Design of Support System for Headrace Tunnel of Upper Balephi ‘A’ Hydropower Project Using Three-Dimensional and Conventional Approach

Abstract

Tunnel Construction is one of the most complicated tasks among various Civil Engineering works. In Hydropower Tunnels, the work becomes even more complicated because we have to deal with huge water pressure when water is conveyed from intake to power station and ultimately back to the river. When a tunnel is dug the then prevalent in-situ condition of the surroundings is disturbed and new set of stresses are induced. This may lead to failure of the rock mass around the tunnel if not properly assessed. The risks and hazards involved are life threatening, so stabilizing the excavation with support system is of utmost importance. When resistance is developed against the impending failure, then permanency can be achieved. Conventionally, Tunnel is considered as a continuous two-dimensional structure and the stresses in the longitudinal direction are not assessed with. But in case of discrete features like bends in the Tunnel, Adit connections and underground surge shaft, the stresses are significantly disturbed in all three directions and only three-dimensional analysis can recommend the best support system in such case. In this thesis, using the actual project information of Balephi Hydropower project, support system is designed using the Empirical, Analytical and Numerical modelling approaches. Support system is designed with conventional two-dimensional methods at five different chainages with varying overburden and rock mass parameters. Three- Dimensional approach is taken at a bent, the adit connectional and the underground vertical surge shaft. The typical geological data of the site formed the input parameters for the aforementioned approaches. All these methods independently suggest support system for various sections based on their scientific principles which are compared and the one which gives the most economical support design with highest factor of safety is recommended.