John Hart Dam Seismic Upgrade: A Robust Seepage Barrier for Cascadia Megathrust Resilience

Abstract

The John Hart Dam, constructed in the late 1940s on Vancouver Island, required upgrades to address seismic vulnerabilities and ensure resilience against a potential magnitude M9 mega earthquake originating from the nearby Cascadia Subduction Zone. The Middle Earthfill Dam (MED), one of three earthfill structures at the site, was determined to be at risk of flow slide failure or significant displacements due to liquefiable and strain-softening foundation soils. Many conventional ground improvement techniques were unsuitable due to environmental sensitivities, constructability challenges, and dam safety concerns, especially given the need to avoid reservoir drawdown and cofferdam installation. To overcome these limitations, a range of solutions—from straightforward to moderately complex—were implemented to rehabilitate the various structures. These included an upstream buttress berm with dredging and replacement under full reservoir conditions, unloading by excavation, and a downstream toe berm reinforced with steel pinning piles. A key element was the construction of a new Plastic Concrete Cutoff Wall (PCCW) to replace the vulnerable slurry trench cutoff wall, providing a robust and seismically resilient seepage barrier. The PCCW was embedded into bedrock or silt and constructed with a suitable mix to ensure durability and reliability. The presentation will outline the overall upgrade strategy and focus on the PCCW’s design and construction, highlighting key challenges and performance verification. The seismic upgrade of the dam is ongoing, with the PCCW completed in early 2024. (https://majorprojects.ca)

Presenter

Thava Thavaraj, Ph.D., P.Eng. Senior Geotechnical Engineer/Principal Klohn Crippen Berger [email protected] CAEES-ACGPS Secretary