Engineering Geological Evaluation Of Underground Works Upper Madi Hydroelectric Project, Nepal
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Nepalese geology is termed as one of the youngest formations in the world. Tunneling in this Himalayan Geology is complex in nature. The very complex geology in the region offers challenge in stability of even the best location of underground structures. Tunneling in weakrock is more challenging in terms of stability and application of support. Upper Madi Hydroelectric Project is a run-of-river hydroelectric plant located north-east of the city Pokhara. The project will have an installed capacity of 20 MW with annual energy generation amounting 119.5 GWh. The project mainly consists of a diversion weir, surface settling basins, approximately 4 km long headrace tunnel, 37 m high surge shaft, a surface penstock, semiunderground powerhouse and a tailrace canal. The prediction of the rock mass has been done optimistically in the feasibility report of Upper Madi project which is correlated with implemented project and correction has been done on that. Also alternative alignment is analysed for the reduction of cost. Vertical drop shaft isreplaced by inclined tunnel and surface surge shaft is replaced by underground one. Afteralternative analysis support is calculated by empirical method and later on verified by numerical analysis using software tool Phase2. Stability analysis is also done for both tunnel and surge shaft and finally cost is calculated for inclined tunnel option. Numerical method of analysis has added advantage over empirical and analytical methods,particularly in complex geometry. The Phase2 code has been adopted and the Hoek-BrownFailure criterion has been used to determine the state of stress, strength factor anddeformations around the periphery and the in-between walls. The result will be useful for the prediction of rock mass in Himalayan condition in the preliminary phase of the project. Also it will be useful for the preliminary costing of the project. It will help for the prediction of support in very weak rock condition and also possibilities of big opening(surge shaft of 13.5m dia. and 33m height) in relatively weak rock.