Cost Risk Assessment of 13 km Long Headrace Tunnel in the Himalayas

Abstract

ELC Electroconsult S.p.A. (Italy), in association with NEWJEC Inc. (Japan), is carrying out the updating of a feasibility study and the detailed design of a storage hydropower project in the Himalayas, under a grant from the Asian Development Bank. The project layout includes two powerhouses. The main powerhouse, with a capacity of 600 MW, is located at some 14 km distance from the reservoir. The headrace tunnel to the main powerhouse has a length of 13.3 km. It is located in a region of high seismicity, with lack of access along the tunnel alignment, which it makes it difficult to get comprehensive geotechnical information along the tunnel route. Despite the uncertainties present, it is necessary to provide and assessment of the headrace tunnel cost and schedule to determine the project feasibility. This paper shows the procedure followed by the Consultant to assess a risk-based contingency cost for the tunnel accounting for the uncertainties present. The uncertainties have been grouped in three types: (1) extent of the geological formations, (2) geo-mechanical properties, and (3) the occurrence of adverse events such as collapses, rockfalls, squeezing and tunnel flooding due to groundwater. The objective is to assess a level of cost contingency associated with a certain probability of not being exceeded, to inform the decision on the amount of the contingency cost that should be considered at this stage of the project.