2010 Volume 51 Issue 1 Pages 19-30
The authors investigated two sites where slope failure occurred by The 2004 Niigata-ken Chuetsu Earthquake. The main aim of the investigation was to evaluate the strength of sliding surface by laboratory tests and to estimate the mechanism of slope failure based on the test results and relevant analysis results. Each failure occurred in the gentle (from 15° to 20° dip; parallel to the bedding plane) slope, and the sliding surfaces were developed in the weak layers along the bedding planes which had almost flat shape. According to the laboratory tests on the undisturbed samples, the friction angles of each sliding surface (weak layer) are from 35° to 40° and the cohesions are about 10kPa in unsaturated condition and 0kPa in saturated condition. The slope stability analyses based on the test results reveal that each slope was stable against the ordinary water level change induced by rainfall, while its stability during earthquake was influenced by the ground wetting caused by heavy rainfall just before the earthquake. In addition, the failure mode with a long-distance movement suggested that the strength of the weak layer should have been weakened during the earthquake. The simple shear tests show that the alternating load weakens the resistance of the weak layer. Thus, it is estimated that the gentle dip-angle of the failed slope enhanced the alternating effect of cyclic loading, resulting into strength reduction during the earthquake.
