Residual Strength of Unsaturated Soils Using a Fully Servo/Suction-Controlled Ring Shear Apparatus

Abstract

Results from a series of suction-controlled ring shear tests conducted on statically compacted samples of silty clayey sand and silty sand are presented. The tests were accomplished in a newly developed servo/suction-controlled ring shear apparatus that is suitable for testing unsaturated soils under large deformations via axis-translation technique. A thorough performance verification of the apparatus has been previously carried out against a conventional Bromhead device. Results from suction-controlled ring shear tests performed in this work reflect the important role played by matric suction on residual shear strength properties of unsaturated soils. Assessment of these properties at relatively low net normal stresses and suction states, as reflected by the range of experimental variables considered in this work, is of critical importance in slope stability analyses that involve potentially shallow failures triggered by rainfall at relatively high degrees of saturation. For the given range of net normal stresses and suction states investigated, the increase in residual shear strength with increasing net normal stress was found to be virtually linear for both soil materials, regardless of the induced matric suction state.