Engineering Geological Study of Punatsangchu Stage I Hydroelectric Project in Bhutan
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Hydropower development has resurged in the developed and in the developing countries due to the global issue of climate change, the need to reduce carbon footprints and the need to achieve energy stability and energy security. The IPCC‟s fourth Assessment Report (AR4) had showed that renewable energy (RE) has the potential to contribute to the mitigation of climate change via the reduction of greenhouse gas emissions in the energy sector. Therefore, hydropower is to remain as the largest renewable source of electricity generation and its development is going to play vital roles in the supply of reliable and sustainable energy for the socio-economic development of many developing countries. The topography and large annual volumes of water flow in Bhutan combine to give huge hydropower potential. With the gross estimated capacity of 30000 MW, the energy sector is expected to play a significant role in the Bhutanese economy as a major source of revenue. Apart from the need to meeting the increasing growth in domestic energy demand, there is a huge possibility of exporting the power to India and generate revenue. The shortage of power and the thermal driven nature of the Indian grid offer ready market for the hydroelectricity from Bhutan. As such, in order to exploit this opportunity, the government has placed high priority on accelerated hydropower development. It has set ambitious target to add minimum of 10000 MW generation capacities by 2020. Several large hydropower projects are under construction and many are in the planning stages. With the accelerated hydropower development taking place in the country, the construction of tunnels and underground works are going to increase considerably in the future. Hydropower plants have to be necessarily put underground in view of the limited stable surface space and the environmental concerns, as more than 51% of the national territory falls within the protected areas of national parks and biological corridors meant for environmental conservation. However, underground construction remains a challenge in the Himalayas. Due to active tectonics and weathering, the rocks are highly deformed, schistose and altered. Predicting rock mass quality and stability of underground constructions is difficult during planning due to high overburden, forest coverage and rugged nature of topography. Hydropower projects have experienced high discrepancies in the predicted and actual rock mass and encountered major adverse geological occurrences in tunneling, resulting in delay and cost overrun. In this thesis, engineering geological study of the Punatsangchu Stage – I Hydroelectric Project in Bhutan has been done with the aim to study the stability assessment and support requirement in Headrace tunnel and Powerhouse caverns. The details of planning and investigation works done during preparation of detailed project report have been utilized in the study. Details on rock mass collected during visit to construction site have been incorporated. The thesis deals with the review of engineering geological investigations which includes geological mapping and assessment of rock mass conditions, laboratory and in-situ tests and measurements; determination of engineering parameters of rock mass; stability assessment and estimation of rock support requirements with wedge analysis, empirical approaches and numerical modeling with Phase2 finite element program. The HRT is a low pressure tunnel and hydraulic fracturing is not occurring even at the point of surge shaft where the maximum hydrostatic pressure is 0.74 MPa while the minimum confining in-situ rock stress is 3.5 MPa at the point with minimum rock cover. But due to stress redistribution after excavation, the confining stress around the tunnel periphery reduces to almost zero, which would lead to opening up of joints and raveling of rocks. Tunnel lining with concrete is necessary. However, this is only an indication as there are still uncertainties with the assumptions in the analysis. The HRT is aligned unfavorably with one minor joint set for more than 2 km stretch towards upstream of adit 1. The wedge analysis shows that in general, with the major discontinuities mapped during construction, there is formation of roof wedge in the tunnel arch back but not in the ribs. The most critical wedge indicates wedge apex height of about 5 m. This is the basis for the lengths of rock bolts in HRT. Goel‟s empirical approach qualifies the HRT at maximum rock overburden (>500 m) for the mild squeezing condition with poor rock mass. Rock burst will not be the problem with poor jointed rock mass. With the Q-method, the ratio of maximum tangential stress and UCS of rock mass estimated by empirical methods is high, indicating heavy squeezing condition. Rock slabbing and popping is possible during heading from the junction of invert and walls due to high stress concentration in the area. During benching, the stress concentration is reduced. Deformations in the plastic analysis even after application of supports are higher than the elastic limits but are considered acceptable. The Powerhouse caverns have also been analyzed with numerical modeling. There are potential wedge formation in the cavern arch back and ribs. The most critical wedge apex heights indicates the requirement of roof bolt length of 8-10 m long and wall bolt length of 12-13 m long. The minimum distance of the rock pillar between Machine hall and Transformer hall needed is 50-53.5 m. The deformation and yielding of rock mass around excavation periphery is huge. The maximum displacement in the Machine hall walls occurs at elevations from 844.5 masl to 864.5 masl while in Transformer hall it occurs from elevation 856.5 masl to 867.5 masl after full excavation. After application of full supports, the maximum displacement in Machine hall is about 174 mm occurring on the walls. This is higher than the elastic limit of 71.8 mm but is considered acceptable. In the Transformer hall, the maximum displacement is about 111 mm and occurs in the downstream wall (R/S of the section). The stage-wise displacements in the cavern excavation vary and are given in Appendix 8-3. The support requirements are heavy but the results are only indicative and have to be verified with instrumentation and back analysis. Parametric study of underground excavation is carried out with the HRT section by independently varying some key rock parameters and observing displacements occurring in the excavation section. GSI is the most important rock mass parameter in the numerical analysis. It has great influence on the tunnel displacement. More importantly, it influences all other engineering parameters used in the analysis. Rock parameters have to be chosen/determined carefully that best represent the actual site condition.