Discrepancies Between Predicted and Actual Rock Mass Conditions in Himalayan Tunnelling: Insights from the Upper Trishuli 3B Project, Nepal

Abstract

Tunnelling in the Nepal Himalaya presents significant engineering challenges due to highly variable geological conditions, where rock mass characteristics can change within short distances. Inadequate pre-construction investigations can result in discrepancies between predicted and actual ground conditions, causing construction delays, increased costs, and inappropriate support designs. This paper examines these challenges through the Upper Trishuli 3B Hydropower Project in Nuwakot District, Nepal. Significant differences were observed between the geological conditions predicted in the Detailed Project Report and those encountered during tunnel excavation. The EPC contract further complicated construction, as certain additional support requirements were treated as contractual obligations, leading to excessive support installation in areas where it was unnecessary. Workmanship-related delays also affected project progress. The findings emphasize the importance of comprehensive geological investigations, including the identification of rock types, geological structures, and shear-band fabric. Rock mass classification should be supported by kinematic analysis and relevant design parameters. Furthermore, introducing intermediate support classes and flexible EPC contract provisions could allow support adjustments based on actual tunnel-face conditions. Independent oversight and clearer separation of technical and commercial decisions are essential for improving tunnelling efficiency in heterogeneous Lesser Himalayan geological environments.