A number of problems are being faced in the construction of the two projects, namely Punatsangchhu hydropower project P-I and P-II. These problems have occurred due to the fact that the projects are located in rugged terrain with complex geological and geotechnical conditions. The rock mass conditions are highly anisotropic in nature and a number of challenges are being faced during the construction. Especially in underground constructions such as powerhouses, desilting chambers and headrace tunnels a number of rock instabilities can occur such as loosening of the rock mass, squeezing and spalling. It is not easy to predict the behaviour of anisotropic rocks due to the variation in mineral assemblages, fabric and geo-mechanical properties. Therefore, a detailed engineering geological assessment of such rocks is warranted. NGI is carrying out engineering geological studies including rock mass characterization and rock support design in underground constructions using numerical simulations.  The client is the Punatsangchhu hydropower authorities in Bhutan through the Ministry of Water Resources.

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NGI is applying analytical, empirical and observational (instrumentation) methods for verification of reinforcement design in the surface and sub-surface structures. The analytical approach involves using numerical techniques such as distinct element modelling (UDEC and 3DEC) for predicting the behaviour of the rock masses in tunnels which is then correlated with the empirical approach using the NGI developed Q-system of rock mass classification. In the observational approach, continued monitoring of the rock movement is planned to obtain an efficient and stable support in both underground structures and in the right bank of of Punatsanhchhu hydropower project P-I. The expert advisory services are planned to be continued till the projects are completed in 2021.

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