Stability of Clay at Vertical Opening

The stability of clay at vertical openings in walls and tunnels has been investigated. Laboratory experiments have shown that a cohesive soil flows through a vertical opening when the total overburden pressure exceeds about six times the undrained shear strength of the soil at the level of the opening. Field data collected from tunnels and pipes constructed in soft clays show that no difficulties have been encountered with soil flowing through openings in the tunnel lining or through the tunnel front when the net overburden pressure has been less than six times the undrained shear strength of the soil. Field data indicate, furthermore, that the force required to push a tunnel shield forward into a cohesive soil corresponds to approximately the sum of the overburden pressure and six times the undrained shear strength of the soil.