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Forestburgh, New York


Swinging Bridge Dam was constructed in the 1920s and is a hydraulic fill earthen structure. The dam is 965 feet long and 135 feet high and retains a water reservoir used for production of power. The dam is traversed by a penstock/tunnel that feeds water to one of the two powerhouses located at the base of the dam.


A 50 foot long, 30 foot wide, by 20 foot deep sinkhole depression appeared in the crest of the dam. ACT was contracted to perform a fast track investigative program to determine the underlying cause of the sinkhole, assess the overall condition of the dam, and determine the remedial measures necessary to meet dam safety standards. The investigation determined that the sinkhole formded as a result of water seepage through piping paths along the penstock/tunnel. The loss of soil along these seepage paths created the subsurface voids that ultimately resulted in the sinkhole formation.


A comprehensive remediation program was implemented immediately following the investigation program. The remediation included repair of the main sinkhole on the crest of the dam, along with other smaller sinkholes discovered on the face of the dam. The unstable materials within the sinkholes were removed to stable subgrade using traditional excavation methods. Engineered materials were used to replace the unstable materials that were excavated form the sinkholes to restore the dam to original condition. and replaced with engineered earthen materials that were placed and compacted.


Access roads, staging areas, and office complex facilities were all installed to facilitate the remedial works. Given the emergency nature of the work, the remedial work was a 24 hour per day, 7 days per week continuous operation.


A conduit filter and toe drain system were also installed to minimize the potential for the development of piping paths, uncontrolled seepage, and further sinkholes. Components of the system included two braced steel sheet pile cofferdams; design and installation of a deep well system to maintain excavation stability; excavation of in place soils; placement and compaction of engineered backfill including fine and coarse sand, gravel bedding, imported fill, and light and medium stone rip rap; and installation of horizontal drain piping. Approximately 40,000 tons of materials were imported and placed.


To address the piping paths along the penstock/tunnel, a detailed grouting program was instituted to fill any existing voids outside the penstock/tunnel. Holes were drilled through the penstock/tunnel and balanced stable grouts were injected, effectively filling the voids and seepage paths. Construction joints in the penstock/tunnel were injected with polyurethane chemical grout to prevent water seepage, and carrying of fines, through the joints. Low mobility grouting of loose soils and larger voids below the penstock/tunnel invert ensured that all seepage paths were addressed during the remedial program.



Client: Mirant NY-Gen, LLC

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