A triaxial cell was developed to investigate the shear strength of unsaturated silt under elevated temperatures and high suction magnitudes. The results from a series of drained triaxial compression tests on compacted silt specimens are presented in this paper. After anisotropic compression, some specimens were heated before suction was applied, while others were heated after the application of suction. The shear stress-strain curves of the soils under high suction magnitudes showed a brittle failure mechanism, with a clear increase in peak shear strength with net confining stress. Heating after suction application led to a greater peak shear strength than reference tests at ambient temperature, whereas heating before suction application led to a lower peak shear strength. Despite the observed path effects, a single peak failure envelope was defined when evaluating the data in terms of effective stress. The suction stress concept was used to define the effective stress, and the values of suction stress were found to be linked with a non-isothermal definition of the soil-water retention curve.

This paper focuses on the results from a new triaxial cell developed to measure the shear strength of unsaturated soils under elevated temperatures and high suction magnitudes. Suction control is implemented by circulating vapor through a soil specimen having an initially low degree of saturation while the temperature of the soil specimen is controlled using heating elements within the cell fluid. A mechanical load frame was also modified to be capable of performing both load and constant displacement control triaxial tests. Volume changes were assessed using axial displacements and by tracking changes in the cell fluid volume. In addition to presenting the details of the new cell, the results from a set of drained triaxial tests performed on compacted silt specimens under different combinations of total suction and net normal stress at both ambient and elevated temperatures are presented.

This paper presents an evaluation of the effective stress concept in unsaturated, compacted silt at low degrees of saturation. A set of isothermal, consolidated-drained triaxial tests was performed on silt specimens under different combinations of total suction and net normal stress. The total suction was controlled using an automated humidity system, and variables monitored during equilibration and shearing include anisotropic volume change, axial displacement, temperature, relative humidity at the top and bottom of the specimen, and the gas pressure difference across the specimen. The results from the triaxial tests were analyzed to examine the applicability of predicting the suction stress characteristic curve (SSCC) using parameters from soil water retention curve (SWRC) models itted to experimental data obtained at low suction magnitudes. The SSCC predicted from the SWRC at low suctions was found to overpredict the suction stress values at high suctions obtained from back-extrapolation of the failure envelope to deine the tensile strength. However, small adjustments in the itting of the SWRC were found to provide a better it between the SSCC and experimental suction stress data. The suction stress deined using the adjusted SWRC was found to provide a good interpretation of the critical state line in terms of mean effective stress over both high and low suction ranges.Abbreviations: CD, consolidated drained; CSL, critical state line; LVDT, linearly variable differential transformer; SSCC, suction stress characteristic curve; SWRC, soil water retention curve.Many of the previous studies on the principle of efective stress in unsaturated soils used the axis translation technique to characterize the shear strength or volume change of soil specimens having relatively high degrees of saturation (Khalili et al., 2004;Lu and Likos, 2006;Khalili and Zargarbashi, 2010;Alonso and Romero, 2011). At these relatively high degrees of saturation, typically greater than 0.5, the water phase is likely connected through the specimen. Khabbaz (1998), Lu andLikos (2006), Kayadelen et al. (2007), andLu et al. (2010) have shown experimentally and analytically that the soil water retention curve (SWRC) and the efective stress are functionally related, and these relationships are assumed to hold over the whole range of saturation. Lu et al. (2010) proved that the van Genuchten (1980) SWRC model can be used to deine the efective stress in both unsaturated sands and clays and found analytically that the efective stress for clays should monotonically increase with increasing suction for the case of constant net normal stress.Despite the strong evidence supporting a relationship between the SWRC and efective stress, this relationship has not been fully veriied at high suction magnitudes (or low degrees of saturation) where the water phase is not continuous. his is a critical gap in the literature because it is common practice to extrapolate the shape of the SWRC itted to data at high degrees of saturation to the entire range of degrees...

The soil water–retention curve (SWRC) is commonly theorized and measured in the positive matric suction or negative pore water pressure domain. However, all soils do not reach full saturation when matric suction decreases to zero. To date, few methods or theories have been developed to understand the SWRC in the negative matric suction domain, which could play important roles in field mechanical stability conditions, such as slopes under heavy rainfall or levees under rapidly rising water table conditions. A method employing both the transient water release and imbibition method (TRIM) and a constant flow method (CFM) is devised to measure a soil’s complete loop of the SWRC under both wetting and drying, and positive and negative matric suction conditions. Although the TRIM is used to measure both drying and wetting paths of the SWRC in the positive matric suction domain, the CFM is used to quantify the soil water–retention behavior in the negative matric suction domain. The TRIM method has been previously validated and extensively tested. The novel feature is the cyclic application of the CFM in the negative matric suction domain. The head loss in the high air entry ceramic stone because of the application of the CFM is calibrated in the range of the applied flow rates. Similar sets of flow rates are used to validate the repeatability of the measured SWRC behavior in the negative matric suction domain. Three different soils, sandy, silty, and clayey soils, are used to demonstrate the applicability of the methodology for various soil types. It is shown that, for the sandy soil, a few kPa of negative matric suction are needed to fully saturate the specimen, whereas for the clayey soil, over 10 kPa of negative matric suction are needed to fully saturate the specimen.

This study presents an evaluation of yielding mechanisms for unsaturated, compacted silt using drained triaxial compression tests with control of elevated temperatures and high suction magnitudes. After anisotropic compression, some compacted silt specimens were heated by approximately 40 °C before a suction of approximately 300 MPa was applied, while others were heated after suction application. A frictional response was observed for the specimens sheared under high suction magnitudes, in the form of a consistent increase in peak shear strength with increasing net confining stress. An effective stress analysis was used to evaluate the trends in the peak shear stress and the role of stress history for the different specimens. A single peak failure envelope was observed when the shear strength data was interpreted in terms of the mean effective stress. Changes in preconsolidation stress were estimated by identifying the intersections between a thermo-elasto-plastic yield function and the experimental peak shear strength values. Soil specimens heated before application of high suction values had lower peak shear strengths than reference specimens at high suction and ambient temperature. This behaviour is consistent with thermal softening trends observed in soils heated under low suction values. However, soil specimens heated after suction application had greater peak shear strengths than the reference specimens. This indicates heating under high suction results in hardening. The impact of suction on the preconsolidation stress was found to be better represented by a power law model at high suction magnitudes than other available models. The estimated preconsolidation stress values were used to evaluate the impacts of stress history on the thermal volume change response, which matched well with data from tests on saturated specimens. Response to Reviewers: Comment 1 : The paper presents interpretation of results from suction and temperature controlled triaxial testing of unsaturated silt. While the experimental results have been published previously by the authors, the interpretation here is original and valuable. The paper is very well written and presented. Background information is covered nicely. Recommendation is to publish. Authors should consider the following comments in a Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation revision. Reponse 1: We thank the reviewer for the thorough evaluation of the paper. We have carefully addressed the reviewer's comments, and we believe that they have improved the presentation of the paper. We have also carefully reviewed the entire manuscript to check for any editorial issues.