In the Nepal Himalayas, hydropower projects having an installed capacity of over 10 MW usually consist of underground waterways. Most of these underground waterways consist of vertical or inclined pressure shafts, which are part of the headrace system of a hydropower project. Excavation of these shafts requires special techniques, and the performance is dependent on the quality of rock mass. Therefore, the selection of an efficient construction method for shaft excavation is most challenging work.
This manuscript evaluates the challenges associated with the construction of shafts for three hydropower projects in the Nepal Himalayas. The achieved construction progress of each method is compared with actual geological conditions. It is concluded that the major challenges associated with the excavation of pressure shafts through the Himalayan rock mass conditions are frequent overbreak, water inflow, debris flow, difficulties in surveying and control of shaft alignment, ventilation, poor visibility, and pilot hole deviation.
Natural events such as rock-soil failure, high rainfall, landslide, tunnel collapses, etc. cause challenges associated with the development of hydropower projects. Appropriate and viable underground excavation methods should be used to address these problems (Panthi 2006). In hydropower projects, unlined or lined underground pressure shafts are constructed either vertically or inclined to carry water from the headrace tunnel to the powerhouse. In the Nepal Himalaya, most of the hydropower plants with installed capacities over 10 MW consist of underground pressure shafts as penstock water conveyance systems.
The construction of shaft with length (depth) greater than 200 m is a challenging task due to uncertainties in the underground excavation (Sunuwar 2016). These uncertainties are categorized as geological factors associated to weak rock mass quality, high weathering, faulting and fracturing of rock mass, rock stress, and groundwater effect; and non-geological uncertainties associated to the level of skill, expertise, and the technology in use. The stepwise geological investigation is crucial to minimize the uncertainties in underground excavation work (Panthi 2006 and Panthi 2007).
In this manuscript, three shafts of hydropower projects from the Nepal are selected to present the case histories of the shaft excavation and challenges faced during the excavation of these shafts. In addition, applied remedial measures are also discussed.
