Effect of fines content of completely decomposed granite on slope stability under rainfall infiltration

Completely Decomposed Granite (CDG) is one of the most typical soils found in Hong Kong. However, limited attention has been given on the effect of fines content on its engineering properties. A series of laboratory tests was conducted to examine the effect of fines content, varying from 0 - 20%, on the engineering properties of CDG. Based on the results from the laboratory tests and the theory of unsaturated soil mechanics, the seepage behaviour and stability of homogenous CDG slopes with different fines contents under intense rainfall were examined using SEEP/W and SLOPE/W. Results showed that fines content plays an important role in the compaction characteristics and permeability of the soil. The coefficient of saturated permeability decreases as fines content increases in the CDG. In addition, an increase in fines content generally results in a decrease in the shear strength of the soil. Numerical analyses showed that the matric suction in the slope with a lower fines content will disappear more easily with faster advancement of the wetting front under intense rainfall. The initial Factor of Safety (FOS) of an unsaturated CDG slope is not much affected by the fines content. However, CDG slopes with a lower fines content shows a more significant drop on its FOS during infiltration process, whereas slopes with a higher fines content are more sustainable to the rainfall infiltration with less decrease in FOS under intense rainfall.