Study On The Dedormation And Stability Of The Shaoguo Tunnel In Jointed Rock Mass

Because of the characteristic of anisotropy for structure and strength of joint rock mass,there exists significant difference in failure mechanism compared with the other rock masses.However,the stratums with abundant of joint fissures are commonly crossed in tunnel construction in China,the investigation on deformation and stability of surrounding rock around tunnel with numerous joint fissures is,thereby,quite important.In this paper,the numerical analyses of the Shaoguo Tunnel of the Chengchi Highway were carried out using the Discrete Element Method software UDEC,and the important results are(1)Under the condition of no supporting,the thickness of the overlying rock has a phenomenal effect on the stability of surrounding rock around tunnel with joint fissures.It is easy to lead to collapse with shallow depth of tunnel.In contrast,the pressure arch can be generated,and then,contributes to the state of self-stability when the tunnel is buried deeper.(2)The distribution of secondary stress of surrounding rock is determined by the distribution of joints after tunnel excavation.Or the pressure arch can be produced on the top of tunnel,which can transfer the gravity load of overlying rock to the both sides of the surrounding rock,which will contribute to reaching self-stability without supporting.Or the pressure arch cannot be produced because of the severe effect of unloading after excavation,which will lead to collapse at the top and both sides of tunnel because of the tensile and shear failure.(3)When the joint surfaces possess lower strength,the surrounding rock of tunnel without supporting with deeper buried depth will turn up tension,slide failure in joint surfaces,which will lead to partial or total collapse of tunnel.However,when the joint surfaces possess higher strength,the failure of joint surfaces will not occur and the self-stability state can be reached without supporting,although there are higher circumferential stresses at the top of arch of tunnel.(4)The secondary stress of surrounding rock around tunnel is largely affected by construction methods.For instance,the unloading effect of secondary stress through entire section method is obvious,which lead to the fact that the pressure arch cannot be produced easily,but lead to partial or total instability of surrounding rock located at the vault of tunnel.Side pilot tunnel method is a good way of easily forming pressure arch with strong capability of self-supporting.Therefore,the latter takes advantages over the former in terms of the total stability of surrounding rock in the vicinity of tunnel.(5)With supporting of linings,the subsidence at the vault of tunnel,the lump at the bottom of tunnel and the horizontal expansion outside at skew and side wall of tunnel can be observed by both methods separately.Additionally,the side pilot tunnel method can effectively suppress the settlement of the vault of tunnel,but can also lead to the adverse influence on the skew back of tunnel because of repeated excavation.Hence,the arch feet should be strengthened if adopting the side pilot tunnel method.(6)In comparison with the structural internal stress of primary lining and secondary lining,the primary lining satisfies the requirements of strength and deformation in tunnel engineering only,and the secondary lining plays a role of safety stock and decoration.