Stability Analysis Of Middle Rock Pillar And Cross-Section Optimization For Ultra-Small Spacing Tunnels

Ultra-small spacing tunnels are increasingly used in urban subway construction,However,due to the complexity of the ultra-small spacing tunnels engineering and the constraints of natural conditions,such as geological conditions,etc.The design and construction of ultra-small spacing tunnels and other technical requirements are not perfectly enough.Therefore,it is a great significance to study the feasibility of the design and the safety and reliability of the construction of ultra-small spacing tunnels.In order to study the feasibility of the construction technology of ultra-small spacing tunnel and the stability of the tunnel in the process of construction,this paper combines theoretical analysis with numerical simulation to analyze and study the three-dimensional dynamic excavation of ultra-small spacing tunnels lining promoting stability distance,the space-time effect of double-track tunnel excavation,the optimization of cross-sections of ultra-small spacing tunnels and the stability of middle rock pillar.The main research results as follows:(1)Three-dimensional dynamic excavation simulation results of ultra-small spacing tunnels show that the tunnel vault subsidence curve obtained from the simulation excavation of left tunnel by adopting the benching tunnelling method,and that the secondary lining should be actively implemented to promote the stability of the key parts of the surrounding rock of the tunnel approximately twice the diameter of the excavation face.Through the simulation excavation of ultra-small spacing double-track tunnel,It shows that three times the tunnel diameter of the right tunnel lagging behind the left tunnel is appropriate.(2)In the optimization of two-dimensional ultra-small spacing tunnel sections,focusing on the comparative study of surface settlement,lining stress,plastic zones and deformation of key monitoring points during the dynamic excavation of three types of cross-sections(P type cross-section,Q type cross-section,R type cross-section).The numerical simulation results show that Q type cross-section should be preferred in the cross-section design of ultra-small spacing double-track tunnels.(3)Based on the generalized mechanical model of the ultra-small spacing double-track tunnel,a simplified calculation method was proposed with the load and ultimate bearing capacity of the middle rock pillar.It can be used as an important basis for judging the stability of middle rock pillar.The above results describe the time-space effect of excavation of ultra-small spacing tunnel,the optimization of tunnel section and the influence of middle rock pillar on tunnel stability.The conclusions obtained in the study are of important theoretical value to provide on-site engineering construction guidance.