ASSESSMENT ON THE POTENTIAL USE OF SHOTCRETE LINED HIGH PRESSURE TUNNEL AT RASUWAGADHI HYDROELECTRIC PROJECT, NEPAL

Abstract

In this thesis, Rasuwagahdi Hydroelectric Project located in central region of Nepal, has been taken as the case study. In this project, approximately 4.2 Km long headrace tunnel with 138m long vertical shaft has been designed and being constructed. Rock types along the headrace tunnel alignment are migmatitic gneisses, quartzite, schists, banded gneisses, and weak or shear zones. In this case study whole tunnel alignment is divided into headrace tunnel and shotcrete lined high pressure tunnel. Major part of this thesis is to proposed the pressure tunnel and analyze it against stability and hydraulic fracturing. And another part of the thesis is to design and analyze surge shaft. Shotcrete lined pressure tunnel is proposed from the construction audit two at chainage 1+850m. Surge shaft is proposed at chainage 2+064m. Along the proposed high pressure tunnel and surge shaft location quartzite is the dominant rock type. In this thesis two different method have been used for the analysis of Hydraulic fracturing. Analytical evaluation has been done with Norwegian rule of thumb. All together 10 section have been selected and analysis has been performed. All sections have factor of safety more than two which suggest tunnel is safe from hydraulic fracturing. Numerical modelling of four selected section has been performed with the help of Phase2 program. Results of the phase2 also shows that pressure tunnel is safe against hydraulic fracturing. Stability analysis of the pressure tunnel and surge shaft has been carried out using Numerical modelling. Input parameter are estimated based on geological report of Rasuwagadhi Hydroelectric Project and lab testing of rock sample collected from powerhouse excavation, through Rocdata. Computer program Phase2 has been used for numerical modelling with generalized Hoek and Brown failure criteria. Analysis of pressure tunnel is performed on 4 selected sections and support are estimated based on the analysis result. It is concluded that the proposed shotcrete lined high pressure tunnel and surge shaft are stable and safe from hydraulic fracturing. Proposed pressure tunnel is good alternative of present headrace tunnel. It is also safe and economical.