The new 7 m diameter (23 ft) Gorge 2nd Tunnel will be constructed next to an operating hydropower tunnel. Observations during design investigations and existing tunnel inspections indicate hydraulic connectivity between the host bedrock and existing tunnel. This paper discusses the approach used to pretdict groundwater inflows into the new tunnel excavation, including analyses to evaluate influence of a constant head source in the existing tunnel on construction inflows into the new tunnel. The approach includes finite element modeling and statistical risk analysis to assess hydraulic input uncertainty. Results are used to design a groundwater management program-including pre-excavation grouting and a water collection and storage system.
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