Numerical Modeling of the Underground Machine Halls in Tala and Chukha Hydropower Plants
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The Kingdom of Bhutan is part of South Asia. It is located at the eastern end of the Himalayas, encircled by India in the south, east, west, and China in the North. A major part of the electricity consumed in Bhutan originates from hydro power. In the western part of Bhutan alongside the Wangchhu River, two of the countries water power plants are located 30km apart. Furthest to the north we find Chukha Hydel, downstream Tala is situated. These power plants lie in close proximity to ?the Main Central Thrust? which contributes to high tectonical stresses and challenges regarding the excavation of the powerhouse caverns. Chukha hydro electrical power plant is the oldest power plant in the country. It was fully operational in 1986-1988, with a maximum capacity of 336MW. The rock mass surrounding the powerhouse cavern is dominated by banded biotite gneisses with minor bands of schists and shear zones 5-50cm in thickness. The joints of the rock mass are mapped, and the is-situ stresses are measured. Basic rock mass parameters are also investigated ? both in the field and the laboratory. The excavation procedures and the securing measures taken are referred to in respect to the literature. During the constructional phase, a major rock fall in the roof arc of the cavern occurred as a consequence of the high vertical stresses and the weakness zones. The accident resulted in material damages, personal injuries, and sadly the loss of personnel. Today it has been operational for 25 years, and the powerhouse cavern has yet to experience problems with stability. Tala hydro power plant is located-, as previously mentioned, about 30km downstream of Chukha. The construction of the power plant complex commenced 1998, and was completed in 2005. It?s made out of two main caverns; a machine hall cavern and a transformer cavern. Separating them is a 40m pillar and several access tunnels connecting the two main caverns. At the moment of completion, Tala was the largest power plant of its kind, with a production capacity of 1020MW. The machine hall cavern is situated in a rock mass dominated by jointed, folded and foliated phyllites with anisotropic properties, phyllitic quartzite and quartzite bands. The main joints are mapped, and the properties of these joints are evaluated. The excavation procedures and the securing measures are described together with the rock mass properties. During the excavation of the roof arc of the machine hall cavern, a 70m stretch of the roof overhead collapsed, resulting with the installation of steel beams to secure the roof. As the excavation of the cavern proceeded, several of the installed rock bolt at the downstream and upstream sidewalls failed. Now that the power plant is in its operational stage, rock bolts continue to fail, and convergence measurements shows deformations still taking place. In this master?s thesis the software ?Phase2? is used to generate numerical models of Tala and Chukha. Then it?s possible to see the reasons for the stability problems Tala is going through from the aspects of rock mass quality and in-situ stress state, and why the powerhouse cavern of Chukha remains as stable as it is. To get more acquainted with the software, ground reaction curves were made representing five rock mass qualities/classes ranging from very good to very poor. Two models were made for each class, showing two values of the residual strength. The results of this investigation helped determining some of the rock mass parameters to be used in the models representing Tala and Chukha. To ensure the reliability of the models, the model covering Tala gets calibrated to displacement measurements already performed. The rock mass parameters in Phase2 get modified ? this way the calculated displacements is as close to the measured displacements as possible. Seeing as displacements measurements from Chukha were not obtained, the model covering Chukha is not calibrated. The geometry, safety parameters and the excavation procedures regarding the models is described in detail, enabling the reproduction of the results.Two models are created for each of the locations; the first model aims to show the rock mass as it is this present moment, the second model attempts to explain a weakened rock mass. This is done by adjusting the GSI value in the software; the higher the GSI value, the stronger and less jointed the rock mass becomes. A lowered GSI value therefore represents the rock mass as it gets more fractured, which is to be expected as the proximity to the excavation boundary increases. Towards the end of the thesis, the results of the numerical analysis are analyzed, and some of the main conclusions worth to mention is:? The overall rock mass of Chukha is stronger than the rock mass of Tala? The stress conditions of Tala are more challenging than that of Chukha, seeing as Tala is closer to the MCT than Chukha. This causes high horizontal and tectonic stresses. When the anisotropic rock mass is exposed to these conditions, the rock mass keeps deforming. ? The geometry of Tala is not optimal when compared to the in-situ stresses ? the machine hall-, and the transformer cavern is too close to each other, and the machine hall is possibly too high compared to the width. As the quality of the rock mass gets lowered and the GSI is reduced:? The number of yielded bolts at Tala is increased.? The zone of affected/yielded rock mass stretches further into the surrounding rock mass.The thesis finally concludes that the numerical models of Tala and Chukha are reliable, and that they can be used to estimate the future stress-, and stability conditions surrounding the caverns of Tala and Chukha hydro power plants.