soil and other simulants.     

Influence of Stress State on Soil-Water Characteristics and Slope Stability

A soil-water characteristic curve defines the relationship between the soil (matric) suction and either the water content or the degree of saturation. Physically, this soil-water characteristic is a measure of the water storage capacity of the soil for a given soil suction. Conventionally, the soil-water characteristic curves (SWCCs) are determined in the laboratory using a pressure plate apparatus in which vertical or confining stress cannot be applied. For investigating the influence on the stress state on the soil-water characteristics, a new stress controllable pressure plate apparatus has been developed. Effects of K0 stress conditions on the SWCCs of an “undisturbed” volcanic soil in Hong Kong are determined and illustrated. The net normal stresses considered in the apparatus are 40 and 80 kPa, which are appropriate for many slope failures in Hong Kong. Experimental results show that the soil-water characteristic of the soil specimens is strongly dependent on the confining stress. Numerical analyses of transient seepage in unsaturated soil slopes using the measured stress-dependent soil-water characteristic curves predict that the distributions of pore-water pressure can be significantly different from those predicted by the analyses using the conventional drying SWCC. For the cut slope and the rainfall considered, the former analyses predicted a considerably lower factor of safety than that by the latter analyses. These results suggest that wetting stress-dependent soil-water characteristic curves should be considered for better and safer assessment of slope instability.     

Role of Matric Suction in Collapse of Compacted Clay Soil

This paper examines the influence of variations in matric suction on the collapse behavior of compacted Bangalore clay soil. The ASTM filter paper method measured the matric suctions of the compacted soil specimens. The matric suction of the compacted clay soil specimens ranged between 50 and 8,000 kPa at the as-compacted Sr values of 90 and 35%. Comparison of the matric suction-gravimetric water content relations of various compacted soils showed that the soil with a higher liquid limit has a higher matric suction at a given gravimetric water content. Variations in as-compacted degrees of saturation at a constant relative compaction or variations in relative compaction at a constant as-compacted degree of saturation notably affected the matric suction of the Bangalore clay soil. Experimental results also showed that the influence of matric suction on the collapse behavior of this compacted clay soil greatly depended on the relative compaction of the specimens and the pressure at which they were inundated.     

Residual Shear Strength for Interfaces between Pipelines and Clays at Low Effective Normal Stresses

The residual shear strength mobilized between pipelines and supporting soils at low effective normal stresses is needed for designing stable pipelines in offshore environments. A tilt table device is used to study the effect that effective normal stress, type of pipeline coating, composition of soil, stress history, and rate of loading have on the drained residual shear strength mobilized at the interface between a variety of clays and polymeric pipe coatings. The drained residual friction angles for both the interfaces and the clays decrease substantially as the effective normal stress increases. Empirical correlations published for predicting the residual strength of clays cannot be readily extrapolated to the pipeline problem because the correlations do not cover the relatively small effective normal stresses acting on pipelines. Residual shear strengths for the interfaces range from 60 to 90% of the residual shear strength for the clay. The residual shear strength for the interface depends both on the composition of the clay and the type of pipeline coating.     

Analysis of a Large Database of GCL-Geomembrane Interface Shear Strength Results

A database of 534 large-scale direct shear test results was assembled in this study to evaluate the interface shear strength between geosynthetic clay liners (GCLs) and geomembranes (GMs). The tests were conducted between 1992 and 2003 by a single independent laboratory using procedures consistent with current testing standards. The number of results in the database allowed quantification of the impact of GCL type, GM type, normal stress, and procedures for specimen hydration and consolidation on the shear strength of GCL-GM interfaces, as well as identification of sources of shear strength variability. The interface shear strength was found to be sensitive to the type of GCL internal reinforcement, GM polymer, and GM texturing, but not to the GM thickness or manufacturer. On average, the GCL internal shear strength was observed to be higher than the GCL-GM interface shear strength when tested using the same procedures. GCLs sheared internally show similar stress-displacement responses and friction angles to GCL-GM interfaces that incorporate a GCL with the same reinforcement type. Hydration under normal stresses below those used during shearing (followed by a consolidation period) led to higher GCL internal shear strength, but lower GCL-GM interface shear strength, than when hydration was conducted under the shearing normal stress. Such different responses are attributed to bentonite extrusion from the GCL into the interface. Good repeatability of test results was obtained using GCL and GM specimens from the same manufacturing lot, while high variability was obtained using specimens from different lots. GCL-GM interface peak shear strength variability was found to increase linearly with normal stress.     

Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas

The apparent cohesion due to soil suction plays an important role in maintaining the stability of steep unsaturated soil slopes with deep ground water table. In this paper, a modified direct shear box is used to determine the relationships between the value of this additional cohesion and the associated soil suction. The apparatus incorporates a miniature tensiometer which allows for the simple and direct measurement of suction during shearing. The soil-water characteristic curves and shearing behavior of intact residual soils, being low-to-medium plasticity silts, as well as silty sand, taken from four landslide-prone areas in Thailand, have been investigated. The relatively low air-entry suctions (0–7 kPa) and bimodality of the soil-water characteristic curves gives an indication of the structured pore size distribution of the materials tested. Samples with higher suction tend to display stronger bonding at particle contacts and thus are more brittle. The shear strength is found to increase nonlinearly with suction, though linearization can be reasonably assumed for suction below around 30 kPa. Prediction of shear strength based on soil-water characteristic curves agrees better with ultimate than peak values. A simple equation is proposed for the minimum ultimate strength that can be expected in an unsaturated residual soil with a suction lower than about 30 kPa.     

