Strength of Unsaturated Granite Residual Soil of Shantou Coastal Region Considering Effects of Seepage Using Modified Direct Shear Test

Granite residual soil, which is formed by weathering of granite, is widely distributed in southern China. Previous studies have shown that granite residual soil has lower compressibility, higher bearing capacity and shear strength in its natural state. However, its mechanical strength is significantly reduced after rainwater infiltration, thus causing instability of slopes. Therefore, it is of great significance to study unsaturated engineering properties of granite residual soils under influence of seepage. The main objective of this study is to investigate the shear strength of granite residual soil under the action of seepage. Soil–water characteristic curves under different pressures (100, 200, 300 kPa) were measured using unsaturated soil quadruplet shear apparatus. Shear strength of soil under different combinations of matric suctions (0, 50, 100, 200 kPa) and pressures (100, 200, 300 kPa) was measured using unsaturated soil direct shear apparatus and unsaturated soil triaxial apparatus, respectively. Microstructure of soil with matric suctions (50, 100, 200, 300 kPa) was observed using SEM. Besides, the shear strength of unsaturated soil under the influence of seepage was measured by means of direct shear apparatus and stress–strain permeameter. The results show that the pore channel tends to become smaller and the amount of pore tends to increase up on an increase in matric suction. In addition, the shear strength of granite residual soil decreases by about 15–30% under the influence of seepage.