Experimental study of the pore pressure variation during EPB shield tunnelling in the saturated sand

During tunnelling with an earth pressure balance (EPB) shield in the sandy soil, the support pressure at the tunnel face depends on the properties of the muck (soil-foam mixture). Five series of foam infiltration tests were performed to investigate the effects of foam expansion ratio (FER), foam injection ratio (FIR), initial pressure gradient at the tunnel face, air pressure in the excavation chamber and the types of soil on the pressure variation in the soil in front of the tunnel face. The sand used to make the muck and sand column is the same. The experimental results show that both higher FER and FIR values lead to faster pore pressure drop. A minimum initial pressure is required to drive the foam further infiltrate into the soil. Moreover, a higher initial pressure gradient over the sand column leads to a faster pore pressure drop. For foam flow from the muck into the saturated sand, the model of foam infiltration can well describe the soil with a relatively high permeability, whereas overestimates the pore pressure but underestimates the water discharge volumes for the soil with a relatively low permeability.