Research On The Deformation And Failure Characteristics Of Steep Dipped Bedding Surrounding Rock In The Tunnel Exit Section And Its Engineering Application

Steep dip bedding rock mass is an unfavorable rock mass with low structural plane bonding strength and high risk of instability.The excavation disturbance of a tunnel crossing a steep bedding rock slope will break the balance of the original fragile geo-stress environment and cause the slope failure.The complex conditions of steep dip and bedding layered surrounding rock will affect the structure of tunnel entrance.Therefore,it is necessary to study the deformation and damage characteristics and treatment techniques during the excavation process of the tunnel passing through a steep bedding slope.This article is based on the Chenjiaping tunnel exit section of Tongxin Expressway,and establishes a three-dimensional simulation geological model using PLAXIS 3D software to simulate the entire dynamic process of tunnel passing through steep bedding rock mass exit.Based on the deformation and failure characteristics of rock mass obtained from numerical analysis,the effectiveness of different construction methods and control measures is studied,and a "zero excavation" exit construction system is designed.The main research content of the paper is as follows:(1)By analyzing and summarizing the typical failure mode and control mechanism of the surrounding rock with steep dip bedding,the failure types and key influencing factors of the surrounding rock of the slope are obtained,the construction measures of tunnel exit through steep bedding surrounding rock are suggested.(2)Through finite element numerical simulation calculation,this study investigates the deformation characteristics and failure modes of rock mass during the entire process of excavation using three construction methods: step method,reserved core soil method,and CD method.Compare and select the construction method with the best quality.The final decision is to use the reserved core soil method for the exit of the Chenjiaping Tunnel.(2)Through finite element numerical simulation calculation,the reasonable blasting charging structure and maximum segmented charging amount for millisecond blasting construction are studied.The research results show that the disturbance of the air uncoupled charging structure is the smallest,and the maximum allowable blasting charge for tunnel excavation under steep inclined bedding rock conditions is 5 kg.(3)One of zero-excavation construction system: millisecond blasting construction.On the basis of summarizing the research on the deformation and failure characteristics of rock masses in the research area,aiming at the ultra shallow buried areas of tunnels that are not suitable for blasting excavation,through analysis of past tunnel engineering construction experience,reasonable construction equipment is selected and the construction process is designed.(4)The second part of zero-excavation construction system: select reasonable support measures.By simulating tunnel construction under different support control measures,the reinforcement effect of support control measures on tunnels crossing steep bedding rock slopes is studied.The research results show that using single-layer advanced grouting,the total displacement of the arch crown and arch waist measurement points is 1/12～1/4 and 1/6～2/3 of the non grouting condition,respectively.The displacement generated by using double-layer advance grouting is significantly smaller than that of single-layer advance grouting,ensuring the stability of tunnels in ultra-shallow buried areas.Using radial grouting,the displacement of the side walls on both sides is 1/5-1/3 of the displacement of the original support combination side walls.(5)Based on the engineering application and site monitoring analysis of chenjiaping tunnel,from the angle of tunnel deformation law and stress development trend as well as tunnel structure stability,the feasibility of zero excavation tunnel construction system is verified.