Anisotropy in a natural soft clayey silt

Undisturbed specimens of a soft clayey silt have been taken vertically, horizontally and at 45° to the vertical at the base of a trench and submitted to consolidation and undrained triaxial compression and extension tests in the laboratory. Undrained shear strengths and compressibilities are found to be strongly dependent on sample orientation, but the effective stress strength parameters are shown to be much more influenced by stress path than sample orientation. The pore-pressure response vs axial strain is shown to be unaffected by sample orientation. Most of the anisotropic effects are shown to be eliminated by successively consolidating the specimens under isotropic pressure increments and re-testing in undrained shear. Some of the evidence from the experimental programme indicates that tension resisting bonds act between the grains of the soil specimens.