Stability analysis and failure mechanism of the steeply inclined bedded rock masses surrounding a large underground opening

The stability of high sidewalls of large-span underground openings is a crucial geological engineering issue when the steeply inclined rock strata form a small angle to the cavern axis. In this study, detailed field surveys, in-situ tests and numerical simulations were performed to investigate the stability and failure mechanism of bedded rock masses surrounding a large underground cavern at the Wudongde hydropower station in China. First, a 3D real time, movable microseismic (MS) monitoring system was installed to analyze the stability of the underground caverns. The micro-fracturing processes and potential failure mechanism of cataclinal layered rock masses were revealed by the spatial distribution evolution and seismic source parameters (i.e., S-wave to P-wave energy ratios, namely Es/Ep) of the MS events. Then, an approach integrating the field surveys, MS monitoring and conventional measurements, is proposed for evaluating the cavern stability and identifying the high risk regions in the bedded rock masses during the staged excavations. Subsequently, discrete element method (DEM) was adopted to depict the progressive failure processes of the layered rock masses. The relationship between the failure in the upstream sidewall and the orientation and spacing of bedding planes was analysed. Abovementioned investigations allow us finally to conclude that the failure mechanism of the steeply cataclinal layered rock strata after cavern excavation is a combination of (1) the composite bending and sliding occurring in the lower part of the upstream sidewall with no supports, and (2) the opening and shear dislocation of bedding planes in the middle area. Timely supports for the rock strata at the toe of sidewall might be an effective way for preventing the occurrence of the studied failure.