Suction-controlled Triaxial Tests Research Articles

Critical state behaviour of an unsaturated kaolin mixture

In this paper, a series of suction-controlled triaxial tests were conducted to investigate the hydromechanical behaviour of an unsaturated kaolin mixture. The studied variables included the mean net stress, suction, and initial dry density. The suction was controlled over a wide range – in the span of 0–390 MPa using the axis translation method and the vapour equilibrium technique, aiming to holistically study the mechanical response of unsaturated soils. The discussion focused on the critical state behaviour and dilatancy characteristics of the tested soil. The experimental results indicate that the ductility of the soil decreases and shifts towards brittleness with increasing suctions. The critical state stress ratio (Ma) of the soil exhibits a significant increase with suction, and this increase becomes more pronounced with higher initial dry density. Conversely, the impact of suction on the critical state line in the specific volume-mean net stress plane (v-lnpnet plane) diminishes with higher initial dry density. Furthermore, suction notably influences the stress-dilatancy relationship of kaolin mixtures, while the effect of confining pressure appears to be marginal. The influence of suction on the stress-dilatancy relationship primarily stems from its impact on the critical stress ratio (Ma), which can be normalized by dividing the stress ratio by the critical state stress ratio. Lastly, the stress-dilatancy equation of the modified Cam-clay model is extended to capture the effect of suction on the dilatancy characteristics of unsaturated soils.

Constitutive and experimental study of unsaturated fine-grained soils over a wide suction range

The instabilities exhibited by unsaturated fine-grained soil, stemming from coupled hydro-mechanical phenomena, represent crucial engineering characteristics essential for geotechnical designs. This study introduces a constitutive model for unsaturated fine-grained soils, incorporating the framework of sub-loading surface plasticity. The model integrates Bishop’s effective stress and capillary degree of saturation as basic variables, while also addressing the distinct influences of soil volume change on capillarity and adsorption mechanisms. Validation of the proposed model is conducted using a diverse set of experimental data derived from suction-controlled triaxial compression tests conducted under low and high suction conditions. The results demonstrate that the new constitutive model can achieve a reasonable agreement with the observed data.

Suction and anisotropy effects on the stiffness characteristics of a compacted lateritic clay from small to large strains

Lateritic clay has distinct properties from other clays due to its high sesquioxide content. Its stiffness characteristics have not been well understood, especially when the soil is unsaturated and anisotropic. This study investigated the stiffness characteristics of compacted lateritic clay through suction-controlled triaxial compression tests equipped with local strain measurements. Both vertically and horizontally cut specimens were tested to determine the evolution of stiffness anisotropy during shearing. Three suctions (0, 10 and 150 kPa) and two confining pressures (50 and 200 kPa) were considered. When strains are relatively small (e.g., less than 0.2%), the secant Young’s modulus E<sub>sec</sub> of vertical specimens is consistently higher than that of horizontal specimens at all suctions and stresses due to the inherent anisotropic structure. The degree of anisotropy increases with increasing suction since suction enhances the stiffness significantly more in vertical specimens than in horizontal specimens. This behaviour may be due to an enhanced force chain in the vertical direction during shearing. As strains increase, the degradation of E<sub>sec</sub> normalized by the maximum Young’s modulus E<sub>0</sub> is almost independent of suction and anisotropy. Lateritic clay has a higher degradation rate than other clays with a similar plasticity index because of its aggregated microstructure. 

Accounting for the microstructure for the prediction of unsaturated shear strength of remolded fine-grained soils

Predictions of the unsaturated shear strength with generalized effective stress-based approaches disregard the non-uniform microstructure of remolded fine-grained soils. The study aims at investigating the adequacy of microstructurally-based effective stress to predict the unsaturated shear strength of remolded fine-grained soils over a wide range of suctions. For that purpose, shear strength data are acquired on a silty clay soil compacted at two different dry densities through suction-controlled triaxial tests and unconsolidated triaxial tests at constant water content. The microstructure of the soil at the as-compacted state is determined with mercury intrusion porosimetry and is directly incorporated in different expressions of microstructurally-based effective stress available in the literature. The experimental data suggest that compared to the generalized effective stress, microstructurally-based effective stress expressions provide better predictions of the unsaturated shear strength, especially at high suctions. Also, the use of microstructurally-based effective stress is particularly relevant for remolded fine-grained soils compacted at high dry densities, i.e., with a low proportion of macropores.

