TUNNELING ASPECTS OF THE WATERWAY SYSTEM AND STABILITY ASSESSMENT OF THE POWERHOUSE CAVERN FOR THE MINAS-SAN FRANCISCO HPP
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The future trend of energy supplies is gradually moving from fossil fuels to renewable and sustainable energy sources. In this context, hydropower is one of the best options to supply the base load and meet the energy demand. For this reason, it has been given priority in most countries with potential for hydrogeneration. Such is the case in the Minas-San Francisco Hydropower Project (MSF HPP) in Ecuador where underground excavations required the use of both the drill and blast method and a tunnel boring machine (TBM).The present work focuses on the analysis of geological conditions along 9 km of the TBM tunnel of the MSF HPP and how they affected the performance of the TBM during excavation. In the course of this study, emphasis was given to the rock mass parameters and cutting technology implemented in the TBM. Very abrasive and strong rocks caused a considerable delay in the progress of the excavation, an issue that was not foreseen during the planning stage. Results show that penetration of the disc cutters depended mostly on the strength of the rock mass. In the same way, records from penetration reveal that the TBM tunnel has self-supporting conditions along most of its length, showing the suitability of the rock for the choice of deep unlined tunnels and shafts. In the same scope of the study, stability assessment of the underground power house was conducted using the actual knowledge of the rock mass properties. Emphasis was given to the stress induced instabilities and this was addressed by analytical and numerical approaches. This analysis was guided by, observations and measurements from multipoint borehole extensometers, conducted throughout the construction of the cavern. It was found that the situation of stress in the cavern is such that moderate spalling in overstressed areas on the contour in fact occurred. However, because of the moderate effect of the spalling phenomenon, manifested by detachments of rock slabs, the instability problem was confused with construction issues and was badly understood. In the same way, a goodness of fit between modelled and measured displacements of the cavern reveal that no time dependent deformations had taken place after the conclusion of the civil works; this indicates the underground cavern is fully stablished in terms of deformations.