Construction of a Jet-Grouted Backup Seepage Cut-Off Wall in the John Hart North Earthfill Dam

To provide additional protection to a 70 year-old dam in case of a seismic event, the local utility company, BC Hydro, required the construction of a backup seepage cut-off wall in the right abutment of the John Hart North Earthfill Dam, using jet-grouting, a well-known technique that uses a high-velocity jet of grout mix at high pressure (300 to 500 bars) to erode and simultaneously mix cementitious grout with the soil in situ. The main challenge was the use of these high pressures during construction of the seepage cut-off wall in the dam fills and foundation soil while avoiding reservoir drawdown, to minimize the risk of hydro-fracturing or hydro-jacking in the embankment. The contractor carried out a field trial in a safe area with similar soil conditions and, with the utility, developed a jet-grouting procedure that minimized the risks. The paper describes the jet-grouting procedures, grout mix used, results of the field trial, and safety and environmental precautions implemented on site to reach successful completion.