Stability Assessment of Headrace Tunnel, Middle Modi Hydropower Project,Nepal
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Nepal is a land locked country with diversified topographical and geological features. Most of its parts are covered by the low hills to high Himalayas. The Himalayan geology is one of the tectonically youngest geological formations in the world. Except the young formations, rock mass in the Himalaya is highly weathered, fractured and weak in strength and is thus challenging for the construction of underground structure like tunnel and caverns especially for the development of hydropower project. Middle Modi Hydropower Project (MMHP) is also facing the problem with weathered, fractured and weak rock mass for the construction of headrace tunnel. The Middle Modi Hydropower Project (MMHP) is a run of the river scheme with installed capacity 15 MW. Design discharge of 25 m3/s is feeding 2 Francis turbines utilizing the grosshead of 75.34m. Geologically, MMHP lies in the Precambrian sequence of the lesser Himalayan meta-sedimentary rock formation of Baglung zone. The extension of Lumle Fault crossing the project area is challenge for the underground structures. The major structures of the project are headworks, approximately 2.8 km headrace tunnel with 34 m height surge tank and semi surface power house. Quartzite and Phyllite are the major rock types covered throughout the whole length of 2.8Km headrace tunnel. Quartzite and Phyllite rock mass formation are highly weathered, fractured and shared, and separated by the Sathi khola fault (Extension of Lumle Fault) about chainage 1+800 is the weakest part of headrace tunnel. The main aim of this study was to analyze hydraulic fracturing, rock support design throughout the headrace tunnel and carry out stability analysis in the weakness zone (Lumle Fault) by different approaches. Analysis of hydraulic fracturing from the different approaches; deterministic, analytical and finally verified by numerical model phase2. The highest hydrostatic water pressure in headrace tunnel was found to be 0.3 MPa. From numerical analysis, possible hydraulic fracturing was shown in tunnel crown in the weakness zone. From the analysis of few sections, rock bolts and shotcrete is sufficient for rock support except for the weakness zone of headrace tunnel. Stability analysis of the major weakness zone in headrace tunnel using empirical, semi analytical and numerical methods have been presented. From both empirical and semianalytical analysis, squeezing problem was found in the weakness zone. Moreover, it was also verified from numerical analysis that about 1 m of displacement was found in the periphery of tunnel contour. Consolidation grouting before tunnel excavation and shotcrete with reinforced concrete in weakness zone and except the weakness zone simply rockbolts and shotcrete has been proposed for the long term stability of headrace tunnel. Finally, based on the empirical, semi-analytical and numerical analysis; conclusion and recommendation have been made for the tunnel excavation, support installation and safety measure, further geological investigation and monitoring during construction.