Plastic deformation behavior in tunnel in schist and schistose mica gneiss based on a case from Nepal Himalaya

Plastic deformation in tunnels is one of the few tunnel stability challenges that are being faced in the Himalayan region. Severe tunnel deformations have been recorded, particularly when tunnel passed through plastically deformed and highly schistose rock mass, weakness and shear zones. Therefore, there is essential to understand plastic deformation behavior of the rock mass and the interaction that rock mass has with the in-situ stress, mechanical properties of the rock mass, applied rock support. Such understanding enables us to make appropriate strategy in minimizing the stability challenges and using the appropriate tunnel construction methodology. The Khimti headrace tunnel experienced instability related to tunnel deformations in several tunnel stretches consisting mica schist and schistose mica gneiss. Monitored convergence pattern indicated that the tunnel deformation continued for a long period of time after excavation. This paper assesses the applicability of both Sulem et al (1987) approach in estimating long term time dependent deformation and Hoek and Marinos (2000) approach in estimating total tunnel convergence. The methods are finally verified by 3D numerical modeling in one of the tunnel stretches. The assessment indicated fairly good match between measured, back calculated and numerically modeled results.