Analysis of a Complex Flexural Toppling Failure of Large Underground Caverns in Layered Rock Masses

Abstract

Flexural toppling failure is one of the most common instabilities in large underground caverns and occurs when the steeply inclined rock layers form a small angle to the cavern axis. This study investigates a typical flexural toppling failure in the underground powerhouse at the Wudongde hydropower station, Southwest China, using in situ tests and numerical tools. The characteristics of flexural toppling failure in the large underground caverns were initially analysed by field observations and conventional testing methods, including the use of multipoint extensometers, acoustic wave tests and borehole television images. A high-resolution microseismic monitoring system was installed in the right main powerhouse to reveal new and important insights into the mechanisms of typical failures that occur in bedded rock masses. The fracturing processes of the rock mass associated with the flexural toppling failure were analysed based on microseismic monitoring data, and the development of flexural toppling failure was related to shear failure. Then, the discrete element method was used to further investigate the effect of geological planes on the formation of flexural toppling failure. Finally, the complex mechanism of flexural toppling failure was evaluated by incorporating field observations, in situ monitoring and numerical modelling. An effective method of preventing this type of failure in underground caverns was analysed. The results can improve our understanding of flexural toppling failure in the underground caverns of the Wudongde hydropower station, and the proposed method can be adopted to study similar large underground caverns.