Analysis And Evaluation Of Stability Characteristics Of Surrounding Rocks In Shallow Buried Tunnels In Soft Layers

In recent years,my country’s urbanization process has been accelerating,and major cities have gradually expanded to the urban fringes.Chengdu is the economic center of the southwest region,with rapid urbanization and wide range,which is echoed by the construction of a large number of underground transportation facilities.When the subway continues to extend to the edge of the city,the geological conditions become more and more complex.The influence of different geological conditions and construction conditions on the stability of the tunnel is multi-faceted.Therefore,it is of great practical significance to study the influence of different working conditions on the characteristics of surrounding rock stability in shallow tunnels.This paper takes shallow buried tunnels in Chengdu as the main research object,and uses similar model test,numerical simulation,stability evaluation and other research methods to analyze the stability characteristics of surrounding rock deformation,surface subsidence,surrounding rock stress and other tunnel stability characteristics under different working conditions.change law,and the sensitivity of tunnel stability to various influencing factors.Finally,with the help of the tunnel stability evaluation model,the influencing factors are applied to the stability analysis of the actual subway engineering,which provides a reference for the actual engineering.The main research contents and results of this paper are as follows:（1）The similar model test box,monitoring system,and data acquisition system of the sand gully tunnel were independently designed,and the test simulated the slope and back of the sand gully tunnel（the surface inclination was set to 15°,30°,and 45°,respectively）.During the slope excavation process,the variation law of parameters such as tunnel surrounding rock stress and formation deformation was analyzed.The experimental results show that the earth pressure of the vault is the largest,and it increases with the increase of the surface inclination angle,and the back slope excavation is smaller than the surface slope excavation.The average growth rate of vault pressure was 27%when the surface inclination was changed from 15° to 30°,and 41% when the surface inclination was changed from 30° to 45°.Compared with face slope excavation,the vault pressure of back slope excavation is reduced by 7.9% on average.The stratum deformation mode can be roughly divided into abrupt type and gentle type.The sudden type is that the soil displacement increases suddenly at the tunnel excavation section,and the gentle type is that the soil layer displacement starts to increase at the tunnel excavation section,and then the rate of increase gradually increases.decrease until stable.（2）The stability characteristics of tunnels with different cross-section shapes excavated in gully terrain are analyzed by ABAQUS simulation,and the surface settlement value is mainly studied.Along the tunnel excavation direction,it is concluded that the surface subsidence curve is a parabola through data fitting,and the reduction rate of the surface subsidence value decreases with the increase of the excavation depth.（3）Establish a large number of finite element models with the geological conditions of Chengdu as the background,and analyze the influence of the section shape,section size,soil layer boundary on the characteristics of the tunnel stability,and the stress characteristics of the lining.Orthogonal tests were designed to study the sensitivity of the stability characteristics to the influencing factors in the project,and the range analysis of the test results was carried out and the conclusion was drawn: the support type of the tunnel,the size of the tunnel section and the burial depth are the key factors affecting the stability of the tunnel.（4）Combine the existing research results,collect a large amount of literature and engineering data,determine the index levels of the factors affecting the stability of the surrounding rock of the shallow tunnel,and select the tunnel excavation span,burial depth,surface inclination,groundwater,management and technical level.,excavation method,section shape,average annual rainfall as evaluation indicators.The entropy weight method is used to determine the weight of each index,and the stability evaluation of the tunnel between Longhuasi Station and Long’an Station of Chengdu Metro Line 13 is carried out in combination with the extension matter-element model.Compared with the measured data of the project and the situation on site,the evaluation result is consistent with the actual situation of tunnel stability.