Triaxial tests on weakly bonded soil with changes in stress path.

The results are presented of drained triaxial tests on a weakly bonded artificial soil in which the stress path direction has been changed partway through the shearing process. The effects of the previous shearing path history on the yield and failure surfaces are examined. Yield of the bonds occurs under each stress path direction followed, even when yield has previously occurred along another path. This demonstrates that bond breakdown is an anisotropic process. The position of yield was found to be independent of the previous shearing path history of the soil and occurred at points that corresponded to a yield surface defined for the current shearing path direction. However, the previous shearing path history of the soil did significantly affect the failure envelope. It is suggested that the bond yield surface is kinematic, in the sense that it is an expandable/shrinkable surface, but that it is not a moveable surface. It is postulated that the yield surface expands when volumetric strains are compressive and shrinks when the volumetric strains are dilatent