Numerical study on the tunnel stability in granular soil using DEM virtual air bag model

Granular soil is commonly encountered during underground constructions in and around urban areas. Due to the lack of cohesion in the granular soil, tunneling through it can easily induce excessive ground settlement and/or even collapse. This study primarily focuses on the tunnel/ground stability during the construction phase before introducing the tunnel lining. The influence of relative tunnel depth and the density of granular soil on the stability and collapse mechanisms of shallow tunnels are investigated using discrete element method (DEM). The tunneling process is numerically realized by an air bag model, which is developed to pressure/depressurize the tunnel. The pressurized air bag in the DEM is calibrated and verified against physical tests. The unloading of the tunnels is realized by reducing the pressure of the fully pressurized air bag stepwise. The soil displacement, soil volume loss as well as the evolution of soil stress are recorded and compared with predictions based on previous experimental studies. Our results show that the tunnel stability and failure mechanisms are significantly influenced by the soil density and relative tunnel depth. The tunnel stability is less dependent on the relative tunnel depth with the increment of the soil density. The soil density is shown to have a notable effect on tunnel stability, especially for relatively deep tunnels. The width of surface settlement trough does not appear to change before the tunnel collapse. However, it decreases dramatically at collapse and keeps decreasing thereafter. The tunnel stability number of different test cases in granular soil is found to be smaller than that in clayed soil but larger than that in saturated sandy soil.