Linings for sub-sea tunnels in rock

The Norwegian tunnelling method implies maximum utilization of the inherent self-supporting capacity of rock for tunnel excavation. This is reflected in the tunnel economics, particularly since rock support is largely based on rock reinforcement by means of bolts. Another consequence is that water and frost have to be dealt with separately by means of special designs that are less costly than watertight, in-situ concrete linings. This applies in even a higher degree for sub-sea tunnels. The designs originally adopted include aluminium linings like crown, single and frost-insulated linings. Installations of 45 mm thick PE foam mats, introduced 10 years ago, offered very flexible and inexpensive linings. However, the fire-protection requirements that have now been adopted demand a more costly solution. Recent developments include concrete elements that can compete in price with the lightweight installations. Other recent developments of interest for low-traffic and sub-sea tunnels are linings of high quality PVC-coated polyester fabric. They may also be insulated with mineral wool for protection against ice.