Swinging Bridge Reservoir
Location
Forestburgh, New York
Client
Devine Tarbell Associates, Inc.
Services Provided
Emergency Response
Filter and Drain Design
Dewatering System Design, Installation, and Monitoring
Grouting
Seepage and Stability Analysis
Specifications and Construction Documents
Construction Monitoring
Project Overview
Swinging Bridge Reservoir Dam is a 120-foot-high dam constructed in the 1920s, located on the Mongap River north of Port Jervis, New York. When the embankment developed numerous sinkholes and subsequent erosion by piping occurred, emergency drawdown of the reservoir behind the hydroelectric dam was necessary. The severity of these conditions and loss of power generation called for quick action.
Schnabel was retained to provide rapid response engineering and remedial design analyses for this dam under contract to the owner’s engineer. We performed detailed evaluations of events and causes and assessed potential for further impacts on the structure. Subsequent tasks included exploration including drilling, sampling and instrumentation; better defining geologic conditions; and evaluating seepage and slope stability. Since the dam is FERC-regulated, all documents and construction activities were subject to FERC review, as well as review by a Board of Consultants.
Remediation design was approached in two phases. Phase 1 included remediation of sinkholes at the crest of the dam; design, installation and startup of construction dewatering at the toe of the dam; and contact grouting along the diversion tunnel to fill voids. Phase 2 included design and construction of a downstream conduit filter and toe drain, blanket drains, and a toe berm. Alternatives included an open cut excavation and a filter or diaphragm filter to encapsulate the diversion tunnel and penstock. Cutoff systems including steel sheets and jet grouting were considered beneath the filters and conduit.
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