Research On The Application Of Pilot Heading Method In Large Deformation Control Of Squeezing Rock Tunnel

In the construction of squeezing surrounding rock tunnel,large tunnel section,poor self-bearing capacity of surrounding rock,fast deformation speed,and support can not be closed in time will lead to large deformation of tunnel,support structure damage,surrounding rock intrusion limit,and seriously affect the safety and schedule of tunnel construction period.In order to effectively implement some special tunnel construction methods for this kind of squeezing surrounding rock tunnel deformation,such as double-layer support method and pilot heading method,it is necessary to deeply study the deformation control mechanism of various methods and clarify their applicable conditions on the basis of accumulating engineering practice.This paper relies on the engineering practice of large deformation section of Yuelongmen tunnel of Chenglan Railway.Firstly,it analyzes the elastic-plastic theory of surrounding rock softening,then uses ABAQUS three-dimensional finite element simulation technology to analyze and compare the longitudinal distribution characteristics of deformation,stress release and deformation velocity after the excavation of squeezing surrounding rock tunnel.Then,numerical simulation with pilot heading tunnel,non-pilot tunnel and double-layer support is carried out to study the effect of pilot heading on deformation control.Finally,based on the results of field monitoring and numerical calculation,the stress and deformation characteristics of pilot heading and double-layer support are compared.The main research results are as follows:(1)It is assumed that the cohesive force of surrounding rock decreases with the increase of plastic strain.Based on the elastic-plastic analysis of surrounding rock softening and the calculation results of finite element 3d model,it is found that the softening strength of surrounding rock has a certain effect on the longitudinal distribution of deformation,stress release and deformation velocity,and it increases markedly with the decrease of the strength-stress ratio.(2)Based on the results of numerical simulation and the analysis of elastic-plastic theory,the longitudinal distribution characteristics of tunnel deformation,stress release and deformation velocity under different strength-stress ratio are obtained.Through the comparison of the longitudinal distribution curves of deformation and stress release,it is found that the proportion of stress release at the palm surface increases and the proportion of deformation has no obvious change.(3)Through the comparison and analysis of the calculation results of the simulated working conditions with or without pilot heading.it is found that pilot heading construction method has better control effect on the deformation stability and plastic zone size of surrounding rock.In particular,the inhibition effect is more obvious on the initial speed of tunnel convergence deformation and the bulging deformation of the tunnel face,they respectively fell by 23% and 31%.(4)Combined with the large deformation monitoring data of Yuelongmen tunnel,and compared the numerical simulation between the double-layer support and pilot heading construction method.The results show that the control effect on the final deformation of tunnel is similar,but the pilot heading method has better control effect on the initial deformation velocity and the bulging deformation of the tunnel face.