Foundation cutoffs for flood-detention reservoirs

In design and periodic safety reviews of flood-detention reservoirs one of the key issues, in terms of sustainability, affordability and safety, is the need for a positive cutoff through foundation strata to prevent internal erosion. Such strata often involve alluvial or terrace deposits, which are of variable composition and lithology. The authors will describe the considerations that affect the decision as to when a positive cutoff such as sheet piles, or a bentonite cutoff wall is required, and conversely when it can be omitted, with the potential consequences of such an omission. The latter is likely to include additional requirements for surveillance and monitoring. The paper will describe the investigations that are necessary to inform that decision, including desk study, understanding geological context, site-specific intrusive investigations and arrangements for inspections and so on during construction. The paper will conclude with key issues to be evaluated in deciding when a positive cutoff is necessary.