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

Abstract

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.