Dilatancy and strength of an unsaturated soil-cement interface in direct shear tests

The dilatancy of a soil is significantly influenced by matric suction, and it affects the apparent friction angle and shear strength of the soil. To examine the influence of dilatancy on interface behavior, a series of direct shear box tests are conducted on a compacted completely decomposed granite (CDG) soil-grout interface in a cast in situ condition under different matric suctions and net normal stresses. The test results indicate that matric suction and net normal stress have significant influence on the hardening-softening and dilatancy of the soil-cement grout interface. The failure envelopes for different matric suctions are observed as linear. The apparent interface friction angle and adhesion intercept increase with matric suction. The suction envelope is found to be nonlinear as the δ^b-angle decreases with matric suction. A modified model is proposed to consider the influence of dilatancy on the apparent interface friction angle and hence on the interface shear strength. Experimental shear strength data are compared with the analytical shear strength results obtained from the modified model using effective interface strength parameters and analytical values of dilation angles obtained from dilatancy curves. The experimental shear strength data agrees well with the analytical results for different net normal stresses and matric suctions.