Rock Properties and Steel Tunnel Liners

The economical design of steel liners for underground pressure tunnels in rock is dependent upon the determination of the properties of the rock envelope and its state of stress. For maximum economy, liner plate thickness is held to a minimum by relying on the load bearing response of the rock to resist part of the internal hydrostatic pressure while restraining the steel liner at a safe level of strain. A method is presented for evaluating the load bearing response of the rock around the tunnel, based upon measurement of tunnel convergence (relaxation) during excavation. A system is briefly described for classifying rock along the tunnel and providing appropriate adjustments of liner thickness and reinforcement. A design analysis based on a criterion of steel strain is presented.