Research On The Excavation Method And Support Parameters Optimization Of Shallow Buried Small Clear Distance Tunnel

In practical projects,due to the influence of terrain conditions and other factors,the double-hole tunnel can not meet the construction requirements,so it is often necessary to adopt a small distance tunnel construction scheme,which is prone to collapse during excavation and affects the construction safety.This paper,taking Hongjiaban tunnel of an expressway as engineering background in Jiangxi province,using the method of theoretical analysis and numerical simulation calculation of shallow buried small clear distance tunnel excavation methods and supporting parameters optimization and security footage of rock blasting excavation research,the results for the shallow buried small clear distance tunnel design and construction to provide theoretical reference.The main work content is as follows:(1)Comparison and Optimization of construction and excavation scheme of shallow buried small clear distance tunnel.In this paper,Hongjiaban tunnel is taken as the engineering background.established the shallow buried tunnel entrance section of the three dimensional finite element model of the whole section method is adopted,the reserved core soil step method and the cross middle wall method(CRD)of three kinds of excavation method to simulate the tunneling process,analysis of the three kinds of excavation of the tunnel surrounding rock and lining stress and deformation of supporting structure law of development.The results show that the method of reserving core soil steps is more ideal in the control of confining pressure horizontal displacement,and it can shorten the construction period and reduce the cost of the project,and is more suitable for the construction and excavation of the entrance section of Hongjiaban tunnel.(2)Optimization of initial Supporting parameters of shallow buried small clear distance tunnels.The method of reserving core soil step is used to simulate the confining pressure excavation process of the entrance section of Hongjiaban tunnel,and the initial support parameters such as the diameter and length of bolt,the thickness of shotcrete and the type of steel arch are numerically simulated and optimized,and the displacement and stress variation of surrounding rock under different working conditions are compared and analyzed.Through the analysis of the calculation results,the optimal initial support parameter combination for the opening section of Hongjiaban tunnel is suggested as follows: the diameter and length of the bolt shall be 28 mm and 2.5m respectively;The thickness of shotcrete is 180 mm,and the steel arch frame model adopts I120 b I-steel.(3)Study on safety footage of blasting excavation of shallow buried small clear distance tunnel.The finite element blasting excavation calculation model of the opening section of Hongjiaban tunnel is established by using the optimized support structureparameters,and the influence law of blasting excavation of the rear tunnel on the initial support of the first tunnel and the vibration velocity at the key position of surrounding rock under different excavation scales is studied.The calculation results show that the influence of the horizontal vibration velocity of surrounding rock of the tunnel opening is greater than that of the vertical vibration velocity,and the influence of the blasting side of the tunnel opening is greater than that of the backblasting side.In combination with the requirements of relevant regulations,it is recommended not to exceed 1.5m for the safety progress of the excavation of the backblasting hole at the entrance of Hongjiaban tunnel.