| With the rapid development of highway construction in Southwest China,the application of tunnel engineering in highway construction has become more and more extensive.In view of the influence of geotechnical mechanical conditions,hydrogeological conditions and engineering geological conditions,the bias and stability of tunnel engineering have become the key to ensure the safety of engineering construction.With the development of computer software,more and more professional software has been applied in the field of geotechnical engineering.The application of numerical simulation in geotechnical engineering has become a hot issue.In this paper,Dacun Tunnel in Yuxi City,Yunnan Province is taken as an example.Combined with the specific engineering geological conditions of the tunnel,FLAC3D finite element analysis software is used to analyze the bias pressure and surrounding rock stability of the tunnel.Among them,the analysis of tunnel bias is mainly through the stress field and stress trace of the two-dimensional model.The stability of surrounding rock of tunnel is analyzed by displacement field,stress field and plastic zone of surrounding rock under two-dimensional model and three-dimensional model.The main conclusions are as follows:(1)Dacun Tunnel is an extra-long mountainside tunnel crossing the irregular piedmont slope in the northwest corner of Qilu Lake Basin.Various data such as the height difference between the tunnel and the upper Qilu Lake watershed,the horizontal distance from the slope,and the slope of the ground directly above the tunnel and the front foot of the mountain indicate that the Dacun tunnel has a certain degree of bias risk.(2)According to the survey data,the strata are generalized into 9 layers,and the elastic modulus,Poisson’s ratio and other parameters of 9 strata are determined according to the suggestions of the formation parameters provided by the existing data.FLAC3D software was used for numerical simulation.(3)Eight two-dimensional models are based on eight sections which are approximately equidistantly distributed along the tunnel axis and approximately perpendicular to the tunnel axis,which can better represent the Dacun tunnel.At the same time,eight two-dimensional models cover the entire slope from the watershed to the foot of the mountain,ensuring the rationality of the bias possibility analysis.In the two-dimensional model,the stress distribution in the tunnel surrounding rock shows obvious symmetry on both sides of the tunnel vault.In the stress trace diagram,the maximum principal stress trace is parallel to the slope near the surface,and has been deflected to the direction of gravity near the tunnel elevation.It shows that the distribution of principal stress around the tunnel is less affected by topography,suggesting that the possibility of bias in Dacun tunnel is low.(4)With the help of《Code for Design of Highway Tunnel Volume I Civil Engineering》(JTG 3370.1-2018)and《Code for Design of Railway Tunnel》(TB10003-2016),the bias problem of Dacun Tunnel is further verified.Highway tunnel design specifications usually believe that shallow tunnels may have bias problems,and deep tunnels generally do not.According to the specification,the boundary depth of deep buried and shallow buried Dacun tunnel is calculated.The boundary depth Hp=19.66 mm,and the minimum buried depth of the tunnel in the eight two-dimensional models is 53.09 m,which indicates that Dacun tunnel is a deep buried tunnel,and there is generally no bias problem.The railway design specification clearly gives the conditions for considering the terrain bias.According to the specification,the minimum thickness of the outer covering layer of the Dacun tunnel is 41 m,which is much larger than the critical value(11 m)of the specification.It can also be determined that the bias problem will not occur in the Dacun tunnel.(5)According to the definition of bias,this paper uses the surrounding rock pressure on both sides of the tunnel vault to determine the bias.Based on the research results of others,the stress value in the numerical simulation is extracted.The calculated stress ratio of the surrounding rock on both sides of the vault of the Dacun tunnel is much smaller than the critical value given by other scholars.(6)In the two-dimensional model,the order of magnitude of the tunnel surrounding rock displacement is within 10~0～10~1 mm.Although the maximum displacement value reaches 28.6 mm,it is still less than the reserved deformation(60～120 mm)of grade IV surrounding rock of three-lane tunnel stipulated in the highway tunnel specification,indicating that the self-stability of tunnel surrounding rock is good.By comparison,the displacement of the three-dimensional model is smaller than that of the two-dimensional model.At the same time,the displacement field stress field and plastic zone distribution of the three-dimensional model are in good agreement with the two-dimensional model,which further indicates the good self-stability of the surrounding rock of the Dacun tunnel and the low risk of encountering bias. |