Investigating the hydro-mechanical behaviour of unsaturated sand-bentonite mixtures using suction-controlled triaxial test

Soil improvement is often done through mixing the granular soils with fine-grained materials to obtain densified soil matrix and eventually increase the bearing capacity of the soil. In this paper, sand was mixed with three different quantities of bentonite. Then, the hydromechanical behaviour of sand-bentonite mixtures was evaluated at different matric suctions. Suction-controlled triaxial test and the pressure plate extractor were used to obtain the experimental data needed to derive the stress-strain curves and the Soil Water Characteristics Curve (SWCC), respectively. The results showed that the apparent cohesion that developed due to the existence of matric suction was increasing with the bentonite content increase. Residual suction value was increasing as the bentonite content increasing. It was concluded that the increase of water retention capacity has made the sand-bentonite samples responds very significantly with changing of matric suction. Therefore, addition of fine materials to granular soils would make the soil prone to wetting collapse upon reaching the saturation condition during the rainy seasons.

Behavior of unsaturated cohesive-frictional soils over a whole range of suction/thermo-controlled stress paths and modes of deformation

The present work documents some of the most recent experimental evidence of the thermohydro-mechanical behavior of compacted soils over a whole range of suction- and/or thermo-controlled stress paths and modes of deformation, including data from a series of triaxial, true triaxial, plane strain, ring shear, and resonant column tests conducted on different types of cohesive-frictional soils in the low-to-medium matric suction range under either room temperature or thermally controlled conditions. The work has been accomplished at the Advanced Geomechanics Laboratory of the University of Texas at Arlington, focusing primarily on the following essential features of unsaturated soil behavior: (1) Loading-collapse and apparent tensile strength loci assessed from suction-controlled triaxial and true triaxial testing on clayey sand, (2) Critical state lines from suction-controlled plane strain testing on silty soil, (3) Peak and residual failure envelopes from suction-controlled ring shear testing on clayey soil, (4) Frequency response curves and cyclic stress-strain hysteretic loops from thermo-controlled, constant-water content resonant column testing on clayey soil, and (5) Residual failure envelopes from suction/thermo-controlled ring shear testing on clayey soil. The work is intended to serve as a succinct yet reasonably thorough state-of-the-art paper contribution to PanAm-UNSAT 2021: Third Pan-American Conference on Unsaturated Soils, Rio de Janeiro, Brazil, July 21-25, 2021.

The relationship between the shear strength and water retention curve of unsaturated sand at different hydraulic phases

This technical paper focuses on evaluating the shear strength of unsaturated sand at drying, wetting and alternate drying–wetting cycles, as well as the correlation between the strength and the water retention curve (WRC) at different hydraulic phases. Results of stress–strain curves and WRCs were obtained from conducting numerous tests using suction-controlled triaxial test. Although the maximum shear strength was obtained from the specimens under drying conditions, the difference between the drying and wetting shear strength was relatively small for the tested soil. Based on analysis of the test results, a multi-surfaces envelope model was proposed to fit the experimental shear strength data under drying conditions and to predict the shear strength under wetting conditions (or any subsequent drying–wetting cycle) based on the drying fitting parameters. The fitting and prediction performance of the proposed equations were checked using experimental data from previous studies, and very good agreement was reported. Moreover, the WRC was found to be capable of not only estimating unsaturated soil property functions but also anticipating the soil shear strength behaviour under different hydraulic phases.

Test Apparatus for Rapid Determination of Soil-Water Retention Curve under Isotropic Loading Condition

Abstract In the present study, a custom-designed test apparatus comprising a plexiglass cell chamber with a base pedestal to accommodate a cylindrical soil sample with the water potential and volumetric water content sensors was developed to measure the soil-water retention curve (SWRC) of unsaturated soils under isotropic loading condition. The developed test apparatus is capable of rapidly measuring isotropic stress-dependent SWRC utilizing preequilibrated unsaturated soil samples. The methodologies for obtaining normalized volumetric and gravimetric stress-dependent SWRC using continuous and spot measurements were discussed in detail. In addition, the need for the soil-specific sensor calibration was illustrated, and an alternate method of developing calibration curves for sensors was discussed. To understand the efficacy of the developed apparatus, stress-dependent SWRC for two different statically compacted low-plasticity clays at isotropic confining pressures of 10, 100, and 200 kPa were evaluated. The stress-dependent SWRC obtained from the developed test apparatus is in good agreement with the results obtained from the suction-controlled triaxial test apparatus, which proves the ability of the developed apparatus to replicate stress-dependent SWRC without involving the axis translation technique. In addition, the apparatus can also be used to obtain the void ratio of unsaturated soil under various isotropic confinements at constant water content. Hence, the developed test apparatus has the potential to simulate both water content and void ratio surfaces with respect to suction under rapid isotropic loading condition, which can be used for constitutive modeling in unsaturated soils.

