Stability analysis of the air-cushioned surge chamber of the diversion system for San Gabán III hydropower station through seepage-stress coupling approach

The design scheme of the air-cushioned surge chamber is adopted for the diversion system of the San Gabán III hydropower station. Due to the combined action of both high internal water pressure and external water pressure, it is necessary to demonstrate the stability of the design scheme of the air-cushioned surge chamber. This paper adopts the seepage-stress coupling analysis method to analyse the two working conditions of the air-cushioned surge chamber, i.e., the construction condition and the normal operation condition. A fine numerical analysis model of the air cushion surge chamber including the drainage holes and the drainage belts is established. The numerical simulation adopts the one-way coupling scheme. Firstly, the actual seepage fields in the air-cushion surge chamber area under the actual waterhead boundary conditions are numerically simulated. Secondly, the equivalent seepage volume forces are calculated based on the seepage fields obtained. After that, the equivalent seepage forces are applied to the surrounding rock of the air-cushion surge chamber to carry out the mechanical simulations. Results show that, most of the seepage loads are borne by the surrounding rock mass because of the drainage effects. While the pore pressure differences between the inner and outer edges of the lining are dramatically reduced, which greatly reduces the load borne by the lining and ensures the stability of the lining structure. In general, the surrounding rock deformations of the air-cushion surge chamber are not large, the stress states are normal, the supporting structures are within the bearing capacities, and the plastic zones of the surrounding rock are within the control range of the rock bolts, indicating that the air-cushioned surge chamber is stable.