CS33C - Concurrent Session 33C: The Site-Specific Intervention Plan – an Indispensable Risk Reduction Tool for Dam Foundation Investigations in Challenging Conditions

During normal dam safety inspection activities, some signs of potential internal erosion in the foundation of two dikes were detected downstream of the structures. These two embankment structures, with dimensions reaching 1 km in length and 15 m in height, are founded on overburden reaching depths of over 100 m and are part of the same facility that was built in the late 1950s. In 2019, Hydro-Quebec initiated a remediation works project to address this safety issue. As part of the remediation strategy, four large scale geotechnical investigation programs, spanning from 2019 to 2023, were launched during the first phase of the project. Several investigation techniques were used for the purpose of establishing a comprehensive understanding of the site stratigraphy and reaching a better understanding of the potential for backward erosion. The most part of the investigations consisted of Sonic drilling (5,800 m total length), Cone Penetration Tests (3,700 m total length) and groundwater pumping tests. The investigations were carried out from the crest of the two dikes to roughly 1,000 m downstream of the dikes.< br> < br>Prevailing site conditions included the presence of artesian pressures and semi-pervious erodible soils. In such a context, a risk assessment evaluation was conducted to identify potential hazards and evaluate the consequences if the identified hazards occurred. Consequently, a Site Specific Intervention Plan (SSIP) was developed to mitigate the risk associated with performing intrusive geotechnical investigations in a particularly challenging setting.< br> < br>The SSIPs were specifically adapted to different types of sensitive investigations to be conducted in critical areas. The SSIPs included an evaluation of all the potential hazards that could materialize during the investigations, a description of the site preparation earthworks to be completed prior the investigations, 3D groundwater flow modelling to establish threshold hydraulic gradients and the plan for daily site inspections and automated monitoring programs. All of this combined to rapid-response procedures adapted for each applicable hazard. On two occasions, Hydro-Québec had to apply the rapid-response procedures to rectify problematic situations, with success on both occasions. This demonstrated that the Site Specific Intervention Plan is an indispensable tool for any site investigation program that could potentially affect the performance of a dam, especially in difficult foundation conditions.