Experimental investigation of face stability of shallow tunnels in sand

The face stability of shallow tunnels was investigated with small-scale model experiments at single gravity. Aim of the research was to detect the evolution of failure mechanisms in dense and loose soil samples with varying overburden above the tunnel. Moreover, the quality of proposed theoretical/numerical approaches for the determination of a necessary support pressure pf was assessed. The results indicate that pf is independent of overburden and initial density of the soil. The experimental investigation with Particle Image Velocimetry revealed a significant influence of initial density on the evolution of failure mechanisms, though: whereas in dense specimen arching and a propagation of the failure mechanism towards the soil surface could be observed, the loose samples showed a much more diffuse failure zone that did not change its shape throughout the failure process.