Big tunnels in bad rock

Tunnels of 10 to 16 in span are frequently constructed for hydroelectric or transportation projects, and many of these tunnels are excavated in rock masses of very poor quality. When the ratio of rock mass strength to in situ stress falls below 0.2, squeezing of the rock mass becomes a problem that can cause instability of both the tunnel and the face. A method for predicting squeezing conditions is presented and the practical options for prereinforcing the face and supporting the tunnel to deal with these problems are discussed. Two case histories are included to illustrate how these prereinforcement and support measures can be incorporated into a tunnel design. Brief discussions are also given on water problems in tunneling, the use of tunnel boring machines (TBMs) in squeezing ground, and the construction costs for large tunnels in varying ground conditions.