Effect of suction and vegetation on the stability of slope along riverbank

Abstract

Changes in sea water level and rainfall patterns can significantly affect the stability of a slope along a riverbank, especially in Singapore. As soil in Singapore is mainly composed of residual soil where the location of groundwater table is deep under the surface of the soil, a large amount of soil above the groundwater table is in unsaturated condition where this part of soil is affected by matric suction. In this project, the effects of suction and vegetation on the stability of slope along riverbank were investigated. The properties of saturated and unsaturated soil with roots of Cynometra ramiflora and Melastoma malabathricum as well as soil without roots were measured in the laboratory using advanced geotechnical apparatuses. The results of experimental works indicated that the soil with higher percentage of roots has higher organic content. Furthermore, the higher the organic content of the soil, the higher the percentage of fine particles in that soil. Results from laboratory tests showed that soil with higher organic content has higher Air-Entry Value (AEV), lower saturated permeability, and higher shear strength. In summary, soil with roots of Cynometra ramiflora has a higher organic content than soil with roots of Melastoma malabathricum. Furthermore, numerical analyses were performed to obtain the variations in factor of safety during and after rainfall. The seepage and slope stability analyses results showed that the variations in factor of safety during and after rainfall are the highest for vegetation located at the entire slope. Soil slopes with higher organic content have a higher factor of safety than those soil slopes with lower organic content. Therefore, the soil slope covered with Cynometra ramiflora has a higher factor of safety as compared to the soil slope covered with Melastoma malabathricum. In conclusion, vegetation helps to improve the stability of slope by minimizing the amount of water infiltration into the soil and increasing the shear strength of the soil.