Stability Analysis Of Surrounding Rock Of Metro Tunnel In Sandy Pebble Stratum With Cavity

The strata in many areas of our country are sandy cobble strata,and the sandy cobble strata are long-term unbalanced strata.Due to the lack of cohesion in the sandcobble rock stratum,the point-to-point force will occur between the sand-cobble rocks when the formation cavity occurs,and the external factors can not interfere with the condition,the cavity in sandy cobble rock stratum has a certain self-stability.Therefore,there will be a temporary balance in the rock formation cavity,when disturbed,will lead to cavity diffusion,and upward transmission to the surface,collapse and so on.So,the stability of surrounding rock of subway tunnel built in the stratum with hollow sand and gravel is the key issue of geotechnical engineers.In this paper,the sand-gravel stratum is taken as the research object,and the sand-gravel stratum containing voids,one of the important factors affecting the stability of surrounding rock,is taken as the main research object.The stability of the surrounding rock of the subway tunnel in the sand-gravel stratum containing voids is analyzed by combining theoretical analysis and particle discrete element software(PFC2D)numerical simulation.The main results are as follows:The influence of stratum cavity on the stability and construction of excavation face is analyzed under the state of support force and unloading of tunnel excavation face.The results show that: 1)tunnel excavation and cavity failure are the main causes of ground surface settlement,and the size of cavity,the location of cavity and the net distance from cavity to tunnel directly affect the stability of surrounding rock and the size of ground surface settlement;2)the main factors affecting formation deformation,particle displacement and ground surface settlement are the size of cavity,that is,the larger the cavity is,the larger the deformation of stratum and the larger the particle displacement is,the larger the ground surface settlement;3)the size of surrounding rock stress is affected by the size of cavity,the net distance of cavity and the location of cavity,the center of cavity is the core location of the stress variation of surrounding rock,and the appearance of cavity causes the stress concentration in surrounding rock,which makes the direction of the magnitude of the principal stress deflect,especially the direction of the magnitude of the principal stress deflect most obviously in the surrounding rock near the cavity;4)the surface subsidence curve map conforms to Gauss’ s normal distribution curve map.The influence of stratum cavity on the stability and construction of tunnel excavation face is analyzed.The results show that: 1)when the support state is unloading state,because the support structure restricts the formation deformation,particle displacement and surface settlement,the larger the distance between the support structure and the excavation face,the larger the stratum deformation and particle displacement,the larger the surface settlement;2)under the support state,the stratum deformation,particle displacement and surface settlement are mainly affected by cavity destruction,the larger the cavity is,the larger the formation deformation,particle displacement and the larger the surface settlement;3)in the study of excavation face unloading,C/D=0.04～0.07,that is,when the distance between the retaining wall and the excavation face is 0.3m～0.45 m,the disturbance of the stratum is the smallest,while under the support state,the deformation degree of the stratum is related to the location and size of the cavity;4)when the cavity is above the tunnel excavation face,it has the greatest impact on the tunnel,which requires extra attention to prevent the construction and workers from harm.