Study On Squeezing Large Deformation Mechanism And Control Technology Of Phyllite Tunnel

Tunnel excavation in fractured soft rock stratum with high geostress can lead to squeezing large deformation,whose performance is that the deformation of the surrounding rock is large,the deformation time is long,and the supporting structure is strongly deformed and even destroyed,which seriously affects the duration and long-term stability of project,and is one of the major problems in the world tunnel construction.The objective factor of these problems is the existing environment of the tunnel.The soft rock under high geostress environment has poor self stability and remarkable rheological characteristics;The subjective factor is a lack of knowledge of deformation mechanism and evolution law of this kind of surrounding rock,and the control technology is lacks of systematic theoretical guidance.In view of the above problems,based on the Maoxian tunnel of Chengdu-Lanzhou railway which is under construction,this paper studies surrounding rock engineering characteristics of the large deformation tunnel,squeezing large deformation mechanism,rules of surrounding rock deformation and supporting stress,and control technology for deformation.The main conclusions are as follows:(1)According to the investigation,it clears that the formation lithology of soft rock through by Maoxian tunnel is mainly sericite phyllite,where the environmental is development of geological structure,complex crustal stress.The rock joints and fissures develop and the rock mass plane intersects the tunnel axis at a large angle.The tunnel,with deep burial depth,high geo-stress and weak surrounding rock,is severely squeezed.Based on the geological evaluation,it analyzes the mineral composition,water physical properties and deformation characteristics of sericite phyllite,and further studies the rheological properties of Sericite phyllite,and the analytical solutions of rock rheological tests based on unified rheological model are derived.Combining the BP neural network of MATLAB,an identification method of rheological model parameters is proposed,which can realize the multi parameter identification of complex rheological model.(2)Through the investigation and field test,it summarizes that the main features of phyllite tunnel with large compressive deformation is the large deformation of surrounding rock,fast deformation rate,long duration,wide broken range of surrounding rock,tunnel face extrusion phenomenon prominent,and supporting structure and various forms of destruction.The main factors leading to these characteristics are complex geological structure,high in-situ stress,development of rock mass structure,low strength of surrounding rock,obvious rheological characteristics of surrounding rock and water.Through theoretical analysis,based on the Burgers model,the visco elastic plastic solution of circular tunnel is derived,and combined with numerical calculation and loose ring test of the surrounding rock,the failure and large deformation mechanism of surrounding rock is analyzed.(3)Through field test,the development rules of surrounding rock deformation and support force is studied and the action mechanism of anchor and steel frame in extruding large deformation tunnel is analyzed.Use '"self-adaptive modeling method of support structures" proposed in this paper,to make numerical calculation for the effect of double layer support in tunnel,which further analyzes the control effect of double layer support system on the rheological behavior of surrounding rock.For the squeezing large deformation tunnel,use "space" instead of "time" to release displacement in prophase and reduce the influence of the rheological pressure on the supporting structure,and use the means of "yielding combined with resisting,flexible combined with rigid,bolt-grouting and surrounding rock common loading" to control the large deformation.(4)Comparative tests of initial supports are carried out through the test hole at 1:1 and the monitoring of the surrounding rock deformation and supporting force was performed for 900 days.The effects of different forms of steel frame and bolt on the surrounding rock was studied.The paper summarized the mechanism of the double-layer sprayed steel frame and the combination of the long and short bolts,and put forward the soft rock tunnel extrusion and large deformation control technology,including the method of stratifying and dividing the steel frame,the combination of the long and short bolt with the short applied at an early stage and the three-step excavation method with reserved core soil and the tunnel face reinforced with grouting.(5)Based on the "surrounding rock characteristic curve",through the theoretical analysis and numerical calculation,"Surrounding rock characteristic surface" considering the rheological characteristics of surrounding rock is put forward and calculated.Based on the "surrounding rock characteristic surface",a multi-layer support analysis method considering the time and space effect is proposed and it is used to analysis the long-term stability of supporting structures for large deformation tunnel in squeezing soft rock.