Unsaturated soil-cement interface behaviour in direct shear tests

A series of interface direct shear box tests are performed between a compacted completely decomposed granite (CDG) soil and cement grout under different matric suctions and net normal stresses by using a modified direct shear apparatus. Axis-translation technique is applied to control the pore-water and pore-air pressure. The behaviour of shear stress-displacement curves of soil-cement grout interface tests is similar to those of soil-soil direct shear tests. Matric suction and net normal stress have significant influence on the hardening-softening behaviour of soil-cement grout interface. The interface-dilation is also greatly influenced by matric suction and net normal stress. The variation of interface shear strength with respect to net normal stress (failure envelope) for different matric suctions is observed as linear. The apparent angle of interface friction and adhesion intercept increase with matric suction. The compacted CDG soil-cement grout interface behaves as a rough interface. The apparent interface friction angles for different suctions are nearly equal to the apparent friction angles of CDG soil under the same suctions. However, the apparent adhesion values are higher than the apparent cohesion values of CDG soil in lower suction range, but are lower in higher suction range. The variation of interface shear strength with matric suction (suction envelope) is nonlinear and δ^b value decreases with increase in matric suction.