Penstock Grouting at Whitehorse Rapids Dam in Whitehorse, Yukon-Paper

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The Whitehorse Rapids Dam in Yukon, Canada, experienced deformation in its two oldest steel penstocks, WH1 and WH2. Investigations indicated that settlement had resulted from voids and poorly compacted bedding sand beneath the pipes, likely caused by difficult construction conditions and cold-weather backfilling. Geophysical slab impulse response (SIR) surveys, visual inspections, density tests, and limited test holes helped characterize the subgrade.<br /><br />After repairing the damaged pipes, the project team used ultrafine cement grout to fill voids and stabilize the loose sand. A full-scale groutability test informed the injection approach and quality-control requirements. Grout was injected through nozzles installed along each penstock, with results monitored through grout volumes, pressure, flow between nozzles, and repeat SIR surveys.<br /><br />WH2 received about 34,500 L of grout through 72 nozzles. Poor support and high grout takes were especially evident near the lower, groundwater-saturated section. Execution challenges included unexpectedly large grout volumes, cement supply issues, and equipment disruptions. Lessons from this work led to changes in the WH1 plan, including revised grout mixes, tighter quality control, and improved termination criteria. WH1 received about 14,800 L through 80 nozzles, including additional tertiary nozzles in areas with low grout take.<br /><br />Post-grouting SIR surveys showed substantial improvement in apparent subgrade support beneath both penstocks. WH2 showed strong improvement across nearly the entire surveyed area; WH1 also improved, though some zones showed only moderate gains. Because SIR provides relative rather than quantitative measurements, further inspection and testing of grouted sand were recommended but could not be completed before the penstocks returned to service. Yukon Energy plans ongoing visual inspections, 3-D scans, and future SIR surveys to monitor performance.

Keywords

Whitehorse Rapids Dam

Yukon

steel penstocks

subgrade settlement

void filling

ultrafine cement grout

slab impulse response

groutability test

quality control

post-grouting monitoring