Analysis Of Surrounding Rock Pressure And Stability Of Tunnel Passing Through Fold Broken Zone

The calculation of surrounding rock pressure is one of the most important contents in tunnel design,which involves tunnel stability analysis and support structure design.With the rapid development of highway,railway and other infrastructure in western mountainous areas of China,a large number of tunnel projects across mountain areas have appeared.At this time,due to the existence of bad geology such as fold fracture zone in many mountainous areas,the tunnel is more likely to occur engineering diseases such as surrounding rock collapse,roof collapse,support structure damage and so on.At the same time,the pressure calculation and stability analysis of surrounding rock under bad geological conditions are more complex than that of stable rock,so it is of great practical significance and theoretical value to carry out the pressure and stability analysis of surrounding rock under the condition of tunnel crossing the fold broken zone.This paper is based on the project Chengbei Tunnel occurred in Hunan Feng Huang Maglev Project through fold fracture zone,in situation of the special geological conditions of fold fracture zone,the tunnel surrounding rock pressure calculation method and its influencing factors were studied through theoretical way,the stability of surrounding rock and th e choice of the reasonable construction strategies were got in combine with numerical simulation,Finally,the rationality of surrounding rock pressure and stability analysis is verified by field monitoring method,which provides a reference for the construction desi gn of supporting projects.The main research contents of this paper are as follows:(1)Based on the engineering characteristic that the structural plane is the weak part in the rock strata in the broken zone,a failure mode of surrounding rock for tunnel crossing the broken zone is proposed considering the deformation characteristics of the surrounding rock in the broken zone.Th is failure mode of surrounding rock can fully consider the compound failure of the rock-soil materials overlying the tunnel with multiple sliding cracks.On this basis,the expression of surrounding rock pressure with different slip plane cross center point(hereinafter referred to as center point)in overlying rock mass is obtained combining with Hoek-Brown strength criterion and the limit analysis upper bound method.Compared with existing theory method and field monitoring shows that the failure mode is rational,the comparison of surrounding rock pressure and failure mode at different central points shows that: a)With the upward movement of the center point,the surrounding rock pressure decreases continuously,and the decreasing ratio of surrounding rock pressure increases with the upward movement of the center point;b)As the center point moves upward,the rock mass loose zone in failure mode shows an inward shrinking trend,and the shrinking trend becomes more obvious with the upward movement of the center point.In addition,the failure mode of tunnel surrounding rock and the influencing factors of surrounding rock pressure under different central points are deeply analyzed,and a general variation law is obtained.(2)According to the actual situation of the tunnel crossing the fold broken zone in the supporting project of Hunan Feng Huang Maglev Cultural Tourism Project,the three-dimensional numerical modeling is completed.Then,based on the above model,the Flac3 D computing software was used to simulate the excavation process of the exit section of Chengbei Tunnel by using the three-steps method and the middle-hole method(CD method)respectively.In the numerical calculation,the fold fracture zone was regarded as the rock layer subject to Ubiquitous-Joint constitutive relation.Numerical results show that: a)With the tunnel’s excavation,the vertical displacement of surrounding rock gradually decreases from the vault to the arch waist,the area from the bottom of the tunnel to the arch waist is mainly uplifted displacement,and the arch waist is mainly horizontal convergence displacement,and the surrounding rock displacement will be affected by the fold fracture zone,so that the displacement of the side near the fracture zone will be larger than the other side.However,this phenomenon is affected by the timing of supporting structure too;b)The stress trace of surrounding rock of the tunnel will be obviously deflected during excavation,which is especially obvious after the excavation of each region is completed.Because the Chengbei tunnel is not affected by bias,the stress trace of t he tunnel is relatively smooth after excavation;c)The plastic zone is mainly distributed in the surrounding rock within a certain range around the excavation tunnel,and the influence of the plastic zone has a strong timeliness.In addition,compared wit h the final numerical displacement data of the three-steps method and the CD method,both of them meet the requirements of the criterion.Considering the construction cost,progress and site environmental factors,it is considered that the three-steps method is safe and reliable for the construction of Chengbei tunnel.(3)Based on the field monitoring results of Chengbei Tunnel,the spatial distribution curves of contact stress between surrounding rock and initial support during tunnel excavation,the settlement displacement curves of tunnel vault and the relative horizontal convergence displacement curves of tunnel circumference are obtained by collating the field monitoring data.The comparison between monitoring data and theoretical results shows that: a)The measured value of surrounding rock pressure is close to the theoretical value of the vertical component,but the measured value of the horizontal component is greater than the theoretical value due to the influence of the lateral pressure coefficient;b)The spatial distribution of surrounding rock pressure is smaller in the arch and larger in the arch shoulder,showing a "cat ear" distribution.The local stress release caused by the fold fracture zone leads to the asymmetry of surrounding rock pressu re distribution;c)The measured displacement data are bigger than the displacement of the numerical simulation results,the reason is that the supporting intensity in the process of numerical simulation meet the design strength value instantaneously,and the intensity of the actual support is growing over time,at the beginning of the monitoring,common deformation of surrounding rock and supporting structure will cause a larger displacement,but all the displacement monitoring data can satisfy the requi rements of criterion.