Analyses of Retrogressive Slope Failures in Terrace Deposits

Slope failures in terrace deposits were explored and studied in this research. In situ exploration and sampling were carried out in Shin-Shan Village of Nantou County after a typhoon. Large direct shear tests in the laboratory were performed to measure the shear strength of the material. The soil-water characteristic curve of the soil was derived from pressure plate tests. The cross-sectional profiles of the failed slope before and after the typhoon in this area were obtained using a digital elevation model and 3-D terrestrial scanner, respectively. The numerical software SEEP/W and SLOPE/W were used to analyze the slope stability under similar rainfall to the typhoon. Different groundwater levels and river water levels were considered to compare the factor of safety values and reveal the failure processes of the slope. The possible failure of the slope may be due to scouring of the slope toe by fluvial erosion caused by rising water river levels, mud flows, or both, based on the investigated profiles and numerical simulations. The estimated length of scouring was about 20 m long. Based on the numerical modeling, the possible mechanism of failure for this length of scouring could be due to multiple retrogressive failures after toe scouring.