EFFECT OF NONUNIFORM STRESSES ON MEASURED DSS STRESS-STRAIN BEHAVIOR

This paper presents results of a laboratory investigation of the influ ence of the nonuniform stresses imposed by the Geonor direct simple-sh ear (DSS) apparatus on measured stress-strain behavior. The investigat ion concentrated on the influence of the apparatus on the measured str ain-softening behavior of cohesive soils during constant volume (undra ined) shear. Special DSS tests were conducted on an clastic material a nd cohesive soils at varying overconsolidation ratios. The experimenta l results clearly indicate that the nonuniformity of stresses imposed by the device during undrained shear causes a decrease in the vertical effective stress with increasing shear strain. This in turn causes a reduction in the measured horizontal shear resistance at large strains . The device-induced ''unloading'' probably has relatively little effe ct on measured peak strengths in K0 consolidated-undrained DSS tests b ut certainly contributes to the pronounced strain softening typically observed.