DESIGN, OPTIMIZATION AND COMPARISON OF HYDRAULIC TUNNELS IN SIWALIKS, LESSER HIMALAYAS AND HIGHER HIMALAYAS OF NEPAL: A COMPARATIVE STUDY OF STRESSES, SUPPORT SYSTEMS AND DEFORMATION BEHAVIOR

Abstract

Tunneling in the Himalayan region presents significant engineering challenges due to complex geological formations, high seismicity, and variable rock mass conditions. This study focuses on the design, optimization, and comparative analysis of hydraulic tunnels in three distinct geological regions of Nepal: the Siwaliks, the Lesser Himalayas, and the Higher Himalayas. The main objective of this study to evaluate stress distribution, required support systems, and displacement in these distinct geological conditions. Understanding these variations is critical to improve design methodologies and optimization of support systems in order to ensure the long-term safety and functionality of hydraulic tunnels in challenging geological conditions. The methodology incorporated in this study includes empirical, analytical, and numerical approaches to assess tunnel performance in the selected regions. Case studies from the Super Lower Bagmati Hydropower Project (SLBHP), Ranma Khola Hydropower Project (RKHP), and Humla Karnali Hydropower Project (HKHP) were analyzed using the Rock Mass Rating (RMR). Numerical modeling was performed using Phase2 software to assess tunnel stability, in-situ stresses, and support system effectiveness. Additionally, hydrological assessments, including flow estimation, hydrograph interpretation, and flow duration curve analysis, were conducted to optimize the hydraulic efficiency of the tunnels. The findings highlight significant variations in stress distribution, displacement behavior, and required support systems across the three geological regions. The comparative analysis indicates that tunnels in weaker rock formations, such as those in the Siwaliks, require more extensive support measures, whereas tunnels in the Higher Himalayas experience higher in-situ stresses. This study provides valuable insights regarding the behavior of rock mass and the required support systems in the respective geological settings contributing to improved tunnel design and construction in Nepal.