Characteristics of Pore Pressure and Volume Change during Undrained Loading of Unsaturated Compacted Granite Soil

A series of triaxial compression tests were performed on samples of compacted granite soils in a modified triaxial cell that can separately control pore air pressure (u_a) and pore water pressure (u_w) in order to examine the characteristics of pore pressure, volume change and stress-strain behavior under undrained loading condition. Unsaturated granite soil samples were prepared by compaction in a mould. These samples were tested at different suction and different confining stresses. The volume change of an unsaturated soil during shearing undrained is much sensitive to the confining pressure compared to the initial water content, and the matric suction. The volume expands during shearing, and the volumetric strain is much larger at the smaller confining pressure and at the higher matric suction. The variation of the internal frictional angle according to the initial water content and the matric suction is negligible, but the effective cohesion increases according to matric suction.