Abstract:
The Whitehorse Rapids Dam is located on the Yukon River in the city of Whitehorse, Yukon and managed by the Yukon Energy Corporation (YEC). The dam generates electricity at four turbines fed by 3.6 m diameter steel penstocks. Two of the turbines were installed during the original dam construction in 1958. Two additional units were constructed 1969 and 1985, respectively. In May 2020, damage was discovered in the two original penstocks during scheduled inspections. Additional inspections, surveying, instrumentation, (non-invasive) geophysical testing and limited invasive testing were carried out to investigate the cause of the damage. Loose bedding sand backfill beneath both penstocks was found to be the cause of the damage. Small voids (up to 1 cm) between the pipe and the surrounding sand were discovered in some locations. Larger voids were suspected in some areas based on geophysical testing. The penstock was repaired by pressing bulges using a hydraulic ram and installing repair plates in damaged areas. A grouting program was proposed to stabilize the subgrade by filling existing voids and permeating the loose sand using ultrafine cement grout. The results of the grouting program were evaluated by repeating geophysical testing and comparing the results from before and after grouting. The paper will describe the geophysical testing carried out before and after the grouting, the full scale groutability tests carried out before the start of the grouting program, challenges encountered during execution of the program, and initial results/findings from the grouting program.