Influence of degree of saturation in the dynamic stress transmission to a deep foundation

The structural design of deep foundations depends on both the applied loads and the soil that will support them. However, during an earthquake this process reverses, and the seismic stresses are transmitted towards the structure through the soil. Proper design of deep foundations must account for the lateral loads imposed on the foundations by the dynamic loading. In order to assess the influence of soil saturation in the transmission of a dynamic load to a foundation, a dynamic lateral load pile design was performed using Reese’s p-y curve method. A series of suction-controlled dynamic triaxial tests were performed to obtain the Modulus of subgrade reaction at different matric suctions and seismic coefficients were back-calculated to perform structural designs. In general terms, it was observed that contemplating a saturated soil in the dynamic lateral load pile design does not represent the critical load case for seismic analysis.

Loading rate effect on mechanical properties of an unsaturated silty sand

This paper presents the results of an experimental study on the effects of testing rate on stress-strain behavior and volumetric changes of soil. A series of suction-controlled triaxial tests has been performed on reconstituted specimens of a silty sand (SM), at different stress-rates and strain-rates, respectively. The stress-strain paths were applied by using a modified version of a Bishop and Wesley device (USPv2), capable of applying independently pore-water and air pressure at both ends of the soil sample. During the isotropic compression stages loading rates of 2 and 32 kPa/h have been applied under constant suction values of 15 and 45 kPa. The drained deviator stages were conducted at the same suction levels under strain rates of 0.25 and 2.50 %/h. Results are presented in terms of applied loading rates as a function of the specimens specific volume, preconsolidation pressure, soil compressibility and deviatoric stress against strain rate. A comparison of results was made to a former study, under similar testing conditions of suction and loading rates at University of Napoli Federico II. The effect of loading rate on the soil behavior seems to have an insignificant effect on the specific volume variations, for the imposed values during the testing campaign.

Compression and wetting induced volumetric behavior of loess: Macro- and micro-investigations

Abstract Volumetric behavior of intact and compacted loess under isotropic compression and wetting was investigated through stress- and suction-controlled triaxial tests. To assist in the interpretation of test results, X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and Mercury Intrusion Porosimeter (MIP) tests were carried out to determine the mineral composition and the microstructure of the tested loess. Results of compression tests showed that yield stresses of intact loess were larger than that of the compacted loess because of stabilization effects contributed by clay particles and calcite in intact loess, as evidenced by SEM and XRD results. On the other hand, constant water content was observed for intact loess during compression at suctions 50 kPa and 100 kPa, while the water content of compacted loess increased by 4.4% and 7.4% respectively. MIP tests revealed that compression of the intact loess mainly occurred in the extra-large pores, but with minuscule effects on the pores governing water distribution at suctions 50 kPa and 100 kPa. For compacted loess, a number of inter-aggregate pores were compressed below the pore diameters responsible for water flowing in the pores at the applied suctions. In addition, intact loess showed wetting induced swelling at an isotropic stress of 50 kPa, while collapse was observed for compacted loess with the same stress path.

Impact of Drying/Wetting on Shear Stiffness and Shear-Induced Volume Change Behaviours of Unsaturated Silty Clay

To further improve the understanding of the impact of drying/wetting on mechanical behaviours of unsaturated soils, this paper presented experimental results of suction-controlled triaxial tests on a slurry-consolidated silty clay with different drying/wetting cycles, matric suctions and net confining pressures. Test results show that, even if the same net confining pressure and matric suction were followed during triaxial shearing, the specimens that experienced six drying/wetting cycles (D–W-6) produced higher shear stiffness in the early stage of shearing and lower volumetric strain compared to those of specimens that have not been subjected to any drying/wetting history (D–W-0) at low matric suction condition. The stress–strain relationship of soils tends to alter from strain-hardening to strain-softening after drying/wetting. This is because the maximum matric suction experienced in drying/wetting process is similar to the pre-consolidation pressure, and thus the specimens became overconsolidated. However, D–W-6 shows a similar stress–strain relationship with D–W-0 under matric suction of 200 kPa due to the shear stiffness arisen from the irreversible volume compression (overconsolidation), which is counterbalanced by that contributed by its lower degree of saturation. The impact of drying/wetting on mechanical behaviours of unsaturated soils is always affected simultaneously by irreversible volume compression and lower degree of saturation. The overall influence of drying/wetting depends on the proportional effects of the two factors.

