Cutoff walls are widely used in dam construction and remediation to control seepage and maintain hydraulic stability during temporary and permanent works. The Site C Clean Energy Project in British Columbia, Canada required installation of a 187m long by 14m deep cement–bentonite (CB) cutoff wall near the northeast diversion tunnel outlet during transition from active river diversion to final operation. The wall was intended to prevent river water backmigration through permeable overburden and provide hydraulic continuity with previously constructed seepage control infrastructure. As a value-added proposal, Canada Geo-Solutions, Inc. (GSI) and Menard Canada (Menard) proposed an excavator slurry trenching approach instead of crane-mounted clamshell panel trenching used in earlier phases. This paper presents a case study describing design, construction, quality control, and value engineering of the excavator slurry trenching approach. It discusses site geology, design criteria, construction sequencing, tie-ins, slurry management, and performance verification. Production rates, constructability, and overall performance are evaluated relative to clamshell panel trenching. Results show the excavator method provided notable schedule and cost advantages while meeting hydraulic conductivity and strength requirements. The case demonstrates that excavatorbased slurry trenching is an effective alternative for shallow-to-moderate depth cutoff walls in constrained environments when proper design controls and quality assurance are applied.