An Adapted Ring Shear Apparatus for Testing Partly Saturated Soils in the High Suction Range

Abstract The paper presents an adapted experimental setup to study the residual shear strength properties of clays under unsaturated state conditions. This issue is of geotechnical concern in stability analysis of slopes in fissured rocks with unsaturated clayey fills and also, in a more fundamental way, as an indicator of microstructural changes of clays undergoing drying paths. To this end, a commercial ring shear apparatus (Bromhead type) was instrumented and adapted to control total suction in a wide range by using vapor transfer technique. In addition, by using simulation-aided techniques, the time evolution of suction at local state and axial deformation was studied to overcome the experimental difficulty associated with the determination of the suction equalization time. The simulated equalization period agreed well with the experimental information based on a mechanical stabilization criterion (time required to attain vertical strain rates lower than 0.1%/day). Relevant test results showing the main capabilities of the adapted apparatus are presented, which bring up the important changes observed in residual friction angle when testing a clayey soil at different hydraulic states (total suctions ranging from nearly 0 up to 140 MPa).