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  Nonlinear analyses of underground power house affected by potential seepage from the pressuriszed power tunnel

  This paper details the analysis and design of support for an underground powerhouse and pressurized power tunnel incorporating the excavation sequence and connection with the other tunnels, which include: bypass and unit inlets and outlets; access adits; gate gallery; surge chamber; and tailrace tunnel. The impact of stresses and seepage pressures from adjacent tunnels, grouting and excavation shape are also discussed. The powerhouse is 94 m by 19 m wide and 38 m high approximately 70 meters below the ground. The power tunnel is some 1500 m long with a diameter of 8 m. Investigation of the powerhouse consisted of boreholes, in-situ and hydrojacking tests. 2D and 3D modeling was done to assess the stresses and deformations as well as the potential for hydraulic jacking of the rock between the power tunnel and the powerhouse. The hydrojacking study was used for the analysis of the length of steel liner required between the power tunnel and the powerhouse. In this study the rock support design is discussed and careful blasting was used to minimize the damage to the rock mass beyond the excavation perimeter.This case history deals with the comprehensive nonlinear numerical analyses; using MIDAS 3D and FLAC 2D to analyze and optimize the geotechnical and structural interaction designs of the underground power house and associated tunnels.The results of these analyses show the stress distribution obtained for the staged excavation and highlights the stress disturbed area between the powerhouse and power tunnel. Based on these analyses, a separation of 32 m is suggested between the powerhouse and the center line of the power tunnel to avoid the potential of hydraulic fracturing. Underground Powerhouse, Hydrojacking, Pressurized Tunnel, Seepage, Nonlinear Interaction.

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