Bearing Capacity of Desiccated Tailings

The development of matric suctions in soils contributes to their shear strength, resulting in an enhanced factor of safety against bearing-capacity failure. In this paper, matric suction profiles of desiccated mine tailings are predicted from a steady-state solution for evaporative conditions, and from an isothermal mathematical model that simulates liquid and vapor water flow through soils. The shear-strength envelope with respect to matric suction is established by testing reconstituted tailings samples in a modified triaxial cell, in which matric suction can be controlled. The contribution of matric suction to the shear strength is interpreted as an additional apparent cohesion for use in bearing-capacity calculations. Because of the nonlinearity of the shear-strength profile, a numerical method of analysis is adopted to predict the ultimate bearing capacity of the desiccated tailings. A subsequent decrease in bearing capacity following 2D water infiltration into a partially capped tailings deposit and accompanying suction loss is investigated.     

Effects of Hysteresis on Steady-State Infiltration in Unsaturated Slopes

Hysteresis is a common feature exhibited in hydraulic properties of an unsaturated soil. For a specific matric suction, water content or coefficient of permeability on a wetting curve is always lower than those found on a drying curve. This paper focuses on hysteresis observed in steady-state infiltration tests in a laboratory slope model. The slope model consisted of a 400 mm thick fine sand layer overlying a 200 mm thick gravelly sand layer at a slope angle of 30°. The slope model was subjected to artificial rainfalls of different intensities. The slope model was instrumented to continuously measure the changes in pore-water pressure or matric suction, volumetric water content, and water balance during an experiment. Two experiments with similar applied precipitation intensities were conducted on soils that experienced adsorption and desorption processes. For the adsorption process, the slope model was first subjected to an antecedent steady-state rainfall with an intensity lower than the intensity of the incident steady-state rainfall. In the adsorption process, the water content of the soils increased during the incident rainfall prior to achieving the steady-state condition. For the desorption process, the slope model was first subjected to an antecedent steady-state rainfall with an intensity higher than the intensity of the incident steady-state rainfall. In the desorption process, the water content of the soils actually decreased during the incident rainfall prior to achieving the steady-state condition. The results indicate that the matric suction distributions in soils experiencing the desorption process were higher than those observed in soils experiencing the adsorption process. The matric suctions within the slope during a steady-state infiltration were affected by the initial water content of the soil prior to the infiltration process. Numerical analyses, employing both drying and wetting hydraulic properties of the soils, were performed to study the difference in matric suctions as observed in the experiments. The results suggest that the hysteretic behavior of the soil affects the matric suction distribution within the slope at steady-state conditions. The appropriate hydraulic properties of the soils (i.e., drying or wetting) should be used in accordance with the process that the soils actually experience (i.e., desorption process or adsorption process) even though the slope is under a steady-state rainfall condition.     

Shear Strength in Preexisting Landslides

Drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces. Some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time. Torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kPa or less. The test results also show that the recovered strength even at effective normal stresses of 100 kPa or less is lost after a small shear displacement, i.e., slope movement. An effective normal stress of 100 kPa corresponds to a shallow depth so the observed strength gain has little, if any, impact on the analysis of deep landslides. This paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses.     

Uncertainties of Field Pullout Resistance of Soil Nails

A large number of field pullout tests on soil nails have been carried out to provide valuable information for enhancing the understanding of pullout resistance of soil-grout interface and for reliability evaluation of soil-nailed slopes. In this paper, a data set of 167 field pullout tests performed in 23 nailed completely decomposed granite cut slopes is used for a statistical evaluation of four factors influencing the pullout resistance of soil nails, namely overburden pressure, grout length, soil suction, and soil dilatancy. For the tests in which nails were pulled out, the measured pullout resistance is essentially independent of the effective overburden pressure. A bias factor r* is defined as the ratio of the measured pullout resistance and the calculated value using a design equation. The mean value of r* is 4.30 and the coefficient of variation is 47%. When the uncertainties in grout length, soil suctions around nails, and the soil shear dilatancy are considered, the mean value of r* can finally be reduced to 0.99. The quantification of the uncertainties provides a better physical understanding of the working mechanisms of soil nails.     

Plastic Limit, Liquid Limit and Undrained Shear Strength of Soil—Reappraisal

The concept that plasticity index of soils can be defined as a range of water contents producing a 100-fold variation in undrained shear strength has been experimentally verified with the help of a large number of tests on soils of diverse nature. This has led to the redefinition of the plastic limit as the water content at which undrained shear strength is around 170 kN/m2. Undrained shear strength of a soil at the liquid limit can be considered to be around 1.7 kN/m2. Accordingly, both the liquid limit and the plastic limit have been determined in the present work by a single consistent method, i.e., the Swedish fall cone method. The undrained shear strength-water content relationship has been found to be log-linear for a wide range of water contents beginning from lower than the plastic limit to higher than the liquid limit. This resulted in the formulation of an expression for predicting undrained shear strength of a remolded soil at any water content based solely on its liquid limit and plastic limit.