Uncertainty analysis of tunnel squeezing for two tunnel cases from Nepal Himalaya

Weak rocks such as shale, slate, phyllite and schist, and the rock mass of weakness/fault zones are incapable of sustaining high tangential stress. Severe tunnel squeezing is therefore common in the tectonically active Himalayan rock mass and is one of the major areas of concern regarding stability. A reliable prediction of the extent of squeezing is essential so that a strategy can be established regarding stabilizing measures and for optimizing the support well in advance (during planning and design). In this paper, a probabilistic approach to uncertainty analysis that focuses on the effect of the variations in each input parameter of squeezing is used for analyzing and predicting the extent of tunnel squeezing for two tunnel cases in Nepal; Kaligandaki “A” (completed) and Middle Marsyangdi (under construction). A semi-analytical method suggested by Hoek and Marinos [Predicting tunnel squeezing problems in weak heterogeneous rock masses. Tunnels Tunnell Int, 2000; 32(11 and 12): 45–51 and 34–36] for predicting squeezing, an empirical formula proposed by Panthi [Analysis of engineering geological uncertainties related to tunnelling in Himalayan rock mass conditions. PhD thesis, Department of Geology and Mineral Resources Engineering. Norwegian University of Science and Technology (NTNU), Norway, 2006] and the Palisade's 2002 version of @Risk statistical software program have been used for the uncertainty analysis. The analysis results for Kaligandaki indicate fairly good correlation between predicted and actually measured squeezing. The same approach has been used for predicting the degree of tunnel squeezing at Middle Marsyangdi tunnel. It is concluded that the methodology proposed in this paper has potential for predicting the squeezing of future tunnel projects in weak rock mass conditions.