Onondaga Lake Superfund Site Dredging and Capping Services

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History & Location Details

Project Description

Project required Sevenson to provide design support to develop a sediment dredging, capping, and transport system. Capping extends over approximately 400 acres. Approximately 2.1 million CY of material was removed using three hydraulic dredges. Upon completion of dredging operations, Sevenson initiated backfilling and capping operations.

In 2008 Sevenson was invited by Parsons to join a team of technical experts from dozens of scientific and engineering disciplines to address remediation and restoration of lake sediments following established state and federal guidelines. Sevenson’s participated over a 2 year period in project planning discussions focused on constructability review issues surrounding dredging and capping methodology as well as pilot studies and field demonstrations.

In December 2009 Parsons released a competitive request for proposal for dredging, capping and related site work associated with the project. In 2010 Sevenson was awarded the multi-year contract. Several work tasks were required prior to initiating dredging and capping operations. These included:

• SITE WORK
  Sevenson’s initial contract required the installation of approximately 20,000 lf of doubled-walled HDPE slurry pipeline, and supporting infrastructure to transport sediment slurry from the lake to the sediment processing area.
• WASTEBED B / HARBOR BROOK EAST WALL CONSTRUCTION
  Upon completion of the slurry pipeline Sevenson was contracted to extend the west wall of the Wastebed “B” / Harbor Brook steel sheet pile wall by installing additional 1,600 lf of sheet piling. A collection system, complete with pipes, valves, vaults, mechanicals, and electrical components was also installed. An additional scope of work item required Sevenson to bypass and relocate Harbor Brook.
• TIE-BACK WALL CONSTRUCTION
  Site work continued with the installation of a steel sheet pile tie-back system to support an existing sheet pile wall. The contract also required Sevenson to construct a concrete platform supported with wood piles over a portion of the bulkhead to ease construction equipment and barge loading and offloading activities. In addition, a timber fender system was installed along the existing sheet pile bulkhead.
• CAPPING DEMONSTRATION PROJECT
  The purpose of the demonstration project was to evaluate the ability to effectively place sand and GAC through the water column, resulting in a chemical isolation layer with GAC distributed throughout the layer. This demonstration was undertaken to support ongoing cap design efforts.
• CAPPING
  Sevenson used its custom-built slurry system for hydraulic placement of the sand cap and armor stone less than 2 inches in diameter in designated areas. The system consisted of a feed hopper, oversized screening, and slurry system that pumps the sand and small stone hydraulically through a pipeline for placement through a diffuser system. The pump used to convey the sand slurry was a booster pump commonly found on dredging projects. The hopper, screening, and slurry system were situated on the shoreline at two potential locations to minimize pumping distances and the need for booster pumps floating on the water. Sevenson placed over 2.35 million CY of sand and rock to form a multilayered cap. Sevenson also placed larger stone materials mechanically and in locations where the spreader system could not reach.
• DREDGING
  
  Sevenson utilized three dredges to remove sediments. Two of the dredges performed the majority of the dredging. They were equipped with 16 inch and 14 inch discharges with over 1,000 HP installed for pumping. The third dredge (8 inch) was available for dredging in shallow water or confined areas. Sevenson‘s two production dredges were each capable of handling the required daily production of 4,500 CY per day. These dredges were selected to maximize dredge production and efficiency for the variable site conditions.