STAND-UP TIME OF TUNNELS IN SQUEEZING GROUND. PART I. A PHYSICAL MODEL STUDY

The effect of tunnel size, advance rate, and depth of cover on the stand-up time of tunnels in squeezing ground was investigated through a series of 12 physical model tests. The stand-up time, defined as the time elapsed before instability develops, was found to be characterized by increasing deformations and deformation rates rather than a catastrophic collapse of the tunnel. Test results showed a 25% increase in stand-up time was realized by halving the size of the opening (from 5.0 m dia. to 2.4 m dia. when scaled to prototype dimensions) or by decreasing the advance rate by a factor of four (from 1.3 m/hr to 0.3 m/hr for the 5.0 m dia. tunnel). Depth of cover was described in terms of the ratio of confining pressure to material strength. Decreasing the depth (or increasing material strength) by 10% also increased stand-up time by 25%. In order to establish a predictive capability, a constitutive theory describing the time dependent behavior of soft clays has been developed.