A comprehensive laboratory investigation was carried out to determine a suitable mix of cement-bentonite for a slurry trench cutoff wall for Caniapiscau Reservoir project in James Bay, Canada in 1980. Specifications required the mix to have a permeability of less than 0.01 μm/s and to be able to sustain a plastic deformation of 10% without fissuring as measured at 90 days in a triaxial compression test with a lateral pressure of 196 kPa. Laboratory samples were prepared with different mixes of cement, bentonite, sand, and water and kept in a curing chamber of 100% humidity. Permeability determination and triaxial compression tests were performed after curing periods ranging from 10 to 120 days. Over 84 specimens were tested. The compression tests included unconfined tests, unconsolidated-undrained tests at a constant rate of strain, unconsolidated-drained tests with varying strain rates and at a controlled rate of stress, and a consolidated-drained test. Permeability tests were performed before and, in some cases, after loading. The specimens were examined for fissures and cracks after a strain of 10%. It was concluded that a mixture of 6% (by weight) Ultra Gel 180 bentonite and 160 kg of cement in each cubic metre of the mixture would meet the required specifications. Recommendations for standardized test procedures are made.