Seismic Reponse Analysis Of Large Span Flat Underground Cavern

With the rapid development of China’s economy and national defense,more and more large span flat underground cavern projects emerge at the historic moment.It is difficult to construct the super-large span flat underground cavern,and its seismic response is still in an exploratory stage.The response and safety analysis of large span flat underground cavern under earthquake load is one of the difficult and hot issues to be solved in the construction of underground cavern in China.In order to reveal the seismic response law of super-large span underground cavern,the ABAQUS finite element analysis software was used to analyze the displacement,stress,plastic zone and other indexes of the super-large span flat cavern under the earthquake action,and the response characteristics were obtained,and the influencing factors were analyzed.The main work and achievements of this thesis are as follows:(1)Two-dimensional and three-dimensional finite element models were established to simulate the responses of large span flat underground cavern under earthquake action respectively,and the differences between the calculation results of two-dimensional and three-dimensional models were compared,and the accuracy of the simulation results of two-dimensional models was verified.(2)The finite element model is established to analyze the influence of different influencing factors,including lateral pressure coefficient,surrounding rock grade,buried depth,span and sagittal span ratio,on the seismic response of the large-span flat underground cavern.The results show that the lateral pressure coefficient has a great influence on the large span flat underground cavern,and the tensile stress will appear on the roof and bottom of the cavern dominated by vertical stress under the earthquake action.When the lateral pressure coefficient is high(λ=3),a large plastic zone will appear at the roof and bottom of the cavern,and the seismic action will amplify its effect and cause the failure of surrounding rock.The seismic effect of the cavern built in the rock mass of higher grade is stronger than that of the cavern of lower grade.Under the earthquake action,Under the earthquake action,when the buried depth is 80m-140 m,the relative horizontal displacement of the roof and floor decreases linearly with the increase of the buried depth.When the buried depth reaches 140 m,the change amplitude of the roof and floor displacement response decreases.The influence of span on seismic response of large span flat underground cavern is limited and the relative horizontal displacement of roof and floor increases with the increase of span.Under the combined action of horizontal earthquake and lateral pressure coefficient,when the span is 50m(the ratio is 0.26),it is the most stable and beneficial to the seismic resistance of the cavern.(3)Input seismic waves with different characteristics,including different peak acceleration of ground motion,input direction of seismic waves and spectral characteristics of seismic waves,and analyze their influence on seismic response of large span flat underground cavern.The results show that with the increase of the peak acceleration of ground motion,the stress on the roof and bottom of the tunnel does not change,but is always controlled by static force,and the side wall of the tunnel is obviously affected by the earthquake.The horizontal and vertical bi-directional input seismic waves have a great influence on the large-span flat underground cavern.With the increase of earthquake intensity,tensile stress will appear in some positions around the cavern and strong shear action will appear at the coupling Angle of the cavern,which is not conducive to the stability of the cavern.Different input frequency spectrum seismic waves show roughly the same rule,but there are obvious differences in the response characteristics of the tunnel,wolong wave has a great influence on the underground tunnel.(4)The seismic response of the tunnel is analyzed under three working conditions:lining reinforcement,anchor bolt and anchor cable reinforcement and unreinforcement.Through the comparative analysis of the displacement of roof and bottom of cavern,the evolution of plastic zone of surrounding rock and the distribution and magnitude of principal stress under different working conditions,it can be concluded that the reinforcement system is very effective in reducing the deformation of surrounding rock,controlling the development of plastic zone and reducing the tensile stress,and enhances the seismic stability of cavern rock mass.