A laboratory investigation was carried out to determine the physical properties of a suitable mix of cement-bentonite for a low-permeability cutoff wall implemented in 1991 under the north dyke at the LG-1 hydroelectric site, which is part of the James Bay Development Project in Northern Quebec. The project specifications required the mix to have a hydraulic conductivity of less than 10-8 m/s and be able to sustain a plastic deformation of 6% without fissuring, as measured in a triaxial compression test with a lateral pressure of 100 kPa on a specimen cured for 90 days. A two-phase laboratory study was carried out. Phase 1 involved the determination of the mix design, while the second phase consisted of the laboratory control of the manufactured mixture. During phase 1, laboratory samples were prepared with different proportions of cement, bentonite, sand, and water. The permeability, triaxial compression, and unconfined compression tests were performed after curing periods ranging from 8 to 120 days. Once the appropriate mix was selected, it was tested to check if it met the specifications. During phase 2, or during actual production or construction, density and viscosity measurements of the cement-bentonite slurry were made routinely at the plant and at the discharge of the slurry pipe. Cylindrical specimens were molded using slurry from the discharge of the pipe and from the trench. Permeability, triaxial compression, and unconfined compression tests were performed on the specimens after curing periods ranging from 3 to 103 days.