Study On CRD Sequence Optimization And Surrounding Rock Deformation Of A Shallow Buried Biased Tunnel In Chongqing

In tunnel construction projects,the in stability of surrounding rock i s the main cause of engineering accidents.In the process of tunnel construction,the mechanical properties of surrounding rock are affected not only by natural factors such as rock formation conditions and geological conditi ons,but also by tunnel excavation methods,support types and support parameters to a great er extent,which makes people pay more and more attention to seeking reasonable excavation methods.Based on a shallow unsymmetrical pressure tunnel project in Chong qing,combined with the actual construction scheme a nd local environment,this paper systematically analyzes and studies the deformation of surrounding rock of the tunnel.The specific contents and main conclusions are as follows:(1)according to the tunn el construction situation,the monitoring scheme,th e arrangement position of measuring poi nts and the measurement method are formulated.by analyzing the field monitoring data,it is determined that the tunnel should be excavated by CRD method(crossing m iddle wall method),rather than by three steps and seven steps.(2)By analyzing the monito ring data of the tunnel excavated by CRD method,the basic characteristics of surrounding rock affected by excavation of each branch are obtained.The results show t hat the vault subsidence increm ent caused by excavation of each branch has a certain propor tional relationship with the total vault subsidence.In terms of horizontal convergence,with the excavation of section(4),the horizontal convergence trend in the tunnel of AC line(1)and BC line(4)is basically opposite,which is because the horizontal convergence changes of sections(1)and(4)are mainly caused by the displacement of the intermediate wall of temporary support.(3)In order to determine the reasonable excavation sequence of tunnel,fin ite element software Midas/GTS NX is used to model four schemes: right hole first,CRD method first excavating right and right holes first,CRD method first excavating left and rig ht holes first,CRD method first excavating right and left holes first and C RD method first excavating left side.By comparing displacement,stress and plastic zone of surrounding rock,it is determined that the reasonable excavation sequence is right hole first and CRD method first excavating left side.(4)In this paper,GM(1,1)grey model and BP neural network model are used to predict the deformation of tunnel surrounding rock.By analyzing and discussing the forecasting examples,the characteristics an d applicability of the two forecasting methods are expounded.The GM(1,1)gr ey model is characterized by its fixed and simple model,which requires less data samples,but its prediction effect on volatility data is not ideal.BP neural network has good per formance for nonlinear system prediction,but it requires a large number of data samples with wide representativeness,which is sometimes difficult to meet in practice.Generally speaking,the prediction accuracy of BP neural network model is higher.It is more appropriate to use BP neural network model to predict the tunnel proje ct during construction,and GM(1,1)grey model can be used as a supplementary means when the data is insufficient.