STAND-UP TIME OF TUNNELS IN SQUEEZING GROUND - 1. A PHYSICAL MODEL STUDY - 2. GENERAL CONSTITUTIVE.

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. To establish a predictive capability, a constitutive theory describing the time dependent behavior of soft clays has been developed. By generalizing existing empirical rules developed for fixed boundary conditions and then unifying these empirical rules with a tensor framework, a multi-axial constitutive equation describing the stress-strain-time behavior of normally loaded soft clays was formulated.