ABSTRACT
A blow-out that occurred during the drilling of the 2nd Heinenoord Tunnel is investigated and the measured pressures are compared with measurements that were obtained during centrifuge experiments performed to investigate the maximum grouting pressure. Although geometry of the tunnel face is different from the geometry of the centrifuge tests, comparable results were obtained. It appeared that the pressure at which a blow-out occurred was equal to the pore pressure plus 2 to 3 times the vertical effective stress. This is more than what is often used as an upper boundary (pore pressure plus 1 time the vertical effective stress), but much less than according to some theoretical predictions.
