Research On Control Technology Of Large Deformation In Construction Of Double-track Railway Soft Rock Tunnel

With the continuous development of China's economic construction and the continuous improvement of transportation,the railway construction in the southwest has entered a peak period.Due to the complex topography and special geological conditions,deep and long tunnels that still pass through high ground stress and weak rock layers in the surrounding area are still emerging.Among them,the problem of large deformation of the weak surrounding rock tunnel is a serious problem that causes construction difficulties,low construction efficiency,and increased economic burden on the project.In many tunnels such as the Lan-Yu Railway and the Cheng-Lan Railway that have been built,problems such as steel frame distortion and spray-mixing spalling caused by large deformation have occurred,which have seriously affected construction safety and progress.Therefore,it is necessary to study the construction control technology of soft rock large deformation tunnels under high ground stress conditions.This paper relies on the Chenglan Railway Yuntunbao Tunnel project,using theoretical investigation analysis,on-site mechanical tests and numerical simulation to analyze the large deformation characteristics and failure mode of the double-track railway soft rock tunnel,from the perspective of excavation method and support system The construction control technology for large deformation of soft rock tunnels is studied.The specific contents are as follows:(1)Based on the Yuntunbao tunnel,the large deformation characteristics of the tunnel are summarized as high initial deformation rate,large total deformation,long deformation duration and short self-stabilization time.Mainly affected by high ground stress,stratigraphic lithology,geological structure,engineering disturbance and groundwater.(2)Different construction methods have different control effects on the deformation of the surrounding rock of the tunnel,which is represented by the three-step reserved core soil method> three-step method> two-step method;the settlement value of the vault increases with the increase of the initial support closing distance;the same Under the condition of the initial support closing distance,as the length of the steps increases,the settlement of the vault gradually increases,and the horizontal convergence of the side walls gradually decreases.(3)Through theoretical analysis and on-site monitoring,it was found that the use of advanced pipe roof pre-support can effectively control the settlement of the vault and ensure the stability of the palm face.The use of double-layer initial support,extra-long anchors and the back grouting technology can effectively control the large deformation of the Yuntunbao tunnel,and fundamentally solve the problem of initial support damage and erosion.(4)Through the on-site comparison test,the analysis found that the contact pressure between the primary support and the surrounding rock is slightly greater than the threestep method under the two-step method.The internal force of the arch is greater than the internal force of the arch under the three-step construction method,but the arch position is opposite;under the construction conditions of the three-step method,the deformation of the surrounding rock is smaller than that of the two-step method,and the total deformation is about 51%-60% of the two.