The upstream section of the headrace tunnel (HRT) of the Setikhola Hydropower Project passes through calcareous and silicious phyllite with scant foliation which is weak and deformable rock mass. Bands of quartzite and metasandstone are intercalated locally within phyllite. These weak rocks may undergo plastic deformation. In such rock mass, there is a high chance that the tunnel might experience squeezing. Approximately 92 percent of the tunnel alignment has an overburden greater than 100 meters, with a maximum overburden above the tunnel of more than 500 meters. The portion from chainage 1+100 to 1+600 m along the tunnel alignment due to substantial overburden and intercalation of silicious phyllite and meta-sandstone, is critical in terms of tunnel squeezing. The tunnel at this section is analyzed for squeezing phenomenon using empirical techniques such as Singh et al. (1992), Goel et al. (1995), semi-analytical techniques such as Hoek and Marinos (2000), and Shrestha and Panthi (2015), analytical techniques such as the Convergence Confinement Method (Carranza Torres and Fairhurst, 2000) as well as numerical programs such as Phase2 and RS3 . According to the empirical and semi-analytical squeezing prediction criteria, there is high chance of significant squeezing particularly in the selected headrace segment at chainage 1+580 m. The numerical analysis was carried out at the section where maximum deformation was anticipated from the prediction criteria. Numerical model also shows considerable deformation at this section to cause heavy squeezing. The support system estimated primarily using empirical methods are applied in the numerical modeling. The support system is inadequate because the model shows considerable deformation even after the application of the support and also failure occurs at the support. To overcome this effect either a heavy support system needs to be applied, reducing the pull length and adopting top heading and benching tunneling method or strengthen the rock mass by pre-injection grouting.

Keywords : Weak And Deformable Rock Mass, Stability Analysis, Empirical Relations, Semi-Analytical Technique, Analytical Technique, Numerical Modeling, Support System.