For 185 years, the Windsor Upper Dam (a.k.a. Ascutney Mill Dam) has stood in the Town of Windsor, Vermont. The structure is on the National Register of Historic Places and is recognized by the American Society of Civil Engineers as an engineering landmark that is “one of the first gravity-arch dams built in the United States, and possibly the oldest masonry dam of significant size” (ASCE Landmark website). Deficiencies noted in a 2010 inspection were observed to be worsening over subsequent annual inspections leading to repairs being initiated. Financial constraints required development of a repair approach that prioritized a balance between funding resources and best-value risk reduction.
Strengthening of the downstream buttress shell and auxiliary spillways was deemed of utmost importance to improve the stability of the dam. The buttress shell is comprised of rubble rock fill with an outer layer of large cut/dressed masonry blocks and provides global stability to the dam but was shown during analyses to have factors of safety significantly below modern-day standards.
Developing a repair required a flexible approach to manage unknown conditions inside the dam, maintain the dam’s historic character, and be completed within a budget fixed by a historic preservation grant and municipal bond with no flexibility for cost overruns.
The implemented solution consisted of consolidation grouting to strengthen the internal rubble fill of the buttress shell and the auxiliary spillways, and reinforcing the buttress shell’s cut-stone masonry facing. Contract documents were developed with both lump sum costs and unit pricing to allow the Engineer and Contractor freedom to make adjustments to quantities and approach during construction where warranted. This allowed the unknown condition of the rubble fill internal to the work to be managed without costly “extras” to the Town. Test programs were used to optimize the design of the repair to actual conditions encountered in the field. Verification cores were drilled along the length of the dam to confirm the effectiveness of the parameters established during the test phase and allow any needed adjustments.
This project exemplifies the benefits of a flexible construction approach with unit pricing and close coordination between the Engineer and Contractor to achieve project goals and meet a budget.
This project was a recipient of a 2019 Vermont ACEC Engineering Excellence Award and was presented as a case study in successful dam rehabilitations at the 2019 Association of Dam Safety Officials National Dam Safety Conference in Orlando, Florida.