An extensive number of slopes failed in the 2004 Niigata-ken Chuetsu Earthquake. Among them, a dip slope containing a weak layer in Yamakoshi Village (currently Nagaoka City) was investigated intensively. Regarding its morphological characteristics, it is argued that the earthquake reactivated a pre-existing failure plane which then formed most of the present sliding plane. In order to reveal the strength properties of the weak layer that formed the sliding plane, including the behavior against cyclic loading, a series of triaxial compression tests and simple shear tests was performed on undisturbed specimens that were retrieved by block sampling from the site. Based on the test results, a stability analysis and the calculation of the earthquake-induced displacement were performed. By extending Newmark's sliding block analysis, while considering the eŠects of the irregular geometry of the sliding plane and its strainsoftening properties, a reasonable simulation of the process of this slope failure could be provided.

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A procedure based on the Janbu method for slope stability analysis and Newmark's sliding block method for displacement calculation is proposed to evaluate rationally the earthquake-induced displacement of slopes containing a weak layer. In the procedure, the eŠects of the irregular geometry of the sliding plane and generation of excess pore water pressure in the weak layer which forms partly the sliding plane are considered. In the computation following the proposed procedure, a negative yield seismic coe‹cient to induce a safety factor of unity can be obtained because of the eŠect of excess pore water pressure, while the residual displacement can become aˆnite value because the sliding plane becomes ‰atter in general after the slope undergoes a large displacement.

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