Engineering geological evaluation of underground works for Nyadi Hydropower Project, Nepal

Abstract

Geologically, Nepal lies in the tectonically active zone and youngest geological formation in the world. Due to complex nature of geology, construction of underground structures is a challenging issue in the Himalayan. A complete geological evaluation is essential in the entire project area for technically viable, economic and safe construction of underground structures. Geological assessment is more significant for the underground structures; unlined power tunnel, settling basins, surge shaft, powerhouse cavern, tailrace and access tunnel proposed for the case of Nyadi Hydropower Project.

Nyadi Hydropower project is a run-of-river type project, located in Lamjung district of western development region of Nepal. Using the additional tailrace water of Siuri Khola Hydropower Project, the project has been planned for an installed capacity of 30 MW with a gross head of 347.5 m. The proposed tunnel length of the project is about 4000 m with 9.14 m2 in cross-sectional area and about 675 m long penstock of 1.75 m diameter to convey 11.08 m3/s design flow from Nyadi Khola to the underground powerhouse. The entire project lies in metamorphic units of higher Himalayan.

This thesis aims to carry out an engineering geological evaluation of all underground works for Nyadi Hydropower Project. In this regards, an alternative alignment has been proposed with the unlined power tunnel about 4270 m long, 15.4 m2 in cross-sectional area in which about 2330 m is high pressure and remaining about 1940 m as low pressure tunnel has been proposed to convey 18.5 m3/sdesign discharge to generate 249 GWh annually. The settling basin has been designed for fibre reinforced shotcrete instead of concrete lining on the basis of cost comparison. The proposedalternative alignment excludes penstock pipe, vertical shaft and also reduces additional tunnel length nearby surge shaft and access road to the adit of surge shaft which is considerable component for the reduction in overall cost of the project. About 5 million USD will be benefited from additional energy generation during wet season with this proposed alternative layout. The entire project is located in themetamorphic units of higher Himalayas. Faults and weakness zones intersect with the alignment of power tunnel at some sections where one parent thrust zone of tectonic thrust crosses at high pressure tunnel. Further stability analysis and rock support estimation has been carried out for the proposed underground structures.

The unlined high pressure tunnel experiences hydrostatic pressure up to 3.5 MPa (35 bars). The Norwegian criterion for confinement has been used to ensure safety against hydraulic fracturing in the pressure tunnel. This has also been performed by numerical analysis using Phase2 program.

Numerical analysis using Phase2 and Generalized Hoek-Brown failure criterion has been used for the stability analysis and rock support optimization of pressurized headrace tunnel, settling basins and powerhouse cavern. Estimation of required input parameters for numerical analysis has been based on the preconstruction phase geological investigations in the project area and lab tested based resultsfrom the similar rock type with almost similar geological conditions projects such as Upper Tamakoshi Hydroelectric Project and Middle Marsyangdi Hydroelectric project have been used to estimate rock mechanical properties.

Finally, conclusion has been made on the stability and estimated rock support for the proposed alternative underground structures on the basis of analytical and numerical analysis and recommendations have been proposed for good results that resemble with the ground conditions.