The Windsor Upper Dam (aka The Ascutney Mill Dam) has stood for 185 years in Windsor, Vermont and is among the earliest storage dams built in the United States. It is recognized by the American Society of Civil Engineers as an engineering landmark and is listed on the National Register of Historic Places. In response to concerns raised in 2010, the town conducted a regular level inspection that identified several structural and hydraulic deficiencies, threatening its future.
Subsequent annual inspections indicated these deficiencies to be worsening—especially following Tropical Storm Irene—in 2011 with significant overtopping and loss of buttress shell masonry. Financial limitations led the town to choose an incremental repair approach that prioritized a balance between funding resources and best‐value risk reduction.
Some of the main challenges of the project included:
- Strengthening the mass of the buttress shell, yet allowing for maintained drainage to limit additional hydrostatic pressure.
- Flexible construction approach to manage unknown conditions.
- Limited room available to work on the narrow crest.
- Maintaining the historic aesthetic of the cut-stone masonry face and surrounding property.
- Staging to perform the work required access through multiple private properties.
- The budget was capped by a state grant and municipal bond, with no flexibility for cost overruns.
The Solutions Unearthed
- The solution consisted of consolidation grouting to strengthen the internal rubble fill of the buttress shell and auxiliary spillways.
- Additional measures such as vertical and inclined dowels and mortar repointing were included to reinforce the cut‐stone masonry facing on the buttress shell. Sub‐horizontal drains were installed to maintain drainage and limit build‐up of hydrostatic pressures.
- Through coordination with multiple property owners, temporary easements were obtained to allow access to the site.
- Drilling was performed through the crest and into the body of the buttress shell and auxiliary spillways to keep grouting work internal to the dam, thus maintaining the historic aesthetic.
- Access to the top of the dam was accomplished by using cranes to place all equipment and materials on a barge within the impoundment to provide room to accomplish the work. This had the added benefit that the town could keep the town beach open during the majority of the work since the contractor was able maintain pond levels higher than initially expected with the barge.
- Contract documents were developed with a focus on flexibility to allow the engineer and contractor freedom to make adjustments to quantities and approach during construction thus allowing the unknown condition of the rubble fill within the buttress shell and auxiliary spillways to be managed without costly “extras” to the town.
- Test programs were used to optimize the design to actual conditions encountered in the field, allowing selection of grout viscosity, injection pressures, and stage volumes that would achieve project goals.
- A sanded, low mobility grout was developed to be adjustable as needed, based on rubble fill conditions and grouting behavior encountered as the work progressed.
- Verification cores were drilled across the length of the dam throughout construction, following the test programs to develop production criteria and during production to confirm the effectiveness of the parameters established from the test phase and facilitate adjustments.
Client: Town of Windsor, Vermont