Modelling Unsaturated Soil Response Beyond Residual Suction State via Vapor-Pressure Controlled Triaxial Testing

Most of the previous research has been focused on developing and validating constitutive models to predict response of unsaturated soils in low-medium suction range. However, there is a scarcity of efforts in developing soil models to simulate its mechanical response in high suction range, particularly above the residual suction. This article presents a new constitutive model introducing net stress and suction as two independent variables. Furthermore, non-associative flow rule incorporating modified stress-dilatancy relationship to take unsaturated state into account is introduced to improve the model results in low-medium to high suction range. The essential soil model parameters are calibrated using suction-controlled triaxial test results for predictions of compacted silty sand response at high values of total suction above residual suction. Preliminary simulations show that proposed model can reasonably simulate the post-peak strain softening response obtained from suction-controlled CTC tests above residual suction value with reasonable accuracy. Although, the proposed model captures initial compression followed by dilation volumetric response with reasonable accuracy, it needs some improvements to be able to capture volumetric response accurately over entire suction range.

Hydro-mechanical constitutive model for overconsolidated unsaturated soils

Based on the concept of subloading plasticity, a constitutive framework, using mean skeleton stress and effective degree of saturation as fundamental variables, is proposed for capturing coupled hydro-mechanical behaviours for overconsolidated unsaturated soils. This framework can be reduced to the Modified Cam-Clay model when suction becomes zero and the initial overconsolidation ratio (OCR) is equal to 1.0. Compared with the models in literature, the main advantage of the proposed model that it is capable of modelling the hydro-mechanical interaction behaviours of overconsolidated unsaturated soils, such as strain-hardening coupled with shear contraction for normally consolidated and lightly overconsolidated unsaturated soils, strain-softening coupled with shear dilatancy for heavily overconsolidated unsaturated soils, and the effect of OCR on the shear-induced variation in the degree of saturation. Meanwhile, the dependence of mechanical behaviours of unsaturated soils on degree of saturation can be adequately described. The performance of the presented model is illustrated through numerical examples with different initial degrees of saturation and overconsolidation ratios. Finally, the proposed model is validated against a variety of experimental data from suction-controlled triaxial compression tests.

Modeling Stress–Dilatancy Behavior of Compacted Silty Sand Under Suction-Controlled Axisymmetric Shearing

A thorough stress–dilatancy analysis of statically compacted silty sand is presented based on results from a comprehensive suction-controlled (drained) triaxial test program. Induced soil suction values, via axis-translation and relative humidity techniques, ranged between 0.05 to 300 MPa, with net confining pressures varying from 100 to 300 kPa. A first companion paper documented the strength, stiffness, and stress-induced anisotropy of the same soil under similar test conditions and variables. This paper focuses on the dilatancy rate, stress–strain and volume change responses, shear band formations, and peak as well as critical strength characteristics of isotropically consolidated silty sand under suction-controlled axisymmetric shearing. Augmentation in the peak stress ratio is found to be heavily dependent on the level of matric or total suction imposed, the maximum induced dilatancy, and the critical stress ratio. Experimental observations are also used to substantiate, or further refine, existing stress–dilatancy theories, which has led to the postulation of a simplified new formulation for predicting peak friction angle over the entire test suction range applied.

Strength, Stiffness and Radial Anisotropy of Compacted Silty Sand Under Suction-Controlled Axisymmetric Shearing

A thorough analysis of strength, stiffness, and stress-induced anisotropy of statically compacted silty sand is presented based on results from a comprehensive suction-controlled (drained) triaxial test program. Induced soil suction values, via either axis-translation or relative humidity techniques, ranged between 0.05 to 300 MPa, with net confining pressures varying from 100 to 300 kPa. A relatively simple model/equation for peak friction angle (ϕpeak), over the entire test suction and net confinement range, is presented. As expected, peak strength response to suction-controlled monotonic shearing is influenced by the level of suction and net confining pressure, as well as the amount of dilatancy experienced by the silty sand. The nonlinear nature of the variation of deviator stress at peak and critical state conditions, with degree of saturation, was fitted reasonably well via fourth order and second order polynomial equations, respectively. A thorough analysis and modeling of stress-dilatancy behavior is presented in a companion paper. Finally, the inherent-anisotropy observed in the original soil fabric, as well as the stress induced-anisotropy observed in the sheared samples, are discussed based on results from suction-controlled isotropic consolidation and axisymmetric shearing, respectively; including the effect of suction on the rate of degradation of stiffness (moduli) of compacted silty sand under monotonic shearing.

