North Reservoir Dam – High Hazard Dam Rehabilitation

North Reservoir Dam is a 780 foot long, 25 foot tall HIGH hazard potential earth embankment dam that is owned and operated by the Town of Winchester. The dam was constructed circa 1875 for public water supply purposes; North Reservoir still serves as the town’s water supply today.

For nearly a decade, the town has engaged Weston & Sampson to conduct the safety inspections and rehabilitations of its dams and dikes, as necessary. The town most recently engaged Weston & Sampson to conduct a preliminary study and engineering evaluation of North Reservoir Dam. This effort included hydraulic and hydrologic analyses, subsurface explorations consisting of soil borings, installation of monitoring wells, excavation of test pits to locate a potential low-level outlet alignment, dive inspections to evaluate submerged features of the gatehouse containing the outlet headworks, structural assessment of the gatehouse and bridge, and geotechnical analyses and assessment of embankment stability. The study showed that the embankment had inadequate factors of safety against slope and seepage instability and consisted of loose sands susceptible to liquefaction and lateral spreading, presenting a significant safety risk in the event of an earthquake. The study also indicated the potential presence of hazardous materials at the dam. A Weston & Sampson Licensed Site Professional (LSP) subsequently conducted a site visit with the town and developed an approach to better quantify the materials. We completed test pitting activities and estimated the volume of debris that will require disposal. In addition, Weston & Sampson evaluated soil re-use/disposal options and developed a cost-effective soil approach.

Weston & Sampson, using its in-house multi-disciplinary engineers, scientists, and surveyors, is currently providing rehabilitation design and permitting services to bring North Reservoir Dam into compliance with current dam safety regulations and preserve this critical piece of the town’s drinking water supply infrastructure. The design includes broadening the embankment crest, flattening, buttressing, and armoring the slopes with considerations for liquefaction, constructing a two-stage graded seepage filter and embankment toe drain, replacing the bridge to the gatehouse, wetland replication, repurposing the existing water supply pipe as a low level outlet, pipe improvements including spray applied epoxy lining, new ductile iron pipe, a new stilling basin and downstream channel improvements, and new gate valves in the gatehouse.
Construction of the North Reservoir Dam rehabilitation is on-going.

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