Failure Mechanism Ahead Of Tunnel Face And Safety Assessment Of Unstable Tunnel Surrounding Rock

" Unstable surrounding rocks " refers to the surrounding rocks with null "self-stability time" at the present scale of traffic tunnel. During the construction process, this kind of surrounding rocks is extremely easy to occur failure ahead of tunnel face, sometimes accompanied by great deformation ahead of tunnel face. However, due to the different processes, mechanisms and degrees of the failure, the impacts of tunnel construction safety are very different under different conditions of unstable surrounding rock. Therefore, it is very necessary to carry out safety assessment of unstable surrounding rock, which is not only the basis for the selection of tunnel construction project, but also the basic work of engineering construction management.In this paper, the safety assessment of unstable surrounding rock is systematically studied. First, a prediction method of failure zone is presented based on the study of the mechanism and modes of the failure ahead of tunnel face. Then influence factors of the failure are analyzed, which provide a basis for the safety assessment of unstable surrounding rocks. Based on the study of the load characteristics of unstable surrounding rocks, the necessity of taking advancing deformation into safety assessment is clarified. Finally, the evaluation index of reinforcement feasibility and the method of the safety assessment of unstable surrounding rocks is presented. The main work and research achievements are as follows:(1) The evolution rule of advancing damage of extremely unstable surrounding rock is studied. From the failure character and form of surrounding rock point of view, the basic model of advanced failure of the extremely unstable surrounding rock is proposed, which are respectively, the positive extrusion type, forward type caving and posterior tilt falling. By making it the core indicators of safety evaluation of the extremely unstable surrounding rock, the corresponding failure mechanism is revealed. At the early stage of positive extrusion type damage, it’s mainly the tensile failure, later it transfers to the tension-shear failure. During all the phases of forward type falling, it’s mainly the tension-shear failure. At the early stage of posterior tilt falling damage, it’s mainly the tension-shear failure, later it transfers to the tensile failure.(2) Based on the catastrophe theory, the prediction method for the extent of advancing failure of excavation face is proposed by the integrated use of dichotomy and curve fitting techniques, and the influencing factors are systematically researched. The longitudinal and vertical range of surrounding rock advancing failure are clarified as the two other key indicators of safety evaluation for extremely unstable surrounding rock.(3) Considering the pre reinforcement and space effect of excavation face, the load calculation method of the extremely unstable surrounding rock is proposed. Focusing on the analysis of load effect under different surrounding rock condition and different pre-strengthening method, surrounding rock deformation is determined as the main consideration of the safety evaluation of extremely unstable surrounding rock.(4) The intension of reinforcement feasibility of extremely unstable surrounding rock. On the basis of analyzing the effects of different type of pre-reinforcement, the evaluation index of reinforcement feasibility including the decreasing rates of deformation, reducing rate of failure zone and stratum groutability is proposed. The corresponding method to determine each index is given in this paper, which provides a basis for the safety assessment of the extremely unstable surrounding rock.(5) Based on characteristics of engineering safety of stability of extremely unstable surrounding rock, a comprehensive evaluation system including nine basic indicators is proposed by integrating the advanced deformation of surrounding rock. The indicators comprise advanced destruction and reinforcement feasibility, which are the longitudinal range of advanced destruction, the vertical range of advanced destruction, the advanced failure mode, the greatest advanced deformation, the range of advanced deformation, the range of rapid deformation, the decreasing rates of deformation, reducing rate of failure zone and slurry diffusion radius. The classification method of security is then put forward, which has been successfully applied in the Hejie tunnel project.