Volume 34, Issue 5 (September 2011)
A Servo/Suction-Controlled Ring Shear Apparatus for Unsaturated Soils: Development, Performance, and Preliminary Results
In recent years, the key hypotheses of most constitutive frameworks postulated for unsaturated soils have been validated via suction-controlled oedometer, triaxial, and direct shear testing. These techniques only allow for the application of loads along limited modes and levels of soil deformation. It is well known today that a vast majority of geotechnical infrastructure made of compacted soil, or 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 good estimations of shear strength, from peak to residual. To date, however, there is limited experimental evidence of unsaturated soil behavior under large deformations while being subjected to controlled-suction states. This paper introduces a fully servo/suction-controlled ring shear apparatus that has been made suitable for testing unsaturated soils at large deformations via axis-translation technique. The paper outlines the full development of the apparatus, including details of its main components, assembling, and performance verification against the original Bromhead device. A preliminary series of suction-controlled ring shear tests was accomplished on several statically compacted samples of silty sand. Residual friction angle with respect to matric suction, i.e., residual beta angle, was found to remain virtually constant, regardless of the applied level of net normal stress. The results suggest that a conceptual residual shear strength framework for unsaturated soils, similar to that postulated for peak shear strength, could eventually be devised as more experimental evidence of this kind is made available.