Interactions at the front face of EPB shield when drilling in sand

The construction of a tunnel with a tunnel boring machine (TBM) influences the soil in front of the TBM. Excess pore pressures are measured in front of the TBM, which influence the stability of the soil. The magnitude of these pressures depends among others on the soil layering, drilling velocity, permeability of the soil and the sand-foam mixture and the pressure in the mixing chamber. The pressure buildup when drilling starts and the decay when drilling stops depends on the plastering properties of the slurry or the foam mixture. This paper describes the interactions between the soil and an earth pressure balance shield (EBP) TBM when drilling in saturated sand. It shows monitoring results and presents theory to describe the results from this monitoring in the field and from laboratory tests. Groundwater flow appears to be of importance on different scales: at the front face, in the foam-water-sand mixture and in the lamella between two foam bubbles.