INTEGRATED ASSESSMENT OF ROCK MASS QUALITY, TUNNEL STABILITY, AND SUPPORT SYSTEM IN THE KARAKAR TUNNEL, BUNER DISTRICT, KHYBER PAKHTUNKHWA, PAKISTAN

Abstract

Tunnelling through structurally complex schistose metamorphic rock masses poses significant challenges for excavation stability and support design. To the authors’ knowledge, a systematic Q-system based assessment linking rock mass quality to installed support has not previously been reported for Karakar Tunnel in Buner District, Khyber Pakhtunkhwa, Pakistan. This study presents an integrated geomechanical assessment of rock mass quality, tunnel stability, and support performance in the Karakar Tunnel, with the specific objectives of quantifying rock mass quality along representative excavation faces using the Q-system and evaluating the consistency of Q-system recommendations with the pre-installed supports (rock bolts, shotcrete, wire mesh and lattice girders). Geological and geotechnical mapping was conducted at seven representative excavation faces between chainage 2+628 to 2+645, documenting lithology, discontinuity characteristics, and groundwater conditions. Rock mass quality was computed using the Q-system and the resulting classifications were compared with the installed supports against the Q-system support recommendations. The Q-values ranged from 0.07 to 4.00, corresponding to the very poor to fair rock mass quality. The lowest Q-values occurred between chainage 2+632 and 2+637, where closely spaced and persistent discontinuities, increased weathering, and locally damp conditions were observed which needs intensive support. The installed rock-bolts length varied from 2.0 to 2.5 m  in the weaker ground, indicating an adaptive reinforcement approach; however, the relationship between bolt length and Q-value should be interpreted cautiously because only seven faces were evaluated. Shotcrete thickness was uniformly maintained at 40 cm across all mapped faces, irrespective of Q-value, resulting in comparatively conservative support in the higher-quality sections. Overall, the installed support was generally equivalent to or more conservative than indicative Q-system recommendations. The results highlight the potential for adaptive support optimization in comparatively competent ground, subject to detailed design verification, as-built assessment, and field monitoring. Continued face-by-face geological mapping and periodic reassessment of rock mass quality are recommended as excavation progresses.