| Under the guidance of China’s energy strategy,a number of large-scale hydropower stations have been constructed or under construction in southwestern China.Due to the mountainous topography of the locations of these hydropower stations,underground cavern complexs are cost-effective choice,sometimes even the only acceptable choice for the hydropower stations.The construction of large scale underground powerhouses in hydropower stations face the challenges of complex spatial relationship of the cavern groups,complicated structure form,and complicated geological environment.Specially,southwest China is located at the junction of the Qinghai-Tibet Plateau plate,where strong earthquakes occur frequently.The complexity of the problems faced by these large underground caverns is ecpected to be further amplified under strong earthquakes;thus,how to ensure the seismic stability of these large underground caverns become the key scientific issues during engineering construction.Currently,the research on seismic response analysis methods for large underground caverns is still insufficient,and far from systematic.Therefore,the researaches on the seismic response characteristics and damage mechanism of large underground caverns under complex geological conditions,are of great scientific and engineering significance for improving the seismic design level and ensuring the stable operation of large underground caverns in China.This paper aims at several essential problems relevant to the seismic behavior of large-scale complex underground caverns,including the seismic response analysis method of large underground caverns,the combined bearing analysis method of surrounding rocks and underground structures,seismic response analysis method of underground caverns in complex layered rock masses,seismic response analysis method of underground caverns acrosed by geological faults,and seismic response mechanism of the complex structure in underground cavern of the Yingxiuwan Hydropower Station.These key issues are studied and discussed,and the analysis methods corresponded to the key issues are proposed and verified by numerical or engineering examples.The detailed contents of this work are listed below:(1)The basic theories of dynamic finite element and the dynamic mechanical behavioris of rock are analyzed and studied,and then the seismic response analysis method for large underground caverns is developed.The explicit center difference method is adopted to solve the system motion equation.The lumped mass matrix and both the local damping and Rayleigh damping are employed to consider the inertia of the structure and the damping effect of the medium.The rate-dependence,plastic deformation and fatigue damage of rock material under dynamic loads are analyzed and concluded,and the constitutive model,which can describes the complex mechanical behavior of rock materials under dynamic conditions,is established.According to the viscoelastic artificial boundary conditions and the wave field decomposition theory,the obliquely incident of seismic wave is presented by transforming the seismic motion into equivalent nodal forces acting on the nodes of artificial boundaries.(2)An dynamic contact model,which can describe the complex contact states of the surrounding rock and concrete structure,is developed.According to the rate dependence and anisotropic damage of concrete materials under cyclic loading,the evolution equations for tensile and shear damage variables of concrete materials under dynamic conditions are established.Furthermore,according to the characteristic of similarity of the constitutive relationship curves of concrete materials under static and dynamic conditions,the specific expressions of the evolution equations of the tensile and shear damage variables under dynamic condition are derived,and the dynamic damage constitutive model of concrete material is established.According to the two failure modes of wear and shear under the cyclic shear load of the contact surface between surrounding rock and concrete structure,the corresponding shear strength formula considering the bonding characteristics of the contact surface is proposed respectively,the wear degradation evolution equation of the strength parameters under cyclic loading is derived,and the nonlinear shear model of the contact surface is established.According to various contact states of surrounding rock and concrete structure such as bond contact,slip contact and disengagement under complex mechanical environment,the boundary conditions related to the contact states are introduced to solve the dynamic contact force,and then the dynamic contact force algorithm considering the complex contact states of the contact surface is formed.At last,the dynamic contact analysis method considering the complex contact states and complex mechanical behaviors of the contact surface is established by introducing the nonlinear shear model of contact surface to the proposed algorithm.(3)A seismic response analysis method of underground caverns in complex layered rock mass is established,and the failure mechanism of underground caverns in layered rock is studied.According to the deformation and strength anisotropy of layered rock mass under three-dimensional stress state,the laws of deformation strength parameters of layered rock mass are studied under different dip angles and confining pressure.The transverse anisotropy theory is adopted to simulate the mechanical behaviors of the deformation anisotropy of layered rock mass under elastic state,and the strength anisotropy is achieved by taking the strength of both the bedrock and layer surface of the layered rock mass into account.The nonlinear dynamic finite element model of layered rock mass is established by considering the influence of strain rate on the deformation and strength parameters,and the strength degradation caused by fatigue damage of layered rock mass under cyclic loading.Different failure modes and failure mechanisms of layered rock mass caverns are analyzed and summarized,and the occurrence conditions and discriminating methods of different failure modes are given.Combined the anisotropic mechanical characteristics of layered rock mass with dynamic finite element model and its failure mechanism,the seismic response analysis method of underground caverns in complex layered rock mass is established.(4)A nonlinear numerical model for geological faults is proposed,and the effects a geological fault on the seismic response and failure mechanism of underground caverns is analyzed.The thin layer element is used to describe the structural features of geological faults,and the mathematical model of the thin layer element is deduced in detail.By introducing a hyperbolic model to describe the tangential and normal fault mechanical properties and considering the wear degradation of the fault strength parameters,a nonlinear finite element model of the geological fault is proposed.The model is used to analyze the influence of fault parameters and peak acceleration on the seismic response of underground caverns.The analysis results show that the adverse effects of geological faults on the stability of the cavern will be further amplified during earthquakes.Combined seismic damage investigation with the results of numerical analysis,three failure modes and their mechanisms of underground caverns containing geological faults are summarized.(5)The damage characteristics and damage mechanism of the underground powerhouse in Yingxiuwan Hydropower Station during the Wenchuan earthquake are analyzed.According to the results of the post-earthquake field investigation,the seismic damage characteristics of the Yingxiuwan Hydropower Station during the Wenchuan earthquake can be concluded as “ground structure damage is greater than underground structure” and “the concrete structure in the underground cavern is the weak part during earthquakes”.The seismic response analysis platform is adopted to simulate the seismic response process of the underground powerhouse of Yingxiuwan Hydropower Station,in which the effects of oblique incident seismic wave and dynamic interaction between the surrounding rock and concrete structure are both considered.Combined the damage investigation with the results of numerical simulation,the deformation mode and damage mechanisms of the underground powerhouse of the Yingxiuwan Hydropower Station are analyzed and concluded.The result shows that the damage distributions of the cavern structure are mainly dominated by the structure feature itself;the main cause of the severe damage in the upper structure is due to the lack of sufficient constraints on it,which leads to a large deformation in the upper structure. |
