Unsaturated Soil Behavior Under Large Deformations Using A Fully Servo/suction-controlled Ring Shear Apparatus

In recent years, the key hypotheses of most elasto-plastic constitutive frameworks postulated for unsaturated soils have been validated via suction-controlled oedometer, triaxial, and direct shear testing. These techniques, however, allow for the application of loads along limited modes and levels of soil deformation. Today, it is well known that most geotechnical infrastructure resting on unsaturated ground involves a wide range of deformations. Calculation of foundation settlement, for instance, requires a good estimation of soil stiffness at relatively small strains. Analysis of slopes, embankments, and soil bearing capacity, on the other hand, requires an adequate estimation of shear strength, from peak to residual. To date, however, there is very limited experimental evidence of unsaturated soil response under mid-to-large strain levels under controlled-suction states. This paper introduces a novel suction-controlled ring shear apparatus suitable for modeling unsaturated soil response over a whole range of deformations using the axis-translation technique. The apparatus features a 113 N-m torque motor along with a 360-degree-range angular deformation sensor. The lower annular shear platen accommodates a 15.24-cm OD x 9.65-cm ID x 1.5-cm thickness specimen and also features custom-made grooves to house a full set of six 5-bar or 15-bar ceramics as well as diffused-air flushing ports. A series of suction-controlled ring shear tests was accomplished on several identically prepared, ring-shaped specimens of compacted silty sand (SM) and silty clayey sand (SC-SM). Matric suction was found to exert a critical influence on soil's peak and residual strength responses under constant-suction ring shear testing. Residual shear strength parameters, residual friction angle and residual intercept cohesion, showed a significant increase with applied suction. A comparison of results with literature allowed determining than high suction has a significant influence on the residual strength and residual friction angle of plastic material, but any effect on the cohesion intercept, which is predominantly close to zero.