Laboratory experimental study of the forming and failure mechanisms of soil arching during EPBS tunnelling in sand

The effect of soil arching often leads to delayed ground collapse disasters due to the tunnelling instability of earth pressure balance shields (EPBS) in the sand. To reveal the formation and evolution of soil arching in the sand, a novel EPBS tunnelling model test was conducted. This model test can simulate the whole process of shield tunnelling, which includes stable tunnelling, tunnel face instability, and ground collapse. The application of multi-field monitoring and three-dimensional scanning showed the effect of shield excavation disturbance on soil arching during the collapse process in the sand. The results indicate that: (1) To ensure the stable tunnelling of EPBS in the sand, the actual discharge rate needs to be close to the theoretical rate; (2) The stress disturbance range expands remarkably during the unstable stage compared with the stable stage; (3) The evolution of soil arching significantly influences the surface subsidence. The variation of surface subsidence is sudden before ground collapse, which is different from the progressive variation of surface subsidence in dry or saturated sand; (4) During the stable tunnelling and unstable collapse processes, the vertical earth pressure response is greater than in the horizontal direction. Analysis of the variation curve of earth pressure in the strata can reveal the evolution process of the soil arch.