Research On Dynamic Response And Aseismic Or Shock Absorption Measures For Tunnel Portal Structure At Interface Between Hard And Soft Rock

It’s common for tunnel entrance passing though surrounding rock with abrupt change in ground conditions, such as the hard and soft rock interface, while the investigation of tunnel damage in Wenchuan earthquake indicates that tunnels suffer severe damage in these areas. Therefore, the research on the seismic response and aseismic or shock absorption measures of tunnels on such conditions is of great engineering significance.In this thesis, the LongXi tunnel, embedded in the Chengdu-Wenchuan highway, with an entrance passing through distinct geologic media of varying stiffness, is taken as a field case. By using the numerical simulation method, the dynamic response of tunnel lining are analyzed. Then, the aseismic and shock absorption measures are studied on such basis. In the end, a suitable aseismatic measures for the tunnel passing through zones with abrupt changing in ground conditions is put forward. The main research work and achievements of this thesis are as follows:(1) Collecting and sorting materials about the historical earthquake damage to mountain tunnels, through which the importance of seismic fortification when tunnel passing through zones with abrupt changing on surrounding rock mass such as the hard and soft rock interface is clear.(2) The dynamic response of lining structure is studied on the different soft-hard interface dip Angle and position. According to the study, when tunnel passing through the hard and soft interface, the lining structure at hard and soft interface area will produce large deformation and stress; When tunnel depth is30m, the portal slope Angle is45o and the hard-soft interface around20m from the portal,the lining structure deformation and stress where at interface position increases with the reduction of interface dip Angle within a certain range; When tunnel depth is30m, the portal slope Angle is45o and the hard-soft interface dip Angle is45o,the lining structure deformation and stress first increases then decreases with the distance of the soft-hard interface increases, when the distance from the portal is more than80m the deformation and stress of the lining has little change and the lining is of the most unsafe when the hard and soft rock interface posited about40m away from the portal.(3) To get the aseismic and shock absorption effect of whole ring grouting and seismic joint when the tunnel cross the junction of hard and soft rock, a new series of simulation is taken and the results show that the whole ring grouting has a better effect on controlling the lining deformation but works not obviously in reducing principal stress of lining, while the exiting seismic joint on the lining obviously decreases the maxima principal stress but increases the deformation at the interface position.(4) In the end of the thesis, a new measure for the tunnel passing through zones with abrupt changing in ground conditions called’gradual grouting’is put forward,in order to make the surrounding rock quality changes step by step and the seismic effect of it is validated. The numerical analysis results show that’gradual grouting’in the deformation and stress of lining is more advantageous than common grouting and seismic joint setting. So this measure is suitable for tunnels passing though zones with abrupt changes on surrounding rock mass, such as the hard and soft rock interface.