Research On Mechanism Of Pipe Roof Reinforcement And Tunnel Face Stability

In order to prevent tunnel collapse, restrain ground movement and control ground surface settlement when tunnels would be constructed in soft or weak geology stratum, several pre-reinforcement methods of heading have been developed. One of such methods is the pipe roof reinforcement method. This method consists on installing, prior to the excavation of a length of tunnel, a series of pipes, either parallel to the tunnel axis or at a certain angle with it. By injecting grout through the pipes, the ground in between the pipes is stiffened and the pipes are connected, creating a kind of 'umbrella' above the area to be excavated. This technique combines the advantages of the modern fore poling system with the grouting injection method. In other words, the improvement of the mechanical characteristics and the impermeability of the ground around the tunnel can be achieved simultaneously. Taking the mechanism of pipe roof reinforcement and the tunnel face stability as the objects to be investigated, using theoretical analysis, numerical simulation, field tests and other research tools, a series of results were obtained.(1) Based on the predecessors surrounding rock pressure calculating method, the equations for pipe roof reinforcement loading were proposed according to Terzaghi theory. Compared with the traditional theory, it can more reasonably describe the effects of the tunnel cover depth, the geological conditions and manners of excavation on the surrounding rock pressure.Considering the delay effect of initial lining, an analytical approach based on Pasternak elastic foundation beam theory for pipe roof reinforcement was put forward. With the example of Erlangshan tunnel excavation, the comparison of the values of longitudinal strain of reinforcing pipe between field monitoring and analytical approach was made. The results indicate that although Pasternak model gives more accurate calculation and agree better with the result of field monitoring at tuunel face than Winkler model, the general trend of decreasing longitudinal strain with increasing distance from the tunnel face for analytical approach and field monitoring are different. A systematic parameter study was conducted to study the effects of important design parameters such as the pipe diameter, pipe length, and overlap length on the mechanical behavior of pipe roof reinforcement. The findings illustrate that there exists a critical value of each reinforcement parameter, with these critical values, the maximum reinforcing effect can be achieved. (2) A three-dimensional elasto-plastic finite element code was employed to simulate the interaction between tunnel excavation and pipe roof in entrance of Qiaozhuang Tunnel. And the convergence deformations of surrounding rocks, the stresses of I-steel supporting, shotcrete and secondary lining were obtained by field test. The mechanical behaviours and deformation characteristics of pipe roof reinforcement, surrounding rock and tunnel lining were individually analyzed in tunnel excavation, and the effect of pipe roof reinforcement on tunnel face stability was also evaluated. The simulated results were compared with field test results, it is verified that not only the field test but also the finite element method be important for successful construction of the tunnel with pipe roof reinforcement.(3) Based on the kinematic method of limit analysis and the shear strength reduction technique, a three-dimensional model for expressing the tunnel face stability was established and was employed to define the safety factor and its corresponding critical failure mechanism for a given tunnel. For a typical example, the solutions computed by the proposed approach are compared with the results given by wedge model, trapezoid wedge model and centrifugal-model test to verify the reasonability of the method. Furthermore, by modifying the mechanics model, the complicated conditions such as tunnel with pipe roof reinforcement and underwater tunnel with pipe roof reinforcement were respectively considered. And the effects of pipe roof reinforcement and seepage force on tunnel face stability were individually examined. The proposed approach was also employed to study how cover depth of tunnel, groundwater level, tunnel diameter and soil parameters affect the tunnel face stability. The studies revealed that the existence of groundwater may seriously affect the tunnel face stability, and there was a relatively large reduction in the seepage pressure by adopting the pipe roof reinforcement technique, but in dry condition, the effect of pipe roof reinforcement on tunnel face stability is not significant.(4) Based on three-dimensional elasto-plastic finite element method, the design parameters of pipe roof reinforcement were analyzed. The effect of the length of bench and core soil, excavation length, length of steel pipe, underwater level and arrangement of pipe roof reinforcement on the squeezing of tunnel working face, the displacement ahead of the face and mechanical behaviours of pipe roof reinforcement are analyzed. Some valuable conclusions for the construction of tunnel with pre-reinforcement were proposed.(5) The limit support pressure at tunnel face was studied by elasto-plastic finite element method. First, BP neural net work was trained with parameters of surrounding rock and ratio of limit support pressure of tunnel face as training samples. After training, the net work can predict ratio of limit support pressure when lots of parameters were imported. The statistics character of ratio of limit support pressure was gained. Limit state function of face stability of tunnel with pipe roof reinforcement was established, and reliability was solved with the particle swarm optimization. Using this method, not only stability of tunnel face can be appraised rationally, but also it was reference to select pre-reinforcement technique rightly.