Optimization Analysis On Construction Of Shallow Buried Biased Pressure Small Clear Distance Tunnel In Hard Mountain Area

With the rapid development of China’s national economy,tunnel engineering has developed rapidly in China because it not only shortens the travel distance of people in space,but also greatly reduces the travel time of people.This paper takes the biased small-clearance tunnel project in the shallow buried entrance section of the Shuangqiao Village of the Ehan Tunnel as the background,combines field survey data,and uses a combination of theoretical analysis,numerical simulation calculation,and on-site monitoring and measurement to study the V-level surroundings.Under the rock condition,the construction stability of the shallow buried and biased small clear-space tunnel is analyzed,and the following conclusions are obtained:(1)Based on the existing theories of the rock pressure around the tunnel with a small clear distance,combined with the bias geology of the entrance section of the Shuangqiao Village of the Ehan Tunnel,a shallow buried bias small surrounding tunnel suitable for this project is derived.The pressure calculation formula can provide a certain reference for the design and construction of such shallow buried and biased small-clearance tunnels in the future.(2)Numerical simulations of the four excavation schemes and analysis of the simulation results show that the middle-separation method and the double-side guid e pit method are both in controlling the surface settlement and the displacement of the key points of the tunnel,and the surrounding rock of the tunnel.The distribution of plastic zone is obviously better than the step method and the core soil method.From the perspective of construction,the construction method of the partition wall method is larger than that of the double-sided guide pit,the construction speed is faster,and the construction cost is relatively low.After comprehensive consideration of the actual situation,it is recommended that the partition wall method beused as the main tunnel construction scheme.(3)Judging from the simulation results of the four different excavation sequence conditions,the conditions of Case 2 and Case 4 are smaller than those of Case 1 and Case 3,both in the settlement of the vault and in the plastic area.Digging the guide tunnel on the same side of the tunnel is beneficial to further release the stress of the surrounding rock of the tunnel arch,which reduces the stress on the surrounding rock of the guide tunnel on the other side.In the end,the guide tunnel will not cause large settlement of the arch during excavation.The grey correlation degree theory is used to quantitatively evaluate the excavation sequence of shallow-buried and biased small-clearance tunnels under various working conditions.Based on numerical simulations,the displacement changes of surrounding rocks and the size of the plastic zone volume for different excavation schemes are analyzed for grey correlation degree.,And finally get the correlation degree under each working condition:Working condition 2>Working condition 1>Working condition 3>Working condition 4.The greater the degree of correlation,the higher the closeness of the evaluation condition to the ideal condition,so the excavation sequence shown in condition 2 is the optimal excavation sequence.(4)The sensitivity analysis method is used to analyze the initial supportpara meters for the surrounding rock displacements at each monitoring point,and the three sensitivity levels of the initial support to the surrounding rock displacements are ranked as follows:spray mix thickness>anchor spacing>anchor Rod length.According to the results obtained by the sensitivity analysis method,when the tunnel support parameters of the shallow buried bias inlet section of the Shuangqiao Village of the Ehan Tunnel were optimized,the support parameters were laid out in an asymmetric distribution.The simulation optimization scheme was compared with the original scheme to obtain the optimization.The later support scheme can ensure the stability of the rock mass around the tunnel during the excavation of the shallow-buried and biased inlet section tunnel,ensuring construction safety.