Tunnel face failure mechanism with sand layer partial collapse

In this paper, a DEM model is proposed based on Particle Flow Codes (PFC) software, which simulates a sand layer collapse process in tunnel excavation of Qingdao subway project. The fluid–particle interaction was solved with the coarse-grid method in order to account for the influence of groundwater on the movement of sand layer. The development of sand layer collapse and the displacement field were acquired, and the variation of porosity and stress were calculated. Influencing rules of mechanical property of sand layer on the stability were revealed. The maximum settlement occurred at the arch crown of the tunnel, and it reduced gradually up to the ground surface. The porosity and sand inflow amount have a positive relation, while the vertical stress with the sand inflow amount has a negative relation. The influence of silty clay on the stability of the tunnel face was also investigated. For the condition with 2% silty clay amount, the inflow amount reached almost 300 m3 after 1 day, while for the condition with 20% silty clay amount, the inflow amount was only 25 m3, which is regarded as stable during excavation.