Response of granite under strain controlled loading and effect of support system on behaviour of large underground cavern

摘要

The hard rocks like granite, basalt, quartzite etc. show the sudden failure due to loading and unloading. The phenomena of rock bursting during underground excavation for such hard rocks under high rock cover is quite common. The rocks in general show strain softening behavior under loading and unloading. The strain softening behavior of rocks is captured using strain controlled tests in the laboratory. The rate of strain softening (residual strain/peak strain) is generally more in softer rocks than that in harder rocks. The paper presents the post peak response of granite under strain controlled loading for confining pressures of 0 to 30MPa in the laboratory. The strength and deformation parameters are determined from laboratory test results. The peak and residual strength parameters are determined using Mohr Coulomb criterion.The variation of cohesion and friction angle with plastic strain is also determined.The rock shows the vertical splitting under confining pressures of 0 and 10MPa and shear failure under other high confining pressures. The rate of softening is less under low confining pressures and high under high confining pressures. The paper also presents the application of softening behavior of granite to the analysis of large underground caverns (Power house and transformer yard) under plane strain conditions using FLAC. The power house cavern is 22m wide and 44m high and transformer hall cavern is 16m wide and 22m high. The analyses of large underground caverns have been carried out using conventional Mohr-Coulomb model and Mohr-Coulomb model with strain softening effect using FLAC. The support system has been installed in various stages in the form of shotcrete and effect of softening on cavern behavior is studied. The effect of support system on the softening behavior is also studied in detail. It is seen from the analyses that there is significant effect of the softening on caverns behavior.