Abstract: In order to resolve the technical problem of tunnel face collapse in the tunnel which is overlaid by quicksand stratum, a typical tunnel below the dynamic water-rich and weakened sand stratum of Qingdao metro No.2 line (the section of Pi-Miao), was taken as a research model, and a failure mechanical model was established according to the actual destructive characteristics. Based on transformation equilibrium from work to energy, the upper bound analysis of tunnel face stability was conducted. By the upper bound analysis and the strength reduction/gravity loading method, the tunnel face safety factor was obtained, and the critical soil rupture range given different soil cohesions, frictional angles, densities, water-resisting layer thicknesses and tunnel excavation heights were obtained as well. The theoretical research showed that tunnel face safety factor increased and stability was enhanced with the increase of cohesion, frictional angle and water-resisting layer thickness, and safety factor and face stability decreased with the increase of density and tunnel excavation height. A series of numerical calculation models was established to verify the accuracy of theoretical research, which obtained typical failure modes and critical parameters of tunnel face overlaid by quicksand stratum. Results from this research provide theoretical guidance and scientific solutions to the transfixion of Qingdao No.2 metro line and other cases with similar geological conditions.

Key words: quicksand stratum, stability analysis, upper bound analysis method, safety factor, numerical experiment