Inspection, Phase II Engineering Evaluations, Final Design and Permitting of the Rehabilitation of the Nepaug Reservoir Dams

Since 2007, under an existing Task Order Contract with The Metropolitan District (Hartford, CT), GZA has been conducting detailed dam safety inspections and engineering evaluations for the Nepaug Reservoir Dams. This work has included the Nepaug Reservoir Dams situated in New Hartford, Canton and Burlington, CT and is comprised of Nepaug Dam, a 156-foot-high (max), 650-ft-long, arched, cyclopean concrete gravity dam; Phelps Brook Dam, a 67-ft high, 1,250-ft-long earthen embankment dam and East Dike, a 30-foot high, 700-ft-long earthen embankment dike. Nepaug Reservoir (9.5 billion gallons) is an integral component of the District’s water supply system providing about 1/3 of their 60 mgd demand. The construction of the dams and dike was supervised by the venerable Caleb Saville and completed 1918.

Inspection and Initial Evaluation

GZA’s work began with a review of existing information including design drawings, design and construction correspondence, construction photographs and several papers/journal articles prepared by the original designers and published by City of Hartford Board of Water Commissioners or the New England Water Works Association. In-depth field inspections of dams and appurtenances including bridge condition survey (Nepaug Dam), diving and ROV of underwater features and water intake structures (Nepaug Dam and Phelps Brook Dam). Recommendations for engineering evaluations were presented therein.

Phase II Engineering Evaluations

Phase II engineering evaluations included subsurface explorations (test borings, packer testing, geophysical logging), installation of vibrating wire piezometers, detailed hydrologic and hydraulic analyses, 2D gravity stability analyses of Nepaug Dam, and seepage, slope stability, and liquefaction analyses for Phelps Brook Dam and East Dike.

Due to inadequate factors of safety against sliding at Nepaug Dam, a subsequent engineering study to refine the stability analysis of Nepaug Dam. A detailed relief well inspection was performed to obtain additional uplift information. A site specific seismic analysis was performed. A refined stability analysis was performed in two phases consisting of an updated 2D analysis for all of the blocks in the dam followed by a 3D analytical stability analysis of blocks with inadequate factors of safety against sliding. GZA presented recommendations implement stability improvements at 5 of the 13 blocks in Nepaug Dam and the addition of overtopping protection at Phelps Brook Dam and East Dike.

Final Design and Permitting

GZA managed the final design and permitting phase of the project which included stability improvements and bridge repairs at Nepaug Dam and embankment improvements at Phelps Brook Dam and East Dike to mitigate embankment erosion/overtopping due to wave run-up during the Probable Maximum Flood. Final design included supplemental subsurface exploration and instrumentation installations including the installation of vibrating wire piezometers inside the Inspection Gallery at Nepaug Dam and open standpipe piezometers at Phelps Brook Dam and East Dike. Laboratory direct shear testing was performed on the recovered cores samples of the concrete/bedrock interface to further refine the interface shear strength parameter in the Nepaug Dam stability analyses. Final design included the development of a final design report, contract plans and contract specifications for the repairs and improvements to the Nepaug Reservoir Dams. The rehabilitation of the Nepaug Reservoir Dams will include:

Nepaug Dam:

  • Installation of 1,500 kip design capacity, post-tensioned, strand anchors in Block 5;
  • Installation of passive bar anchors in Block 9 and 10;
  • Rehabilitation of the bridge deck over the spillway;
  • Removal/abatement of PCB caulking; and
  • Installation of the concrete reinforcement erosion protection in the plunge pool downstream of the overflow sections of the dam.
  • Adding upstream riprap to mitigate wave run up during the Probable Maximum Flood Spillway Design Flood.
  • Permitting has included filing with CT DEEP Dam Safety Division, US Army Corps of Engineers, and CT DEEP – Natural Diversity Data Base.
  • Final Design was completed by the fall of 2011 with construction starting in spring/summer of 2012. The estimated construction cost is approximately $4.5M.

Phelps Brook Dam and East Dike:

  • Raising the dam crests and installing new pavement.
  • Adding upstream riprap to mitigate wave run up during the Probable Maximum Flood Spillway Design Flood.
  • Permitting has included filing with CT DEEP Dam Safety Division, US Army Corps of Engineers, and CT DEEP – Natural Diversity Data Base.
  • Final Design was completed by the fall of 2011 with construction starting in spring/summer of 2012. The estimated construction cost is approximately $4.5M.