Assessment of Peak/Residual Failure Envelopes of Clayey Soil via Suction-controlled Ring Shear Testing

A thorough experimental program has been undertaken to assess the suction-controlled peak and residual shear strength responses of statically compacted, moderate plasticity clayey soil over a relatively wide range of suction states. The experimental program was accomplished via a servo/suction-controlled ring shear apparatus suitable for testing unsaturated soils under large deformations via the axis-translation technique. Results further substantiate the crucial role that has been observed to be played by the imposed matric suction on residual shear strength behaviour of compacted clayey soils. For the range of net normal stresses (0-200 kPa) and matric suction states (0-300 kPa) investigated, the increase in both peak and residual shear strength, with increasing matric suction, was found to be markedly nonlinear. Furthermore, a readily distinct correspondence was observed between the nonlinear nature of either peak or residual shear strength envelopes, with respect to increasing matric suction, and the very nature of the soil-water retention curve of the test soil. Results, in general, confirm that a conceptual residual shear strength framework for unsaturated soils, analogous to that postulated for peak shear strength, can be formulated with reasonable reliability.