Excavation and Support Challenges in an Exceptionally Weak Shear Zone in the Lesser Himalaya: A Case Study on Tunneling at UKHPP (28.1 Mw), Taplejung Nepal

Abstract

This paper presents a case study on the challenging excavation of an exceptionally weak, 25.5 m thick shear zone encountered during the construction of the Headrace Tunnel (HRT) at the Upper Kabeli Hydropower Project (UKHPP). The weak section, located between chainages 4+075.75 and 4+032.25, is composed primarily of light greenish gray silty clay (90-95%) with highly weathered phyllite fragments, resulting in an exceptionally low Q-value (<0.01) rock mass classification (Class VI) and it has oblique relation with the tunnel direction and the bedrock foliation. To safely manage the adverse geological conditions, sequential Heading and Benching method was adopted, complemented by the immediate installation of robust support measures.

Significant tunnel squeezing (up to 30-40 cm over a 5 m stretch) was observed, which was effectively controlled by the installation of the invert strut and its bracing. The weak section was excavated slowly over 50 days without major overbreak. Furthermore, non-core recovery probe drilling was strategically employed ahead of the face to investigate and pre-judge the onward rock mass condition. This case study underscores the importance of adaptive excavation techniques and comprehensive support systems for safely mitigating high-risk, weak geological structures common in the Nepal Himalaya.