| Bias pressure refers to the phenomenon of asymmetric distribution of surrounding rock pressure along the centerline of the tunnel section caused by topographic,geological,or construction,which can lead to greater bending moment internal force and stress concentration in the lining structure.In mountain tunnels,significant topographic biases are often encountered at shallow burial sites due to the influence of mountain slopes.When the surrounding rock here is an inclined layered rock mass,the tunnel will also be subjected to geological bias caused by the sliding of the rock mass along the inclined plane after excavation.It is clear that this mechanical behaviour of the tunnel after excavation under the combined effect of topographic and geological bias is much more complex than when it is influenced only by a single topographic or geological bias.Currently,research on the mechanical behavior of biased tunnels mainly focuses on topographic biased tunnels,and there is relatively little systematic research on the deformation and failure characteristics of layered rock tunnel excavation under topographic biased pressure.For layered rock,due to the differences in structure and mechanical properties between each rock layer,as well as the uncertainty of the interlayer medium,their strength and deformation have a high degree of anisotropy,which is significantly different from the surrounding rock considered as isotropic and homogeneous in the study of only considering topographic bias.In addition,the layered rock will experience slip and dislocation after tunnel excavation,and directly using traditional continuous medium methods for simulation analysis also has certain limitations.In view of this,this paper first proposes a discretization simulation method based on continuous medium element,and then establishes a model,carries out numerical calculation and combines theoretical analysis to carry out the research on the excavation mechanical behavior of tunnels with underlying surrounding rock affected by topographic bias from the aspects of excavation deformation characteristics,failure modes and main influencing factors.And based on this,optimization analysis was conducted for anchor support.Finally,demonstration and supplementation were conducted through typical engineering.The conclusions obtained can provide reference for the design of excavation and support technology for layered rock mass tunnels.The main research work and achievements of the paper are as follows:（1）Based on continuum media element simulation,an interface element based discretization numerical simulation method for continuum media is proposed,and its accuracy was verified by comparing with the model test results.This method utilizes the advantage of plastic strain being able to remember and describe the development and failure evolution process of plastic zones in the model,and proposes to judged the failure of surrounding rock elements through plastic strain;By adding interface elements after the failure of the surrounding rock,the tensile and shear detachment of the solid elements is achieved,achieving precise simulation of the deformation and fracture process of the layered tunnel surrounding rock after excavation,which is more accurate than traditional use of plastic zone to judge the failure of the surrounding rock.（2）Considering four influencing factors such as the thickness of the side cover thickness（t）,a total of 80 models were established to analyze the excavation deformation and failure characteristics of the tunnel under different working conditions.The results show that changes in the side cover thickness and interlayer thickness only change the size of the displacement field and do not change the displacement trend,and the side cover thickness has the smallest impact on tunnel deformation;There is always a large deformation in the arch shoulder of the shallow buried side of the tunnel.（3）Based on the simulation results of the generalized model,the failure types are divided into five types according to different stress control and different occurrence positions,such as inclined plate tensile failure and side wall tensile spalling failure.The statistical results of failure types under all working conditions show that multiple failure modes often coexist,not a single failure;among all the failure types,the proportion of side wall cracking and spalling failure is the highest,followed by inclined plate tensile failure,but the side wall cracking and spalling failure has little effect on the instability and collapse of the tunnel.In view of this,the theoretical analysis method is used to simplify the rock mass into a cantilever beam model to analyze the tension failure of the inclined plate.（4）The theoretical analysis method is used to simplify the rock stratum into a beam model,and the bolt support effect is analyzed as the change of interlayer bonding force.Then,FLAC3Dsoftware is used to simulate the bolt support under typical vault tension failure conditions and typical spandrel slip failure conditions.By comparing the effect of unreinforced and symmetric anchor reinforcement,the reinforcement optimization direction is given.On this basis,the asymmetric anchor layout scheme is proposed,and the supporting effect of asymmetric anchor is analyzed from different anchor lengths and reinforcement ranges.（5）Taking Songjieya-ⅡTunnel as a typical engineering example,the simulation calculation conclusions were verified and supplemented by analyzing the support effects of symmetric support,conventional asymmetric support,and the optimized asymmetric support scheme in this paper. |
