Tunneling Through the Rock-Soil Interface

Constructive methods, tunnel lining, and soil treatment and conditioning depend fundamentally on ground and hydro-geological conditions. Regardless of specific difficulties, construction in homogeneous ground becomes, after adequate definition of the parameters above and the initial learning curve, a repetitive and uniformly controlled process, either through conventional or mechanical tunneling methods. Difficulties often arise when varying ground conditions are encountered along the tunnel alignment, especially if ground behavior presents significant contrasts in deformability, shear strength and permeability. In geological environments where soft ground overlays rock and tunnels have to be built crossing this interface, the above-mentioned contrasts normally occur at the same location. The most significant recent tunnel failures in Brazil occurred close to rock soil interface, showing the necessity of a review of current design and construction practice This paper intends to discuss main challenges associated to the rock-soil interface in the light of recent tunnel failures and present suggestions for robust design and construction methods.