Mechanical behavior of unsaturated soils from suction controlled ring shear tests.

The shear strength behavior associated with a large shear deformation of both the fine- and coarse-grained unsaturated soils is important in interpreting and forecasting the initiation and movement of landslides. For this reason, a suction-controlled ring shear apparatus was designed by introducing modifications to the conventional Bromhead ring apparatus extending the axis translation technique. A series of suction-controlled ring shear tests were performed on fine- and coarse-grained unsaturated soil specimens subjecting to large shear deformation. The experimental results are presented and interpreted for highlighting: (i) the shear stress/void ratio-shear displacement relationships; (ii) the envelopes of the residual shear strength; (iii) the void ratio, water ratio and degree of saturation of the fine-grained soil specimens sheared to the residual state under different net normal stresses and matric suctions. These results provide valuable information toward understanding and interpreting the behaviors of landslides in unsaturated soils that experience the first failure and the reactivation along a pre-sheared slip surface. • Results of suction-controlled ring shear tests on unsaturated soils are presented. • Unsaturated clay softening in first and subsequent failures is discussed. • Unsaturated sand softens in first failure and hardens in subsequent failures. • Residual shear strength of unsaturated soils are interpreted. • Residual void ratio and degree of saturation of unsaturated soils are discussed. [ABSTRACT FROM AUTHOR]