Abstract
From the experience with many tunnel projects in rocks it is suggested that the ground arch effect in rock can be obtained by controlling a loosening zone. The loosening is considered as the post-peak behavior of rocks. The authors investigated the motorway tunnel which was driven through the tertiary sedimentary rocks in the northern Osaka, and confirmed as follows: Such a small change as 0.1 MPa in the confining pressure, which acts onto the tunnel wall as the resultant effect of support elements, remarkably influences the extent of loosening zone. A magnitude of loosening zone can be quantified through the rock classification and the strain-softening model proposed by Tanimoto et al.
