Effect of Geological Factors on Highway Cut Slope Instabilities: A Case Study from Beliatta on Extension of Southern Expressway

Cut slope instabilities are becoming a major natural hazard during rainy seasons in Sri Lanka. Among them, the slope instabilities along highways have increased during the past decade as a result of road construction and development projects. The Southern Expressway (E01) is a main and frequently used expressway which is from Colombo to Galle, Matara, and Mattala. When considering the natural factors affecting slope instabilities, geological factors as a part of terrain factors play a major role. By understanding these factors the risk involved can be reduced. For this study, one such location was selected at Beliatta (chainage 19+000 km). The analysis was mainly based on borehole data and discontinuity data. The occurrence of a highly weathered pegmatite body, high plasticity of the soil and the presence of a shear zone was the main geological factors affected. By kinematic analysis, it was found that there is a slight probability for a wedge failure and the optimum slope angle was determined as 34°. The slope mitigation design can be improved and modified using these findings.