Abstract

Potentiality of plastic deformation in tunnels in the Himalayan rock mass is considerable. Tunnel 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 controlled 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 Himalaya. Numerical modelling of selected sections has been done to evaluate the effect of tunnel shape and stress anisotropy.

1. Introduction

Tunnels in weak and tectonically deformed rock mass face many challenges. Stability becomes a concern particularly when the rock mass is schistose, sheared, folded and thinly foliated exhibiting high degree of anisotropy. Tunnel passing through plastically deformed rock mass with high overburden may experience large deformation in the periphery of the tunnel contour. In extreme situation tunnel may get partial or full collapse, which may be difficult to handle while tunneling through such rock mass. In order to control excessive deformation and limit the tunnel collapses, heavy supports are normally installed in the world, and the Himalayan region is no exception.

Displacement in tunnel walls in weak rock mass is a common phenomenon. The deformation starts before and immediately after the tunnel is excavated, and even before and after the rock support is applied. Since application of heavy rock support is costly and time consuming, it is advantageous that rock support optimization is made to limit displacement to an acceptable range. Rock support interaction assessment is normally made using Convergence Confinement Method (Carranza-Torres & Fairhurst, 2000; Panet 2001).

This paper deals with Kaligandaki ‘A’headrace tunnel that suffered severe tunnel stability challenges. The tunnel passes through highly schistosed, sheared, thinly foliated and weathered rock mass. In order to control and to limit tunnel deformations, heavy rock supports in the form of closely spaced steel ribs, thick shotcrete and rock bolts were applied. Rock support interaction and effectiveness of the applied support is a main area of assessment in this endeavor.

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