Discussion on method for calculating general safety factor of underground caverns based on strength reduction theory

Abstract

The safety of underground caverns is a key problem of large substructure work for designer. A new method to calculate the general safety factor of underground multi-cavern is put forward,which is based on the strength reduction technique. The method adopts the Mohr-Coulomb rule as the strength yield criterion and takes the transfixion of equivalent plastic strain in wall rock between caverns as the critical failure criterion of caverns. By reducing rock material strength parameters(cohesion c and friction angle φ) step by step,the general safety of caverns is obtained when the result of simulated excavation shows that the equivalent plastic strain of wall rock becomes transfixion,which is the critical state of caverns destabilization. In the course,the calculation program trends to emanate and the deformation of wall rock also tends to rise suddenly and sharply also. The Laxiwa underground hydropower plant,a large cavern group in the northwest China,consists of main powerhouse,auxiliary powerhouse,main transformer chamber,caudal shiplock chamber,tailrace tunnels,and so on. The high geostress and interaction of caverns lead to very serious stability problem. Using the underground caverns strength reduction technique,the general safety factor of Laxiwa underground caverns is gained,which is 1.45. The assistant tunnels of Jinping II hydropower plant are important tunnels also. The general safety factor,calculated by the method,is 1.73. Both of them prove that the method of underground caverns strength reduction technique is very useful for analyzing the general stability of caverns and design of substructure quantitatively.