Collapse Mechanism And Safety Control Of Large Cross-section Tunnel

With the rapid development of tunnel construction scale and the complex and changeable terrain and geological conditions in China,tunnel collapse accidents occur from time to time.Tunnel collapse will bring grave threat to the construction safety,and cause huge economic loss and adverse social impact.The causes and prevention of tunnel collapse have aroused great concern.Therefore,it is necessary to carry out profound research on the safety problems of tunnel collapse to master the occurrence mechanism and evolution law of tunnel collapse,and formulate scientific and effective control measures which is hoped to provide basis for prevention,assessment and treatment of tunnel collapse and to improve the safety status of tunnel construction.In view of the safety problems of large cross-section tunnel collapse,this paper takes the two typical tunnel collapse accidents covering collapse caused by excavation face failure(excavation face failure collapse)and roof collapse with a certain distance behind excavation face(closed collapse)as the research object.The mechanism of tunnel collapse and safety control are studied using theoretical analysis,numerical simulation,indoor model test and field measurement.The targeted and systematic safety control measures of tunnel collapse are given,and the safety control process of tunnel collapse in soft and broken surrounding rock is put forward,which has been applied successfully in engineering.The main research work and results are as follows:(1)The typical failure models of deep buried and portal tunnel face are put forward,and the evolution mechanism of tunnel face failure is revealed.Using the finite element limit analysis numerical method,three typical models of deep-buried tunnel face failure are put forward which are forward caving failure,backward caving failure and positive extrusion failure based on characteristics of tensile and shear strength of surrounding rock and the unsupported section of tunnel.Based the the disturbance and failure evolution characteristics of the tunnel excavation to the slope in the portal section,the sliding failure model of tunnel face in the portal section is proposed.Meanwhile the theoretical model of tunnel face critical failure is established based on the upper limit method of rigid body,and critical limit load and depth of longitudinal failure are obtained.(2)The evolution mechanism of tunnel closed collapse is revealed,and the prediction methods of surrounding rock damage and tunnel roof collapse are put forward.Based on combination of macro and micro damage mechanics,the evolution process from damage to collapse of tunnel surrounding rock is revealed.Based on the strain softening model and damage mechanics theory,the prediction method of surrounding rock damage parameters including damage zone depth and damage degree is proposed.The collapse mechanism and prediction of tunnel vault in deep and shallow overburden ground are studied using the upper limit variational method.Based on the characteristics of progressive collapse of tunnel vault,the prediction model of progressive collapse is established,and the whole process curve of progressive collapse of tunnel vault is obtained.Considering the distribution characteristics of longitudinal virtual support force,the method to determine the location of tunnel collapse is proposed.(3)The safety control principle of tunnel collapse with advance pre-support and process control is clarified.In view of excavation face failure collapse accident,the three function models of the advance pipe shed are put forward which are longitudinal beam function,transverse micro-arch function and grouting reinforcement function.The analysis model and evaluation index of advance support effect of pipe shed are established respectively,and the recommended values of design parameters of pipe shed are put forward.The safety connotation of scaffold system composed of pipe shed and steel arch is clarified.The analysis model of bearing effect of the primary steel arch is established considering the foundation bearing capacity and the reinforcement effect of foot pipe anchor.In view of closed collapse accident,the bearing mechanism of pre-control and process control support measures is revealed based on the progressive collapse characteristics of the tunnel roof.The prediction method of surrounding rock load using pre-control and process control support is established,and the design parameter requirements of pre-control and process control support measures are put forward.(4)The safety control points of tunnel collapse in soft and broken surrounding rock are put forward.Based on the different causes of two typical tunnel collapse accidents,the control tasks and measures of various collapse accidents are defined.The safety control points of tunnel collapse in soft and broken surrounding rock are proposed and the core of control points is the determination of design parameters.The research results are applied to two typical tunnel projects of Beijing-Zhangjiakou high-speed railway,and the monitoring data are analyzed to verify the safety control effect of tunnel collapse.