ABSTRACT
The face stability of an EPB-shield in saturated sandy soil is investigated. Recent research on the behavior of foam at the interface is combined with the knowledge on the influence of excess pore water pressures in the soil on the face stability. A simple model is described to illustrate this influence. It appears that the processes at the tunnel face for an EPB are quite comparable with the processes for a slurry shield. ‘Clean foam’ leads to more reduction of the permeability at the face than a foamy sand that can be expected during drilling. Micro instability is not very likely, except for large diameter tunnels and when drilling in a confined aquifer. Excess pore water pressures in front of the EPB can require a roughly 2 times higher pressure difference between the mixing chamber and the pore pressure far away from the tunnel than calculated with traditional methods.
