Effectiveness of applied support in tunnels in plastically deformed rock mass - an analysis based on a tunnelling project from the Nepal Himalaya

Potentiality of plastic deformation in tunnels in the Himalayan rock mass is considerable. Tun-nel convergence and squeezing phenomena have therefore been serious issues for successful tunnelling in the region. Large tunnel deformations have been recorded in many occasions, particularly in tunnels passing through schistose and highly deformed rock mass. Theoretically, the tunnel deformation is believed to be con-trolled after applied rock support has become effective. However, monitored tunnel deformations show that the tunnel contour continues to deform over a long period of time despite considerable quantity of rock support is applied. This article analyses tunnel deformation to assess effectiveness of the applied tunnel support. Convergence Confinement Method is used to analyze rock support interaction and evaluate the effectiveness of applied support at 198 different tunnel sections of Kaligandaki ‘A’ headrace tunnel from the Nepal Hima-laya. Numerical modelling of selected sections has been done to evaluate the effect of tunnel shape and stress anisotropy.