Probabilistic approach in assessing tunnel squeezing - a discussion based on tunnel projects from Nepal Himalaya

Tunnel squeezing is a common phenomenon in the Himalayan rock mass with high degree of schistocity. Weak rocks like mudstone, shale, slate, phyllite, schist, highly schistose gneiss and the rock mass of the tectonic fault zones are incapable of sustaining high stresses. A reliable prediction on the extent of squeezing is therefore essential to make strategy regarding stabilizing measures and optimization of tunnel rock support well in advance (during planning and design). Failing to do so will result to the construction delay, cost overruns and unnecessary disputes between the client and the contractors involved in the execution of tunneling projects. In the paper, probabilistic approach of uncertainty analysis is used in assessing tunnel squeezing for the two tunnel cases from Nepal Himalaya. The main focus is given to look on the applicability of squeezing analysis using Hoek and Marinos [4] approach in combination with the equation proposed by Panthi [1] for the estimation of rock mass strength for highly schistose rocks of the Himalaya. Measured tunnel convergence (squeezing) and lab tested mechanical properties of the rocks from headrace tunnels of Kaligandaki and Middle Marsyangdi hydropower projects from Nepal Himalaya are used to verify the applicability of the proposed methods and also the uncertainty analysis approach on squeezing introduced by Panthi [1].