Study On Response Characteristics Of Large Underground Cavern Group Under Earthquake

Seismic investigation shows that the underground caverns always suffer some damage under earthquake. The dynamic mechanical tests of granite samples from Dagangshan hydropower station are conducted, and then the seismic propagation in the rock field is simulated by the software FALC3D. Considering the depth attenuation effect, the underground caverns of Dagangshan hydropower station is modeled and simulated to explore its seismic response and damage evolution. The main achievements are summarized as follows:(1)The important fault zones and potential seismic source zones are determined due to seismic geological condition of Dagangshan hydropower station region, and then the peak accelerations under various exceedance probabilities are obtained according to the intensity attenuation relation. Based on the dominant frequency of Panzhihua seismic records, some nonstationary artificial seismic motions are simulated by trigonometric series superposition method.(2)The static and dynamic compression tests of granite samples are conducted to find some changes of dynamic elastic modulus and Passion rate with strain rate and ambient pressure. The test results show that the dynamic elastic modulus of granite sample increases with strain rate, and the compression strength also increases with strain rate. Then the relation between dynamic elastic modulus and strain rate within some given domain are programmed. The full stress-strain of Dagangshan granite under static triaxis test and the damage mechanics analysis show that the brittle rock mainly suffers lateral damage, with damage value 0.7-0.8 near the critical fail. Assuming the linear relation between dynamic damage and static damage, the dynamic damage constitutive function is established.(3)The seismic propagation of mountain& gorge terrain is simulated by FLAC3D. Both the actual seismic records and the numerical simulation shows that the seismic accerlation increases as the earthquake propagates from rock field bottom to top. In other words, the seismic records demonstrate depth attenuation downward and and slope amplification. The attenuation index of model bottom decreases gradually with damping rate and slope ratio; the peak acceleration has no influence on the depth attenuation; the depth attenuation will get intense if the dominant frequency of input seismic motion approaches the natural vibration frequency of the field model. The simulation results basically fit actual seismic records, and it is feasible for FLAC3D to simulate the seismic propagation of rock field.(4) The underground caverns of Dagangshan hydropower station are numerically modeled according to geological conditions, and then the seismic response of the caverns is simulated by FLAC3D. The simulation results show that the acceleration time history, velocity time history, relative displacement time history and stress time history of the monitoring points are similar with and leg behind slightly that of the input seismic motion. The relative horizontal displacement will increase by about 32% without considering the depth attenuation effect.(5)The simulation results show that geostress field and elastic modulus have little influence on the maximum horizontal relative displacement between vault and floor of underground caverns; this maximum displacement increases with damping rate; the peak acceleration has great influence on this maximum displacement, the two indicating a good linear increase relation; seismic motions with different frequency spectra, even considering the depth attenuation, have notable influence on this maximum displacement. The maximum horizontal relative displacement between vault and floor of underground caverns will increase if the dominant frequency of input seismic motion approaches the natural vibration frequency of underground caverns, and vice versa.(6)The damage zone extends deeply from the middle of the cavern wall. The damage zone gets maximum area soon after the input seismic motion reaches the peak acceleration, and then maintain stable subsequently. The maximum damage area will increase by about 32% without considering the depth attenuation effect.The maximum damage area will increase if the dominant frequency of input seismic motion approaches the natural vibration frequency of underground caverns, and vice versa.