Preliminary field investigation, support design and observational construction method for the Kyogoku Hydroelectric Power Plant Project

摘要

This paper addresses the preliminary field investigation method for in-situ rock stress estimationrnand support design based on the investigation results. Observational construction methodrnimplemented in the excavation of a powerhouse cavern is also discussed in detail. The KyogokurnHydroelectric Power Plant is located at a depth of approximately 430m below the ground surface. Thernpowerhouse cavern is bullet-shaped in the cross section with a longitudinal length of 140m, a heightrnof 45.8m, and a width of 24m. The site geology is characterized by Miocene tuff breccia, withrnoccasional tuff blocks of several tens of meters in size. The uniaxial compressive strength of the tuffrnbreccia is low proximal to the faults and varies from 30 to 40MPa, while it is typically 100MPa forrnthe intact rock.rnThe powerhouse cavern of Kyogoku project is to be constructed in difficult conditions in terms ofrnthe structural stability in comparison with those of other underground power plant projects. At thernKyogoku construction site, the strength parameters and deformation modulus of rocks were notrnexcessively large in spite of thick overburden and high in-situ rock stress. In-situ rock stressrnmeasurement was performed at a total of 16 points in 4 boreholes using two kinds of measurementrnmethods to provide input for excavation analysis of the powerhouse cavern. However, the large scalernof the cavern and limited data available prior to the excavation generated uncertainties inrncharacterizing the geological structures and rock mass properties. Therefore, the cavern constructionrnwas managed using the observational construction method in order to consider actual rock response tornexcavation and to ensure structural support for the cavern to be optimum. The final analysis modelrnwas empirically derived from field observations and monitoring. The excavation was completedrnsafely in December 2010, owing to observational construction scheme and robust support design.