Excess pore water pressures in front of a tunnel face when drilling in a semiconfined aquifer

When drilling with a TBM (Tunnel Boring Machine) in saturated sand, excess pore water pressures occur in the sand in front of the TBM. The magnitude of these excess pore water pressures depends on the permeabilty of the sand, the drilling velocity, the layering of the soil. These excess pore water pressures will decrease the stability of the tunnel face and influence the plastering of that tunnel face with slurry or foam. The excess pore water presssures can be calculated using anlytical or numerical methods. Analytical methods are only valid for 1-dimensional flow or homogeneous soil conditions. However, they have the advantage that the influence of various parameters can be calculated very quickly. This paper describes a new analytical method for the situation that a tunnel is drilled in a semi-confined aquifer. It will be shown that in case of drilling a tunnel in a semi-confined aquifer the leakage length of that aquifer also determines the hydraulic gradient at the tunnel face and thus the time needed before the bentonite slurry makes a relatively impermeable cake in case of a slurry shield TBM. Consequences for stability of tunnel face and the soil around the tunnel will be discussed.