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Penn Forest Dam Replacement

Penn Forest Dam is the largest roller-compacted concrete dam by volume in the eastern U.S.
  • Reinforced concrete pier bents on 9-foot-diameter drilled shafts embedded into bedrock support this structure-Gannett Fleming

    Fast-track design and construction and on-site RCC mixing assisted completion of project in 31 months.

Bethlehem Authority

Bethlehem, Pennsylvania

Our Role
Geotechnical, Design, Construction, Construction Management.

145 feet high, 1,930 feet long
Construction Cost
$65 million
2 years 6 months
  • Proposed and designed a new RCC gravity dam
  • Saved $20 million recycling embankment, intake tower, and spillway from original dam 
  • Computerized monitoring of grouting saved two months’ time and $1 million.

Constructed in the 1950s, the original Penn Forest Dam was a large earthfill embankment dam that impounded the City of Bethlehem's largest water supply reservoir. In 1960, during the first filling of the reservoir, a large sinkhole developed on the upstream embankment slope. Emergency repairs were designed and constructed. Our firm's involvement with the project began following emergency repairs in 1963 with the installation of more than 270 piezometers to monitor the dam‘s performance. In late 1994, increased seepage from the dam and sudden changes in the piezometer readings in the proximity of the old sinkhole area presented warning signs of a developing dam failure. The pool was lowered and an extensive investigation and evaluation program was undertaken to reassess the dam’s condition.

The decision to construct a roller-compacted concrete (RCC) replacement dam was made to eliminate problems and risks associated with the existing dam. Safety issues resulted in loss of service of the original Penn Forest Dam and created a critical water supply issue for the owner. In order to minimize the risk of water shortages during drought periods, it was essential to fast-track the construction by utilizing a unique approach in which 11 construction contracts were issued allowing the design to be completed in stages while construction activities were underway for previously completed design elements. 

What We Did

Gannett Fleming provided design, construction, and monitoring of the new 180-foot-high, 2,050-foot-long, RCC dam. The new dam is buttressed on the downstream face by earth and rockfill excavated from the embankment of the original dam. It is the largest RCC dam by volume in the eastern U.S. Existing spillway, intake tower, and outlet works improvements were also made for installation of new low-flow monitoring and automated-flow measurement facilities. Innovative project features included fast-track construction using 11 different contractors; 1,400-foot-long, 25-foot-high cofferdam; removal of 1,200 cubic yards of a concrete plug from the center of the existing 12-foot diversion conduit; and computerized monitoring of the foundation grouting.

Key Features

  • New 370,000-cubic-yard RCC gravity dam
  • Storage capacity of 6 bgal
  • Provides 62 percent of total raw water storage
  • Fast-track design and construction – 31 months from initial construction through initial refilling
  • Computer monitoring of foundation grouting, saving two months’ time and $1 million
  • Triple-line grout curtain
  • On-site mixing and total conveyor delivery of RCC.

Sustainability Features & Outcomes

  • Recycling of existing earthfill dam and appurtenances saved $20 million
  • No displacement of residents, public facilities, or endangered species
  • No impact on wetlands
  • Enhanced fish and aquatic habitat through cold water release facilities.

Awards & Recognition

  • National Rehabilitation Project of the Year Award, 1999, Association of State Dam Safety Officials, National Rehabilitation Project of the Year Awards
  • Honor Award, 2000, American Consulting Engineers Council of Pennsylvania, Engineering Excellence Awards
  • National Grand Award, 2000, American Consulting Engineers Council, 2000 Engineering Excellence Awards.

Similar Projects: Water/Wastewater: Dams & Levees, Dams & Levees: Dams Investigation & Design, Geotechnical: Foundations, Geotechnical, Geotechnical: Seepage Remediation, Geotechnical: Site Evaluation, Water/Wastewater