Predicted versus actual rock mass conditions: A review of four tunnel projects in Nepal Himalaya

The Himalayan region possesses enormous potential for hydropower development. However, there exists great challenge for the successful underground excavation due to complex geological set up of the region. It is generally accepted that the cost and time are of main concerns in any tunnelling project as these two factors influence greatly on the economic viability of the hydropower projects. Consequently, the accuracy of predicted geological conditions during planning phase plays an important role during its implementation. The degree of accuracy in predicting, evaluating, and interpreting the quality of rock mass along the tunnel alignment is thus a key for the successful completion of any hydropower project. This paper assesses and compares the predicted and actual rock mass conditions of the four recently constructed hydro-tunnels in Nepal Himalaya. Further, an evaluation is made on the extent of pre-construction phase engineering geological investigations. The effect on the overall cost and construction time caused by the variations in rock mass quality of these tunnel projects are discussed. A recommendation is given on the minimum level of geological investigation that may help improve the predictability of geological conditions and reduce the discrepancy between predicted and actual rock mass conditions to an acceptable level. (c) 2006 Elsevier Ltd. All rights reserved.