Study On The Face Stability Around Shield Tunnels And Numerical Simulation Of PFC

Metro shield is widely used in the construction of urban subway because of its high degree of automation, fast driving speed and small effects on ground structures, etc. In case of inadequate control of excavation face supporting force during shield tunneling, it may lead to differential settlement of ground buildings, damage of municipal pipelines and other adverse consequences. The failure mode, the relation between face supporting force and displacement and soil arching effect of face around shield tunnels in sand were investigated in this paper, through the combination of small scale model tests, numerical simulation and theoretical analyses. The main research contents and results are described as follows:(1) A series of face stability around shield tunnel tests were performed with varied sand density and depth ratio based on digital image processing technique, and the deformation development process of sand in front of tunnel face, faliure mode and the relevance between face displacement and supporting pressure were studied respectively.(2) Stress-strain curves of triaxial compression tests simulated by means of PFC2D were compared with the correlation curve obtained via laboratory tests for the macroscopic and microscopic parameters calibration. Based on the calibration results of macro-micro parameters, the macro-mechanical response of sand simulated by particles flow turned out to be reliable and reasonable.(3) Discrete element models of PFC2D were established to simulate the development process of shield tunnel face failure. The contact force among soil particles, regional soil porosity, soil stress and other micro-parameters in front of the tunnel face were analysed, thus the soil arching effect during the process of face faliure was revealed from the mesoscopic perspective. In addition, the relevance between the face displacement and the face supporting pressure was validated.(4) Simplified formulas for calculating the limit supporting force at excavation face around shield tunnels were derived and validated against the results of laboratory tests and numerical simulations.