PERFORMANCE OF A SMALL TUNNEL IN CLAY SHALE

The tunnel described in this paper was hand-excavated through Upper Cretaceous clay shale and temporarily supported by steel ribs and spruce laggings. A cast-in-place concete lining was installed for permanent support. Tunnel wall convergences and support stresses and strains were monitored for more than 100 days. In addition to the field instrumentation, in situ pressuremeter tests and laboratory triaxial tests on specimens taken from a block sample were performed to determine the rock mass properties. These properties were used to interpret (by finite element analysis) the measured tunnel wall deformations. In general, the short-term rock properties determined from laboratory testing agreed well with the observed in situ behavior. The long-term response was, however, difficult to predict from laboratory measurements and this was further investigated by application of the convergence-confinement method. It was found that the equivalent, long-term modulus of the rock mass decreased with time to between 50 and 70% of the short-term modulus. The deformation rates decreased significantly after 70-100 days and the support load leveled off accordingly.