Design Optimization Of Shallow Buried Hydraulic Tunnel In The Soft Surrounding Rock Section Of Xinyang Water Supply Project

With the opening of the 14 th Five-Year Plan and the policy demand of "carbon peak in2030 and carbon neutrality in 2060",the construction of water conservancy and hydropower projects in China has been vigorously developed,and the construction demand of hydraulic tunnels has also increased.Hydraulic tunnels are generally in a shallow buried state,and the strength of the rock mass in the strata where they are located is weak,and the structure is complex make the project often face different difficulties.In order to improve the construction safety and stability of the surrounding rock of shallow buried hydraulic tunnels,this paper uses theoretical analysis and numerical simulation methods to study the excavation method,support parameter optimization design and reasonable support timing of shallow buried soft rock tunnels,taking the water transmission tunnel of Xinyang Sishui Tongzhi Project as the engineering background,so as to provide technical support for the design and safe construction of shallow buried hydraulic tunnels.The main research contents and achievements are as follows:(1)Aiming at the optimal design of shallow buried tunnel excavation scheme,this paper establishes a three-dimensional finite difference model of the water transmission tunnel through the shallow buried section of the V.surrounding rock by the four-water Tongzhi project,and uses the full-section method and the three-step method to simulate the excavation support of the tunnel,and compares and analyzes the development law of the four aspects of tunnel surrounding rock displacement,tunnel surrounding rock stress,initial support bolt stress and initial support spray mixed displacement under the two construction methods.The results show that the two excavation methods make the deformation of the surrounding rock of the tunnel and the force deformation of the supporting structure different,and the comprehensive analysis of the full-section method is faster in the construction efficiency,shorter in time,smaller in the influence range,and more ideal in the control of surrounding rock.(2)According to the optimization design research of shallow buried tunnel support parameters,the optimization design simulation is carried out from the spray mixture thickness,bolt diameter and bolt spacing of the initial support,and 4～5 working conditions are designed for each optimization idea,and the full-section method is selected to simulate the tunnel excavation support process,and the displacement and stress of surrounding rock under different working conditions are compared and analyzed,and the maximum deformation displacement curve of surrounding rock under different working conditions is made.The results show that all three optimization ideas will affect the stability of the surrounding rock,among which the spray thickness has a greater influence and the bolt diameter and row spacing have a smaller influence.Through simulation analysis,it can be concluded that under the premise of ensuring the stability of the surrounding rock,the spray mixing thickness is 170 mm,the bolt diameter is 20 mm,and the bolt arrangement distance is 2.0m,the safety and economic benefits can be better taken into account,which can maximize the benefits of the project.(3)Aiming at the design and research of the timing of the secondary lining support of shallow buried tunnel,by simulating the excavation of the tunnel and monitoring the bulk strain of one of the advancing surrounding rocks,the strain-time history change curve of the advancing body is obtained,and the results show that the overall trend of bulk strain decreases first and then increases,and the strain of the surrounding rock mass will expand with the disturbance of tunnel excavation,and when excavation reaches the monitoring section,the surrounding rock undergoes plastic deformation,the amplitude of body strain change increases,and secondary lining support is required.Five working conditions of 75%,80%,85%,90% and95% of the total volume of bulk strain were selected,and the remaining supporting parameters remained unchanged for numerical simulation,and the comparison and analysis of the simulation results showed that the effect of applying secondary lining to control the deformation of surrounding rock was best when 85% of the total volume of body strain.