Interface direct shear behavior of unsaturated decomposed granite soil and cement grout

The behavior of the interface between compacted soil and structure is a very important phenomenon for various geotechnical engineering projects like soil nails, retaining walls, shallow foundations and pile foundations. The interface behavior depends on many factors such as way of forming the interface, soil type, void ratio, normal stress and water-cement ratio. Besides the above mentioned factors, matric suction is considered as an important factor which may influence the interface strength as shear strength of soil is highly influenced by that. The behavior of compacted soil-structure interface is studied by different types of tests. In present study, 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 by using a modified direct shear apparatus. Axis-translation technique is applied to control the pore-water and pore-air pressure. The behaviors of shear stress-displacement curves of soil-cement grout interface tests are similar to those of soil-soil direct shear tests. Matric suction has significant influence on the hardening-softening behavior of soil-cement grout interface. The interface-dilation is also greatly influenced by matric suction. At higher suction levels the interface-dilation value is lower compared to soil-dilation. Similar to CDG soil, the interface shear strength increases with suction. The interface shear strength is higher than the soil shear strength within lower suction range. But, the shear strength becomes slightly lower than the soil shear strength at higher suction range. The variation of interface shear strength with matric suction (suction envelope) is nonlinear.