ABSTRACT
As a part of a measurement campaign the pore pressures were measured in front of a tunnel during drilling with a slurry shield in sand. It was found that during drilling the pore pressure in front of the tunnel up to a distance of 2 tot 3 times the tunnel diameter is higher than the hydrostatic pressure. This is caused because drilling prevents plastering of the bentonite at the tunnel face. Calculations show that this excess pore pressure increases the minimum pressure necessary for stability of the tunnel face and has also consequences for the maximum allowable pressure at the tunnel face. The paper describes the measurements, what mechanism prevents the plastering in front of the tunnel during drilling, speed of plastering by the bentonite slurry during stand still and consequences for stability of the drilling process
