A new approach for ground reinforcing design in tunnelling

The convergence confinement method has been widely applied to tunnel design due to the simplicity of the numerical approach. A solution, able to take the existence of an area with different mechanical properties around the tunnel into account, is proposed in this paper. The ground behaviour considered for this analytical formulation includes a strain softening law and a Hoek and Brown yield criterium both for the reinforced area and for the natural ground. A new approach for the design of bolt reinforcement is proposed using the convergence-confinement formulation. The bolted ground has been modelled by changing, the gradient of the softening branch of the natural ground stress-strain curve and the residual strength criteria parameters in a zone around the tunnel as thick as the bolt length. This approach has been tested on the basis of the physical model results proposed by Indraratna and Kaiser (1989, 1990) and has been applied to the case of a railway tunnel in weak rock reinforced with bolts and steel cables.