Effects of unsaturated properties on stability of slope covered with Caesalpinia crista in Singapore

Rainfall plays an important role in affecting the slope stability in a tropical country such as Singapore. The soil in Singapore is mainly residual soil with a deep groundwater table that is commonly found in the unsaturated condition in the presence of matric suction. This paper presents the unsaturated properties of soil with roots of Caesalpinia crista and their effects on slope stability during rainfall. Laboratory tests were carried out to obtain the index properties, saturated properties (i.e. permeability and shear strength) and unsaturated properties (i.e. unsaturated shear strength and soil–water characteristic curve) of soil with and without roots of C. crista. Seepage analyses were performed to obtain the variations in pore-water pressures during and after rainfall. Slope stability analyses were conducted to obtain variations in the factor of safety during and after rainfall. The results indicated that soil with roots of C. crista had a higher saturated volumetric water content, a higher air entry value, a lower saturated permeability and a higher shear strength compared with soil without roots. In addition, the soil with roots of C. crista was effective in reducing the water infiltration into the slope during rainfall. Hence, slope stability can be maintained during rainfall.