Research On Seismic Response And Seismic Resistance Measure For Large Rock Cavern Under Earthquake Motion

Along with the deeply advancement of the western development drive, a lot of huge hydraulic power plants are being built or will be built in China's western regions which are seismically active zones with high earthquake intensity. For the reason of the restriction of geological condition, the diversion structures of the plants are mainly constructed in the mountains beside the rivers. Thus, the large rock cavern groups is formed. Its main features of this kind of large rock cavern groups are large span, long axial-length and high side walls. Therefore, how to ensure the safety of rock cavern groups under the action of earthquake, especially under the action of strong earthquake is one of the most important problems that should be solved properly and quickly. Taking the dynamic nonlinearity program ABAQUS as the numerical tool, firstly, the numerical model and other influenced factors are fully studied for the rock cavern groups under the action of seismic motion. Then, a seismic resistance measure is proposed for this large scale rock cavern groups.Firstly, in the precondition of using infinite elements as dynamic boundary condition, the comparison between the numerical solution and analytical solution proved the feasibility that ABAQUS can effectively simulate the wave propagation in half space scattered field. A coupling model of finite element and infinite element for the seismic response analysis of rock cavern groups is presented using the nonlinear finite program ABAQUS. On the basis of the propoed coupling model, the influence of geologic conditions such as embedded depth, geotress, strength of rocks and fracture zone on the seismic response of rock cavern is studied. The simulated results indicate that the geologic conditions motioned above can affect the seismic response of rock cavern groups in certain degree.Secondly, from the view of theoretical analysis, the applicability that the concrete damaged plasticity model could be used to study the dynamic property of surrounding rocks is analyzed. On the basis of this damaged plasticity model, the seismic response of rock cavern in a hydraulic power plant is studied. The numerical results indicate that the damaged plasticity model is suitable for the dynamic analysis of rock cavern subjected to earthquake. The model can also consider the stiffness recovery effects and rate sensitivity during cyclic loading. The results of the influence of cavern spacing on the stability of large cavern groups in a hydraulic power station indicate that the cavern spacing does have some influence on the stability of large cavern groups under the action of earthquake. If the cavern spacing is less than one cavern characteristic length (taken to the biggest width of a cavern in the group), the stabilities of adjacent caverns are significant affected by their spacing. Once the spacing exceeds the critical cavern characteristic length, the spacing hardly influences the stability of rock cavern groups. The stabilities of the cavern groups can be assured if the thickness of rocks between the adjacent caverns exceeds the critical cavern spacing in the conditions of adopting appropriate supporting schemes and effective anti-seismic reinforcement.Then, the dynamic nonlinear response of the rock cavern is studied subjected to seismic motion considering wave passage effect. The research shows that the wave passage effect of earthquake can result in phase difference of seismic response along the axial direction of the rock cavern. The amplitude of earthquake motion is the critical factor for the damage of rock cavern. For the rock cavern with its axial length around 300 m～400 m, the wave passage effect of weak earthquake can hardly influence the damage of rock cavern and the wave passage effect of strong earthquake can dramatically aggravate the damage of rock cavern.After that, the blast resistance of rock cavern is studied under the action of explosion loading. Numerical results indicate that the weak performance of blast resistance for the shallow cavern and the strong performance of blast resistance for the deep embedded cavern. Numerical results also indicate that when the geo-stress lateral pressure coefficientλis less than 1, no obvious variation of the blast resistance performance appears for the cavern. Onceλis great than 1, the blast resistance performance of the cavern is dramatically decreasing along with the increasing ofλ. For the effect of arch crown, the locations of spandrel, arch crown and arch footing should be enforced properly in practical engineering, especially the location of spandrel. The simulated results can be as references to the design of this kind of underground rock cavern in a hydraulic power station.Finally, the seismic resistance measure is proposed that is adopting soft-seismic isolation layer, reforced grouting layer and seismic isolation gap to enhance the seismic resistance performance of rock cavern. Sensitivity analysis of seismic resistance measure is conducted for the rock cavern. Simulated results show that soft-seismic isolation layer is better for the earthquake resistance of hard rock cavern and reforced grouting layer is suitable for the earthquake resistance of soft rock cavern. The seismic isolation gap is benefit for the seismic resistance of rock cavern. However, despite the dynamic features of rock cavern have not been changed essentially by adopting soft-seismic isolation layer, not only the self-supporting performance of surrounding rocks could be realized, but also the seismic response of concrete liner could be reduced in certain degree. Reforced grouting layer can reduce the dynamic response of liner in soft rock cavern. The structural joint not only can decrease the desiccation cracks during the period of concrete shrinkage but also can result in the reduction of seismic response for concrete liner. The seismic resistance measure proposed here can lead a way for the similar projects in the design of earthquake resistance.