Numerical Simulation For Seismic Joint Response Of Surrounding Rock And Supporting Structure Of Underground Caverns

With the continuous development of hydropower construction in our country,a large number of hydropower stations have been planned and built in the southwest area with abundant water resources.Restricted by the alpine canyon terrain,the powerhouses of these hydropower stations can only be built as underground powerhouse,then a large number of underground caverns are formed.At the same time,the southwest region where these large hydropower stations concentrated in is a high seismic intensity zone in China.Once a large magnitude earthquake happens,it is bound to bring a serious threat to the safety and stability of these underground caverns.The seismic characteristics of underground caverns are directly related to the normal operation of hydropower stations and the safety of production personnel,which makes the related study is of great significance and be widely concerned by domestic and foreign engineers and scholars.However,there is still a lack of systematic study on the seismic response of underground caverns.Therefore,it is of great practical significance to study the seismic joint response analysis method of the surrounding rock and supporting structure of underground caverns of hydropower stations,and this is beneficial to provide intellectual support for practical projects.In this dissertation,several key problems that be centered on the seismic joint response analysis of surrounding rock and support structure of underground caverns are studied and discussed.The specific contents are as follows:(1)The basic analysis method of 3D finite element method for underground cavern is studied,and the corresponding dynamic finite element program of underground cavern is developed.Damping effects are considered in the form of local damping.And the elastoplastic constitutive relation of rock material is established,which takes into account the influence of three dynamic characteristics,including rate dependent,plastic deformation,fatigue damage.The boundaries of the computational model are set based on the theory of viscoelasticity,free field artificial boundary and the distribution characteristics of seismic wave field in underground cavern.And the means of filtering,baseline correction and amplitude reduction are used in the pretreatment of the measured seismic wave,then the seismic wave is input in the form of displacement and velocity.Finally,the system is solved by the explicit central difference method with variable step size.The correctness of the program is verified by two numerical calculation examples,including a numerical simulation of shaking table test and the comparison between the program and commercial software.(2)The interaction mechanism between fully grouted anchor bolt and surrounding rock is studied.And the complex anchoring unit and the mechanical analysis model of grouted anchor bolt,considering bolt's physical reinforcement and mechanical restraint effect,are established.The physical reinforcement effect of bolt is simulated by calculating the additional stiffness matrix of the bolt unit or improving the mechanical parameters of the anchored rock mass.Based on the bolt's neutral point theory,the basic differential equations of load transfer considering the complex failure of the anchored element are deduced.And the axial shear stress of the anchoring body,which is obtained by solving the equations,acts as the supporting reaction force and is applied on the rock mass to reflect the mechanical restraint effect of bolt.These form a bolt support calculation method.The calculation method is implemented and embedded into the dynamic programming,and the parallel computing,optimization of bolt segment,multi-step solution of bolt force are adopted to decrease the computing time,then a high-efficiency analytical method for the interaction between surrounding rock and anchor bolt is established based on dynamic explicit finite element method.Finally,three numerical examples are provided to verify the rationality and validity of the bolt's high-efficiency analytical method.(3)The two kinds of shear failure modes of contact surface are summarized based on artificial joint shear test.And by consulting data,the formula of peak shear strength considering bonding property is proposed.Aiming at the vibration deterioration characteristics of contact surface shear behavior,the mathematical expression of the vibration deterioration coefficient of contact surface is deduced.Then the complex shear strength formula of surrounding rock and lining interface under seismic action,considering the interfacial bonding characteristics,shear failure mechanism and seismic dynamic deterioration effect,is formed.At the same time,based on the contact characteristics between the surrounding rock and the lining of the underground cavern under the seismic action,a dynamic algorithm of the contact surface considering complex shear strength is put forward.The dynamic analysis model of the interaction between surrounding rock and lining structure is established by coupling the complex shear strength formula of contact surface with the dynamic algorithm of contact surface.Then the model is programmed and embedded in the dynamic program,and the dynamic joint bearing analysis method of the surrounding rock and the lining structure of underground cavern is formed.Taking the underground powerhouse of Yingxiuwan Hydropower Station as a project example,and through the comparison of the numerical results and the post-earthquake investigation results,the rationality and validity of the dynamic joint bearing analysis method are verified.(4)Combining the dynamic analysis method of bolt and lining supporting and the dynamic finite element program of underground cavern,a complete numerical analysis platform for the seismic response of surrounding rock and supporting structure of underground cavern is built.Aiming at the problem of stability evaluation of surrounding rock of underground cavern under seismic action,two new methods are proposed to evaluate the local stability of surrounding rock based on the relative displacement dynamic magnification factor and the damage coefficient of surrounding rock,respectively.Based on the wave equation of seismic response analysis of surrounding rock and the principle of simultaneous reduction of tensile and shear strength,the safety factor method for evaluating the global relative stability of underground caverns under seismic action is presented.Based on the numerical calculation platform,the sensitivity of seismic support effect of surrounding rock mass on the change of parameters of bolt and lining support is calculated,then the optimum design of seismic support for underground caverns is studied.And the damping effect of lining structure with consolidation grouting reinforcement of surrounding rock and flexibility cushion layer are also calculated to put forward the concept of seismic design of underground powerhouse cavern structure.(5)The numerical analysis platform for joint seismic response of surrounding rock and support structure of underground cavern is applied to the seismic time history calculation of underground caverns of Huangdeng Hydropower Station.The seismic response characteristics of the surrounding rock and supporting structure of the underground cavern are analyzed,and the seismic stability of the surrounding rock of the underground caverns is evaluated.The results show that,the global stability of the underground caverns of Huangdeng underground powerhouse under earthquake action is relatively good from a qualitative point of view,while the damage degree of surrounding rock in the local area is larger,and it may occur local instability.During the earthquake process,the stresses of the bolts at different position are mainly manifested in two forms:fluctuation development,cumulative growth.And after the earthquake,only a small number of bolts' stress are reach to the yield limit,the stresses of the other parts of the bolts are small and with a large margin of safety.The local stability of the surrounding rock is evaluated quantitatively by adopting the two methods of the relative displacement dynamic magnification factor on key points and the damage coefficient of rock mass element,and the local loosening of the surrounding rock is consistent with the seismic response characteristics of the surrounding rock failure zone and displacement.Based on the principle of tensile and shear strength reduction,the integral safety factors of underground caverns under two conditions of excavation and earthquake are calculated,then the influence of seismic action on the global stability of the underground caverns is illustrated,meanwhile the integral safety margin of the underground caverns is quantized.