Impact of hydraulic hysteresis on the small strain shear modulus of unsaturated sand

The results of previous studies on silt and clay indicated that variations in the small strain shear modulus, Gmax, during hydraulic hysteresis had a non-linear increasing trend with matric suction, with greater values upon wetting. However, due to differences in material properties and inter-particle forces, a different behavior is expected for the Gmax of unsaturated sand. Although considerable research has been devoted in recent years to characterizing the behavior of the Gmax of sand during drying, less attention has been paid to the effect of hydraulic hysteresis on Gmax and its variations during wetting. In the study presented herein, an effective stress-based semi-empirical model was developed to predict the variations in the Gmax of unsaturated sand during hydraulic hysteresis. The proposed model incorporated the impact of the possible changes in volume through an empirical void ratio function as well as the effect of the degree of saturation through the use of suction stress. The effective stress was also defined using the concept of suction stress. The efficiency of the proposed model was evaluated by comparing the model predictions with the results of an experimental testing program involving the measurement of the Gmax of sand with different grain size distributions during hydraulic hysteresis. Specifically, a suction-controlled triaxial testing device, equipped with a pair of bender elements, was used to define the hysteretic trends in Gmax for different values of mean net stress. The model was found to provide satisfactory predictions of the trends in Gmax with matric suction, as well as its peak value and the suction corresponding to the occurrence of the peak Gmax. It also provided satisfactory predictions of the variations in Gmax upon subsequent wetting.

Investigation of the mechanical behaviour of an unsaturated soil mixture using a digital image measurement system

A triaxial apparatus that includes a digital image measurement system (DIMS) is used to investigate the mechanical behaviour of a compacted soil mixture of tailing sand and speswhite kaolin. The experimental programme involves a series of soil–water retention curve (SWRC) tests, isotropic compression tests and suction-controlled triaxial tests. A non-contact method for measuring the sample volume is developed based on the strain field obtained from the DIMS. Particular attention is given to the volumetric behaviour of the soil, and three distinct stages are observed during the consolidation of the suction-controlled triaxial tests. Then a void ratio-dependent shear strength model is proposed, incorporating the influence of the change in void ratio on the SWRC pattern. The model is validated against the experimental data from the tests, showing that it can provide a more accurate prediction of shear strength than that of conventional predictions using a unique SWRC. Moreover, the proposed model is capable of representing several important aspects of the shear behaviour of unsaturated soils, such as the dependence of suction-induced strength on net normal stress and density effect on the shear strength.

Modeling critical-state shear strength behavior of compacted silty sand via suction-controlled triaxial testing

Most of the recently postulated unsaturated shear strength models have been calibrated only for a short variety of soils. In addition, these models are yet to be extended and calibrated over a wider range of matric and total suction states. The present work focuses on further refinements of previously proposed shear strength equations in light of newly obtained experimental evidence of shear strength behavior of compacted silty sand at a critical state from suction-controlled triaxial tests conducted between 0.05MPa to 300MPa suction range. A refined and rather simple equation comprising two independent functions, is introduced and validated, including a thorough parametric investigation, to predict the unsaturated shear strength of compacted silty sand at a critical state for a wide range of matric and total suction states. The experimental program included a total of 21 consolidated drained (CD) triaxial tests conducted on statically-compacted specimens of silty sand under strain- and suction-controlled conditions. Experimental results show that the angle of internal friction (ϕ′) remained virtually constant over the entire range of induced suction states; however, the shear strength increased while the angle of internal friction with respect to suction (ϕb) decreased with increasing suction, with both varying non-linearly. Finally, a gradual increase in brittleness of the test soil at peak-failure condition, as well as an increasingly marked strain-softening post-failure, was observed with increasing suction.

Small strain shear moduli of unsaturated natural and compacted loess

Suction and microstructure play critical roles in the mechanical behaviour of loess, yet their effects on small strain (from 0·001 to 1%) shear moduli are not fully understood. In this study, shear moduli of loess with different microstructures and at different suctions are investigated and reported. Suction-controlled triaxial tests were carried out on two compacted and two intact loess specimens. Testing procedures include isotropic compression, wetting and shearing at constant mean net stress and constant suction. In addition, the soil microstructure was studied through scanning electron microscope images. At a given suction, the intact specimens have larger shear moduli than the compacted specimens at deviatoric strain below 0·025%, mainly because more clay aggregates accumulate at grain contacts in the intact loess. As suction decreases (i.e. wetting), contrary to the typical behaviour of compacted soils, the small strain shear moduli increase in the compacted loess. The increase is most probably because of significant wetting-induced contraction during the wetting process. In contrast, the shear moduli of the intact specimens remain unchanged, owing to slight swelling observed during wetting. The behaviour of the intact loess is likely to be dominated by the soil microstructure, and suction effects are relatively minor. With continued shearing, the differences resulting from the specific microstructures and wetting-induced volume changes (i.e. suction) diminished rapidly.