Abstract
High head tunnel plugs must be designed to prevent plug failure as well as hydrofracturing of the surrounding rock. If the water pressure behind the plug exceeds the minimum rock stress, a fracture can develop in the rock which will propagate outward toward the hillside after which this fracture will become a leakage pathway. General criteria for positioning a plug to avoid rock splitting are discussed. The overall problem of sufficient cover for unlined tunnels in rock is thoroughly reviewed via case histories and various design approaches are described. Several reported plug failures are also discussed. Finally, experience with the Tyee Lake Project plug is presented. Operating under a water head of 1,338 ft (580 psi), this plug is 12 ft wide by 10 ft high by 60 ft long and has an 84-in. diameter penstock pipe passing through it. Mini-hydrofracturing tests were used to select the final plug location. Upon filling the tunnel, a water leakage rate of 31 gpm along the rock/concrete contact occurred. This was reduced to 11 gpm by re-grouting. Piezometer measurements taken along the plug perimeter showed a sharp drop in pore pressure over the first 10 ft of plug.
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