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Scope of Work:
¨ Dredging objectives required the removal of 2,350 cubic yards of pure coal tar and tar contaminated sediment from three Hazard Areas located just off shore. River conditions and depths (4’-24’) varied significantly with location and the 9’ tidal range. Tide changes created bidirectional flow with high channel velocities, making turbidity controls a challenge, and tug operations were severely restricted during low tide.
¨ Turbidity control was maintained utilizing a crane mounted environmental clamshell and a barge mounted ‘dredge cell’ turbidity curtain. The dredge cell approach provided mobility and flexibility during sediment removal, allowing the operation to re-open the navigation channel at the end of each shift or as needed for freighter access.
¨ Dredging tolerances were limited to a 6” overdredge depth. Tolerance and quality control was confirmed using a proprietary dredge positioning system based on the Hypack model. The coordinates (x,y,z) for each bite were reported daily and utilized to monitor and direct the dredging progress. Cut and fill tolerances were critical on this project due to the required thickness of the capping media to be installed.
¨ Dredge spoils were placed into a 200 CY hopper scow and offloaded at a temporary bulkhead. Preliminary dewatering was performed prior to solidification with Portland cement. Primary solidification was performed on an inland material handling and stockpile pad prior to shipment for off-site disposal.
¨ At the completion of the dredging phase, a subaqueous cap was installed to cover the Hazard Areas and contain residual tar located below the 3’ dredging limit. Over 4,800 tons of capping media, a mixture of pond fines and compost, were installed over the Hazard Areas to a 3-4’ thickness. Installation of the capping media required precision placement to confirm lift thickness and horizontal coverage. Additional efforts were also required to create a relatively flat cap surface.
¨ The final phase of the cap construction included the installation of 16,500 square feet of scour control mats. The mats were installed manually by a team of divers and individually anchored through the capping media into the native sediments below.
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