Stability Analysis for Underground Power House at Tala Hydroelectric Power Project, Bhutan
MetadataVis full innførsel
Concerning socio-economic development of Bhutan, Hydropower development plays a vital role. Envisaging a total theoretical potential of 30,000 MW, according to Power System Master Plan, 2002, the government now has taken up measures for accelerated development to harness 11777 MW by year 2020, and more beyond this time frame. Nevertheless, there is always a difficulty to harness the great hydropower potential due to the uncertainties and challenges faced in tunneling through Himalayan Geology. The challenges can be attributed to the difficulty in obtaining a reliable & better early investigation results due to inaccessibility of the area. The unpredictable geological conditions in the Himalayan region in general, and the difficulty for geotechnical studies have been the greatest setbacks to find better options for locating and aligning the underground openings. These have caused severe stability problems during construction of underground cavities or tunnels in Bhutan, mainly related to rock mass quality and rock stresses. Tala Hydroelectric Project with an installed capacity of 1020 MW on the river Wangchu Basin of Western Bhutan is so far the biggest project constructed (1997 – 2007). This project has suffered rock engineering problems during construction to post construction phases and continuing still in the operation period. The constructions of almost all the major components of the project have met with stability challenges. The machine and transformer caverns are still experiencing two main problems: i) the failure of rock bolts which fly out of caverns’ walls and, ii) convergence of walls, since the construction period and are continuing during operational phase too. These are serious problematic issues, creating hazardous situations and uncertain long term stability may affect the operation of the plant. In this thesis, efforts have been made to study the problems encountering in the machine hall of Tala Hydroelectric Project and finally determined the main causes of the resulting instabilities of the case. After having addressed the problems in Tala, this work started on reviewing the case based mainly on geology, rock mass quality, in-situ stresses, geometry, and excavation & supports. Chukha Hydroelectric Project lying 2 km upstream of Intake for Tala, along the same river basin (1974 – 1988), on the other hand, has not faced such problems during operation period. A review study is conducted in a similar manner on Chukha project as well. The works throughout are focused on machine halls of the two projects. Qualitative studies of the machine halls of two projects have been carried out to compare and identify the most likely aspects of stability problems. The work involves theoretical study and application of engineering geology and rock engineering in reviewing of the cases. The engineering geological investigations of respective phases have been reviewed and through evaluation of deviations, the causes of the instabilities are ascertained. It is found that Tala Cavern is located in closer proximity to MCT and narrower valley than Chukha powerhouse, which signifies high stress regime. Weak rocks such as phyllite & quartzitic phyllite dominate the rock mass in Tala, while Chukha cavern constitutes better grade of metamorphic rock like biotite gneisses. The joints are normally clean in Chukha but infilling materials prevail in Tala with puckered and folded foliations. All the rock mass properties and classifications reveal weaker rock mass quality in Tala than the later project. Based on the above studies, this thesis presents a discussion on the probable reasons of stability problems being faced in Tala cavern. It is evaluated that the powerhouse is constructed in a high stress regime influenced due to tectonic activity, topography and overburden. Besides, the rock mass quality is found to be poor and deteriorated further during excavation. The failures of rock bolts in sections are mainly due to excessive overburden loads and existing weak rock mass conditions in folds enabling development of stresses under compression and tension in varying depth. The creeping nature of the rock mass prevail due to high horizontal stress, time-dependent behavior of highly jointed weak rock mass, reduced shear strength and unfavorable orientation of the cavern with respect to discontinuities. Geometry for shape and dimensions play important roles for the stability of the underground caverns. Beyond qualitative analysis, Examine2D program has been used to investigate the influence of geometry and induced stresses due to excavation. The analysis results indicated higher relative stress regime situation and more relative deformations for Tala compared to Chukha, which has quite favorable stress regime with negligible elastic deformation along the walls. The zones of influence of induced stresses and failure trajectories expand in larger area and in fact they intersect between the two caverns of Tala. At the end, some recommendations are put forth which may be used for successful construction and implementation of future projects under similar conditions in the vicinity of tectonic thrusts. This study has found that fixing location of underground cavity must be studied and options to be explored through qualitative judgment of investigations. The results of the investigation phases show great variations. These make the input parameters for any analysis unreliable and uncertain to a greater extent. An attempt to improve the quality judgment through involvement of experienced and right professional is mandatory for any future project. The designs pertaining to shapes and dimensioning under such conditions need serious attention. Most importantly, proper sequential and careful excavation must be incorporated at all times in such geology. Implementation of any future project must be documented properly and experiences from such cases may be made available to achieve successful tunneling in the Himalayan region.