Underground Excavation Support System for Hydropower Tunnels in Bhutan
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Hydropower is the major natural resources known in Bhutan with theoretical total potential of 30 000 MW, and 23 660 MW from seventy six sites all above 10 MW is found to be techno-economically feasible for development according to Power System Master Plan,2002. The development of hydropower is very much delayed in the country and realizing that the income from the few existing plants contribute to 50% of the total national revenue, the government has given top priority to the development of hydropower in the country and came out with Hydropower Policy, 2006 with the measure for accelerated development. At present many hydropower projects construction are being undertaken in Bhutan under this accelerated hydropower development mechanism. Vast majority of the hydropower construction job involve underground excavation where local geology and rock supports play vital role both in terms of construction cost and completion time. Poorer the geology higher the exaction cost and longer the construction time required which are significantly increased for extremely poor quality rock mass. The correct choice of type and timing for rock supports is very important for the stability of the rock mass and the safety of the working crew. Installation of heavier supports will make the project expensive and it is unnecessary where rock quality is good since surrounding rock itself will bein position to withstand the induced stresses. On the other hand insufficient amount of support will result in catastrophic situation of collapse of excavated tunnel and cavern leading to significant reconstruction cost and even the loss of lives. If the failures of supports occurs later after the project is commissioned, there will be huge loss of revenue as shut down of plant will be inevitable for repair of the tunnel, and sometimes this failure may affect other connected project components. Therefore the right choice of tunnel supports during construction play dominant role for the stability of the structure and keeping the cost at minimum. The concrete lining of hydropower tunnel is very common in Bhutan. Complete water tunnels of existing power plants are concrete lined and the design of all the projects underconstruction designed by Indian firm foresee concrete lining. This practice clearly indicates that the concrete lining is rather followed as traditional practice of safe design methodology rather than on the basis of understanding the mechanics of surrounding rock, which is making the project unnecessarily costly for the poor country. The estimation of rock supports based on Q-method (Barton et al., 1974) requires concrete lining for extremely poor quality rock and equivalent dimension of more than five. For better quality than this, alternative lighter supports using combination of rock bolts and shotcrete will meet the desired stability requirement. The invert of the tunnel will however require concrete lining. The rock bolts and shotcrete are not only cheaper, but can be installed very quicker than steel ribs and concrete lining. As far as possible the combination of rock bolt and shotcrete should be the preferred option for supporting the rock mass both from the time and direct cost aspect. Further these two being most flexible will allow adjustment later if at all additional support is required. The case study of Tala HRT revealed that only 4.1% of total length of 23 km of HRT actually required concrete lining in accordance with Q method. The benefit-cost analysis of concrete lining on account of reduced head loss during the life time of the project indicated benefit-cost ratio of 0.52 and 0.76 for two cases of supports as per Q method and optimal options using minimum of 200 mm thick SFRS respectively. The extra cost of providing concrete lining is Nu. 1,185.62 million ($ 26.35 million) and Nu. 406.78 million ($ 9.04million) respectively for the same two above support cases. As indicated above and found out from numerous other studies, it is not economical to install concrete lining solely for the purpose of reducing friction loss as the cost offsets the lifetime benefits due to reduced head loss obtained through concrete lining. Numerical analysis at four sections along HRT was done using different combinations of rock bolts and shotcrete thickness. The minimum thickness of shotcrete required is found tobe 200 mm, which is adopted as minimum thickness in combination with rock bolts as basis for alternative lighter support case analysis for concrete lining. The parametric analysis for UCS,GSI, Field stress and Young’s modulus has been conducted varying the values of these parameters up to maximum of +/- 50%; two steps each on either side of the base case of 100%,at 25% interval. GSI has significantly higher effect than other three on total displacement and yielding of both finite elements and the supports. For instance 25% increase in GSI reduces the total displacement by 47%, while decrease of GSI by same percentage increases the totaldisplacement by 158%. The interesting to note is that increase of values of GSI, UCS andYoung’s modulus tend to decrease the total displacement and yielding and decrease in values of these parameters will increase the total displacement and yielding. The field stresses have opposite effects than other three parameters. Increasing the field stresses increases total displacement and yielding, and decreasing it has reducing effects. The deep weathering of Himalayan geology associated with high level of water seepage and prevalence of major active tectonic fault and thrust zones pose serious challenges to underground excavation and tunneling in Bhutan. These problems are further compounded by poor accessibility to project site for proper scientific investigations resulting in inaccurate prediction of potential problems.