Authors: Reza Ahmadi Naghadeh; Nabi Kartal Toker
Response of unsaturated soils constitutes an important consideration for many problems in geotechnical engineering. A comprehensive laboratory-based study has been carried out to investigate the effects of matric suction on shear strength of compacted specimens; using a triaxial setup modified with axis translation for suction control, cell fluid volume measurement for monitoring specimen volume changes, and diffused air volume indicator for tracking the water content (Ahmadi-Naghadeh 2016). The results of a test programme on a non-plastic silt show that the shear strength of the studied soil increases nonlinearly as suction increases. The response of the specimens change from strain-hardening in saturated condition to strain-softening as suction increases. For the same net confining stress, specimens with higher suction values reach a peak deviator stress at lower axial strain. In general, total volumetric deformation behavior becomes more dilative as the suction increases.
The laboratory test results were also interpreted under elasto-plastic framework for unsaturated soils and following conclusions were drawn:
The critical state lines were generated through the results of the constant suction triaxial tests. The gradient of the critical state line, M, was determined for each test series. Consistent with Barcelona Basic Model (Alonso et al., 1990), the gradient of the critical state lines was found to be a constant value and independent of the suction (Fig.1).
The intercept of critical state line, µ(s), was determined for each set of constant suction triaxial tests. Consistent with Wheeler and Sivakumar (1995), a nonlinear relationship between critical state line intercept µ(s) and matric suction were observed (Fig.2).
Yield points were determined from saturated and unsaturated triaxial test results. The estimated yield values were employed to generate yield locus in deviator stress – mean stress plane for each suction value. No elliptical yield curve whose axes are parallel to the axes of stress was found to satisfactorily fit each series of constant suction yield points. This is postulated to agree with near-elliptical yield curves that have inclined axes (Fig.3), as suggested by (Cui and Delage 1996). The inclination of the yield locus can be attributed to inherent anisotropic nature of the one-dimensionally compacted specimens. It was found that, the size of the yield locus is suction dependent and expands as the suction increases. This observation is consistent with hardening effect of the suction, as included in Barcelona Basic Model by Alonso et al. (1990).
The volumetric and deviatoric plastic strain increments were calculated to examine the flow rule. It was found that, for compacted specimens the associated flow rule (normality) was not satisfied (i.e. the plastic strain increment vectors were not perpendicular to yield locus).
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