Former Plant Site Remediation
Significant Project Features
- Mechanical dredging of Hudson River sediments. Sediments stabilized with Portland cement for either use onsite as backfill or offsite T+D. Final disposition of sediments determined through analytical testing
- All water encountered during remedial action required collection, treatment, and sampling prior to discharge to the local POTW
- Removed sediments and inspected manholes, catch basins, and approximately 9,500 lf of various sized storm sewer pipe ranging from 4 inches to 48 inches in diameter
- Restoration of all disturbed areas
History & Location Details
The project, at this plant site located in the Village of Sleepy Hollow, New York, was executed in accordance with Brownfield Cleanup Agreements with the New York State Department of Environmental Conservation (NYSDEC). The site is situated on the shore of the Hudson River and occupies an area of approximately 100 acres within the village. The remedial action was focused on a 3-acre area. The site is scheduled for mixed commercial and restricted residential development with public open space.
Sevenson was awarded the remediation based upon a competitive bid proposal. The project was separated into two (2) components. The first required Sevenson to mechanically dredge near shore sediments for placement in scows; transfer sediments to a staging pad for processing; stabilize sediments; and load into trucks for onsite reuse as fill material to raise the grades in low areas of the site. The second required Sevenson to clean sediment from manholes and catch basins and approximately 9,500 lf of sewer pipe ranging from 4 inches to 48 inches in diameter. All water generated from cleaning operations was collected, treated, sampled and discharged to the local POTW.
- Sediment Dredging
A temporary steel sheet pile wall was installed to protect an existing bulkhead prior to initiating mechanical dredging. A 100 ton barge mounted crane equipped with a vibratory hammer was utilized to install the wall. 16 inch diameter pipe piles were also installed 25 ft on center to set and anchor silt curtains and containment booms around the perimeter of the proposed dredge area. Upon completion of sheeting operations a second barge mounted crane rigged with a GPS equipped environmental clamshell bucket was used to dredge contaminated sediments. Two staging pads were constructed to provide an area for sediment handling operations. One housed dredged sediments offloaded from scows for gravity dewatering. The second served as a stabilization pad where sediments were conditioned with Portland cement prior to load out for onsite placement. The GPS equipped crane enabled the operator to monitor dredging progress in real time to ensure the required dredging locations and depths were achieved. Dredging operations were aided by a tug boat that jockeyed the barges in tight work areas which were contained using silt curtains and containment booms.Upon completion of the dredging operation a licensed surveyor prepared as-builts. The final as-built drawing was compared to the preconstruction survey. This provided final dredge elevations and volumes. Dredged areas were capped using virgin stone fines from local quarries in order to meet the material specifications. Cap material was delivered by truck and placed using the crane and environmental clam bucket.
- Sediment Stabilization
A 150 ton crawler crane located on shore transferred dredge sediments to the stabilization pad. An excavator was used to blend Portland cement with sediment to solidify the dredged material. Once the material “set up” so it could be handled, it was relocated and stockpiled in 500 CY piles for profile sampling and additional curing. Once the analytical results were received and the material passed paint filter tests, a second excavator transferred the materials into a 30 ton off road articulated haulers which transferred the material for placement in low spots around the site to raise the grades.
- Water Treatment
Sevenson collected all water generated from remediation operations in a series of 20,000 gallon storage tanks. All water was treated, sampled, and analyzed prior to discharge to the local POTW. The system was installed on a lined containment pad approximately 75 ft x 75 ft and fabricated with a 40 mil liner covered on both sides with a cushion layer of 8 ounce non-woven geotextile fabric, and a 6 inch protective layer of stone for the equipment to rest on. The waste water system was run as necessary to keep up with the influent volumes during dredging and sewer cleaning operations. After the initial batch sample was processed, reviewed and accepted, continuous discharge commenced with monthly discharge monitoring thereafter.
- Storm Sewer Cleaning
9,500 lf of various sized storm sewer required sediment removal and cleaning. This included manholes and catch basins. All water and sediment was collected and contained. Once the sewers were cleaned, a video camera was placed in the sewers for final inspection and documentation. The sewer cleaning equipment was selected based on the diameter of the sewer line and the access to the pipe and manholes. A water jet hose line was used to clean the smaller size pipes. A combination drag line and a vacuum truck was used to clean the large diameter pipe.It was necessary to prevent water flow from the Hudson River from entering the sewer system from the outfall. The manhole leading to the outfall was temporarily plugged with pneumatic air plugs. Work commenced upstream. Sections were isolated as necessary to prevent back flow of water and to capture sediment and water.A bypass pump system was installed from one manhole to another to maintain water flows. All water and sediment removed was transported via vacuum truck to the onsite staging area. The material was dumped into the containment area for processing. The silts and fines settled out and the water was pumped to the temporary onsite water storage facility for filtration prior to discharge. The sediment was staged for gravity dewatering and then blended with Portland cement prior to stockpiling.Once a run of pipe was cleaned the bypass pump system was removed and relocated. The sewer line plugs were also removed and relocated to the next work area.
This project is tagged under:
Health & Safety Overview
- Unique Characteristics: As work was performed immediately adjacent to an apartment complex, care and consideration had to be incorporated into the project planning to minimize any impacts to the community. The size of the dredging work area was restricted as work was not allowed to infringe onto the navigation channel that bordered the entire west side of the targeted excavation area
- Health and Safety Measures: Modified Level D was utilized during all phases of work