Research On Deformation Characteristics And Mechanism Of Soft Rock In Wenbishan Tunnel Under High Ground Stress

In the construction of tunnels,the problem of large deformation of weak surrounding rock tunnels is increasingly prominent,and its safety design and construction have become a major problem in tunnel engineering construction.Since the excavation and support of tunnel surrounding rock is an extremely complex stress redistribution process,large-scale crushing deformations have occurred in some high-stressed weak surrounding rock tunnels at home and abroad.If the tunnel surrounding rock is deformed and stressed,If the rules are not understood or the supporting structure is unreasonable,the tunnel will often undergo excessive deformation and increase the risk of tunnel construction.Therefore,for weak surrounding rock under poor geological conditions and high in-situ stress,it is important to study the deformation characteristics and mechanism of surrounding rock after tunnel excavation and support.In view of the difficulties faced by the current high-stress soft rock tunnel construction process,based on the Wenbishan Expressway Tunnel in Lijiang,Yunnan,this paper combined the initial in-situ stress measurement data and monitoring data to study the deformation and surrounding rock of the surrounding rock.According to the measurement results,the damage of the tunnel during the construction process was analyzed,and the characteristics of the tunnel’s in-situ stress field and the monitoring and measurement results of the typical tunnel cross section were analyzed.Through theoretical analysis,the load-bearing mechanism and characteristics of the tunnel’s typical deformation large-section plastic zone are studied.The main conclusions are as follows:(1)The initial in-situ stress test of Wenbishan tunnel was carried out by small hydraulic fracturing method,and the in-situ stress characteristics are as follows: Shmax>Sv>Shmin,the maximum horizontal principal stress is 8.78 MPa,The maximum lateral pressure coefficient is approximately Shmax/Sv=1.55.Combined with the rock strength parameters of the tunnel and in accordance with my country’s current in-situ stress classification standards,it is determined that the excavation section of the Wenbishan tunnel is in a state of high in-situ stress or extremely high in-situ stress.(2)Through the analysis of the displacement and pressure results of the surrounding rock monitored at the tunnel site,it is found that the Wenbishan Tunnel has the characteristics of large displacement of the surrounding rock,high deformation rate,long convergence time,and various forms of deformation and failure,etc.Tectonics,surrounding rock lithology,groundwater conditions,and on-site construction conditions are responsible for the large deformation of the surrounding rock of the tunnel.(3)The surrounding rock parameters of the typical section of large deformation of Wenbishan tunnel were studied by displacement inverse analysis method,and the positive parameters of the surrounding rock to be inversed(elastic modulus E,Poisson’s ratio ν,cohesive force c,internal friction angle φ)were selected and determined.Based on the combination of tests,a BP neural network training model for inversion of tunnel surrounding rock parameters under strain softening conditions is established,and the surrounding rock parameters of typical sections with large deformations are obtained from the inversion,which provides parameter basis for the subsequent theoretical analysis of tunnel deformation mechanism.(4)Introducing the rock mass strain softening theory and the tunnel excavation stress redistribution theory,the theoretical formula that can reflect the change of the load tunnel after the tunnel excavation is derived,and the surrounding rock parameters obtained by inversion are used to study the surroundings of the typical large deformation tunnel The rock load mechanism is compared with the field rock pressure measured on site.It is found that the theoretical load of the load under strain softening is closer to the true load of the surrounding rock of the tunnel,which is then calculated based on the tunnel excavation support theory under strain softening.The radius of the plastic zone of the typical section with large deformation has grasped the plastic failure characteristics of the tunnel surrounding rock.(5)Based on the mastery of the large deformation characteristics and deformation failure mechanism of the Wenbishan Tunnel,adjust the step excavation method,change the advanced support type,increase the amount of reserved deformation,change the grouting method,and increase the initial support strength,etc.Large deformation control technology.The subsequent construction site conditions show that the large deformation of the tunnel is successfully controlled.This deformation control technology can provide a basis for subsequent construction and can provide a certain reference value for similar tunnel engineering large deformation control research.