Stability analyses of diversion tunnel portal slopes at the Cidanau Dam, Banten, Indonesia

The inlet and outlet portal slopes of the Cidanau Dam diversion tunnel were designed with 55° and 46° overall slope angles, respectively, based on stability chart method static analyses. However, the performance of tunnel portal slopes under earthquake loads has not been verified whereas the research area is located in a high earthquake hazard zone. This research was conducted to verify the slope performance under static and earthquake load using limit equilibrium (LE) and finite element (FE) methods. The engineering geological mapping and rock cores evaluation showed that both portal slopes comprised four materials: residual soil, very poor-lapilli tuff, poor-lapilli tuff, and fair-tuff breccia. The stability analyses results showed that the designed tunnel portal slopes were in a stable state under static and earthquake loads. The lowest safety factor (FS) values at the inlet section under static and seismic load are 1.9 and 1.24, and at the outlet section are 2.29 and 1.69, respectively. However, the weathered pyroclastic volcanic rocks comprising the slope is required to be protected from severe degradation upon exposure during and after construction. Results of this study are expected to contribute to the safe design of rock slopes in tropical areas, particularly in Indonesia.