Mechanical Mechanism Study Of Fault-Crossing Mountain Tunnel With Flexible Joint In Strong Seismic Areas

The area where tunnels across the fault is one of the most serious earthquake damage areas.With the development of transportation infrastructure in China,more and more tunnels will inevitably cross the active fault zones.It is of great scientific and engineering significance to carry out the research on anti-seismic and shock absorption technology of the fault-crossing tunnels in high intensity seismic area.Based on the theory of vibration mechanics,a mechanical model of shear-torsion flexible joint is proposed.A mechanical model of tunnel withs flexible joints is established.Meanwhile,the mechanical model of the fault-crossing tunnel with flexible joints is established.A simplified method for longitudinal seismic responses of fault-crossing tunnels with flexible joints is proposed.The influence of different lining stiffness,surrounding rock properties and fault displacement on the response of tunnel lining is analyzed.The damping effect of different joint types and stiffness of joint on tunnel lining is studied.It provides a theoretical basis for the design of fault-crossing segmental tunnels with joints.Finally,the model tests of the fault-crossing tunnel with flexible joints are carried out based on the Longxi tunnel project of Du-Wen Expressway,which was seriously damaged in Wenchuan earthquake.The specific research work and conclusions are as follows:(1)The Timoshenko beam model is used to simulate the lining segment.The sheartorsion flexible joint model is established by setting extensional spring and torsional spring.Meanwhile,the mechanical model of tunnel with flexible joints is established and the corresponding steady-state dynamic response of the flexible joint is derived using Green's function method.Comparing the analytical solution with the finite element results,it is found that the analytical solution of displacements response and rotational responses of lining is in good agreement with the numerical solution,which verifies the correctness of the analytical solution.Results shows the setting of shear-torsion flexible joint increases the allowable deformation capacity of lining at the position of the joint.The effect of extensional spring on tunnel responses is greater than that of torsional spring.(2)The mechanical model of the fault-crossing tunnel with flexible joints is established and the analytical solution of steady-state response of fault-crossing tunnel with flexible joints under earthquake motion is derived.The effects of different boundary,lining stiffness,fault characteristics and different kind of joints on lining displacement response,rotational response and internal force response are studied in depth.The results show that setting free boundary at both ends is more suitable for the dynamic response analysis of the tunnel with segmental joints.The peak values of the internal force of the fault-crossing tunnel are mainly distributed at the middle of the fault and the interface between the hanging wall and fault and the interface between the fault and footwall,which are the key positions for seismic fortification.Increasing the stiffness of the lining can reduce the displacement responses and torsional responses of the lining,but increase the internal force responses of the lining.With the increase of the width of fault fracture zone,the range of displacement responses,torsional responses and internal force responses of tunnel affected by fault increased under earthquake motion.Strengthening the weak surrounding rock of fault zone can reduce the influence range of the internal force of tunnel by earthquake.The allowable displacement difference and angle difference between the segmental lining are obtained under earthquake motion.(3)By introducing Volterra dislocation theory,the analytical solution of steady-state longitudinal response of tunnel with flexible joints under fault movement is derived.The influence of different fault displacement,different lining thickness,different excavation outer diameter,different lining material stiffness,different fault surrounding rock properties and different joint stiffness on tunnel lining deformation and internal force are analyzed.The larger the dislocation of fault is,the greater the differences of displacement of the joint.The flexible joint can effectively reduce the torsional response and internal force response of lining under fault movement.Although the deformation of tunnel lining is reduced by increasing the lining thickness,excavation outer diameter and lining material stiffness,the value of internal force of lining is increased at the same time.With the increase of lining thickness,excavation outer diameter and lining material stiffness,the decrease amplitude of torsional response and internal force response of lining with flexible joint increases than that of ordinary lining,and the damping effect of the joint is more significant.The allowable displacement difference and angle difference between the segmental lining are obtained under fault movement.(4)Whether the tunnel is affected by fault dislocation or ground motion,reducing the stiffness of the joint can effectively increase the deformability of the lining segments and increase the allowable displacement differences between the segments of lining,thereby reducing the internal force of the tunnel lining segments and reducing the influence range of the dynamic response of tunnel lining.From the analysis of the internal force of lining,it can be seen that with the decrease of joint stiffness,the internal force of the lining is declining.Therefore,to select the appropriate stiffness value of the flexible joint,it is necessary to consider the bearing static load ability of the tunnel,and meanwhile to reduce the influence of the fault on the tunnel under earthquake.(5)A series of shaking table tests were conducted on reduced scale tunnel models under two steps of the loading process,fault movement and subsequent seismic excitation.A new seismic design concept of ‘‘guiding'' and ‘‘yielding'' was proposed.Two joints based on this design concept can reduce the transmission of seismic energy along the tunnel axis and clearly reduce the damage of lining.The idealized behavior—step-like deformation—appeared along the longitudinal direction of the tunnel in the two tests.No shear failure of lining was found in two model tests and the imposed shear deformation was accommodated by the joints.The seismic damage of fault-crossing tunnel is not only controlled by the fault dislocation,but also by ground motion.Fault dislocation is the main factor of tunnel damage,but seismic vibration will aggravate the damage.