Residual shear strength of unsaturated soils via suction-controlled ring shear testing

• Results from suction-controlled ring shear tests on silty soils are presented.
• Increase in residual shear strength with suction was linear in silty sand.
• Increase in residual shear strength with suction was nonlinear in silty clayey sand.

Results from a comprehensive series of suction-controlled ring shear tests, conducted on statically compacted specimens of silty clayey sand and silty sand, are presented. The experiments were accomplished in a newly developed servo/suction-controlled ring shear apparatus suitable for testing unsaturated soils under large deformations and suction-controlled conditions via the axis-translation technique. The present work focuses primarily on two crucial aspects of compacted unsaturated soil behavior, namely, the behavior of silty clayey sand under suction-controlled ring shear testing, and the effects of pre-shearing and suction histories on unsaturated residual shear strength of compacted silty sand. Test results corroborate the important role played by matric suction on residual shear strength properties of unsaturated soils. For the range of net normal stresses and suction states investigated, the increase in residual shear strength with increasing suction was confirmed to be a linear trend for silty sand, but significantly nonlinear for silty clayey sand. Results from multi-stage ring shear tests confirmed that the residual shear strength of unsaturated soils is virtually independent of the pre-shearing and suction histories experienced by the soil.