Challenges of shaft excavation in Nepal Himalaya: case study from hydropower projects
The excavation of the long blind shaft is one of the most difficult and risky in Nepal Himalaya for hydropower development. Excavation of the shaft greater than 200 m length is a challenge due to vertical or inclined orientation, limited space, available technology and changing geological conditions. Amongst Alimak raise climber, shaft sinking and raise boring excavation methods, Alimak raise climber is most commonly used in Nepal. Selection of excavation method depends on geological conditions, size, length of shaft, access and of course the cost. Pressure shaft of Jhimruk, Khimti I, Chilime, Kaligandagi, Kulekhani, Khani Khola, Upper Tamakoshi and other hydroelectric projects were excavated with the help of conventional Alimak raise climber method. Severe overbreak problems were faced in Khimti I hydropower project during construction of upper inclined pressure shaft whereas recently 214 m vertical pressure shaft of Upper Tamakoshi and 280 m vertical pressure shaft of Khani Khola hydroelectric projects in Dolakha have been constructed successfully. Raise boring method was tried in Upper Tamakoshi Project but could not succeed due to deviation of pilot hole and hence shaft sinking and Alimak raise climber were used.
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