Treatment Measures And Stability Analysis Of The Inlet Section Of Biased Tunnel

Our country is a mountainous country,the mountainous area takes up about two thirds of the land area.In the process of building mountain tunnel,due to the influence of engineering geology,hydrology,human activities,engineering economy and other factors,the entrance section of the tunnel is usually in the state of shallow burial and bias,and the entrance section is generally fractured in lithology and rich in fissure water.Therefore,it is very important to take reasonable control measures for the stability of tunnel entrance section.The paper combines the theoretical analysis and engineering application,Mengsong 2 bias tunnel entrance construction as the backing,using FLAC3D to simulate the three regulation schemes in turn,and the regulation after the circular excavation reserved core construction method of tunnel stability analysis,content analysis including the stability of entrance section,the stability of surrounding rock and lining structure,the main research work and achievements include the following aspects:(1)The definition methods of shallow burial and bias,the stability analysis methods of tunnel entrance section and the instability criteria are summarized.(2)Through the simulation of pipe shed and surface grouting treatment scheme,it is found that the advanced support of pipe shed effectively supports the soil in the excavation area and has a good inhibition effect on surrounding rock and surface settlement.Surface grouting measures can improve the physical and mechanical properties of the surrounding rock,improve the integrity of the strata,and prevent the horizontal slip of the surrounding rock.After using pipe shed and grouting scheme,the slope,wall rock and primary support structure of the entrance section have not reached the stability requirement.(3)Through the simulation of the upper retaining pile and upper retaining+lower supporting pile regulation scheme,it is found that the upper retaining anti-slide pile mainly plays a role in reducing pressure,while the lower retaining anti-slide pile mainly plays a role in limiting position.The shear safety coefficient of the surrounding rock of the left wall does not meet the requirements of the code after the treatment with the upper retaining pile scheme,and the shear zone extending from the left wall to the slope still exists in the cloud chart of the maximum shear strain increment of the surrounding rock,indicating that there is a potential risk of instability in the project.After the treatment with the scheme of upper retaining pile+lower retaining pile,the shear zone in the cloud chart of maximum shear strain increment of surrounding rock is blocked,and the slope,surrounding rock and primary branch structure of the entrance section meet the stability requirements.However,compared with the scheme of upper retaining pile,the main stress of surrounding rock increases,so the scheme of upper retaining pile+lower retaining pile is further optimized.(4)Single factor method was used to study the influence of pile section size and pile tunnel spacing on tunnel stability in the scheme of upper retaining pile and lower supporting pile,and the optimal value of pile section size and pile tunnel spacing was given.Compared with the scheme of upper retaining and lower supporting pile,the optimization scheme is recommended because it can save 788,300 yuan in engineering cost and better meet the technical requirements on the basis of stability.