Squeezing Assessment Along 2.84-km-Long Headrace Tunnel of a Small Hydropower Project in KPK, Pakistan: Comparison of Different Methods

This paper follows a deductive approach for the assessment of squeezing potential along 2.84-km-long headrace tunnel situated in Mansehra, Khyber Pakhtunkhwa (KPK), Pakistan. The assessment was made with the combination of empirical, semi-analytical and analytical approaches followed by a numerical simulation. The rock masses along the tunnel were classified as poor-quality rock using tunneling quality index (Q system) and geological strength index (GSI) based on the discontinuity survey data. Two sections of 200 m stretch of the tunnel were identified to be affected by squeezing based on empirical and semi-analytical approaches available in the literature. The detailed analyses of these sections were carried out by an analytical approach such as convergence confinement method (CCM) after Carranza-Torres and Fairhurst, while RS² (rock and soil 2D) was utilized for numerical modeling. Apart from these sections, a non-squeezing section was also analyzed by employing empirical and semi-analytical approaches available in the literature, as a representative for the whole non-squeezing length of the tunnel. The squeezing prediction criteria showed minor squeezing for tunnel section 0 + 600 m to 0 + 800 m. Detailed squeezing analysis was executed for three sections analytically and support pressure was also estimated using CCM and RS² modeling. Both analytical and numerical approaches, considering the rock mass to be an elastic material, gave identical results, that is, about 0.5% convergence. While considering the rock mass as a plastic material, both analytical method and RS² gave higher value of convergence.