Formatted Title:

Tidal Creek System Hydraulic Dredging, Subaqueous Capping, and Landfill Closure

Background/Objectives:

Langan provided oversight and construction management of sediment remediation within a shallow tidal estuary creek system and subsequent incorporation of the impacted sediment in an adjacent on-site inactive industrial landfill in southern New Jersey. The sediment remediation consisted of hydraulic and mechanical dredging of impacted sediment, and subsequent subaqueous capping of select portions of the creek where impacted sediment remains at depth and near critical structures. The landfill closure actions included dewatering and stabilizing the dredged sediment, followed by closure of the landfill. Sevenson Environmental Services, Inc. (Sevenson) and ENTACT, LLC (ENTACT) were contracted to execute the selected remedy. Estimated volumes of impacted sediment, predicted post-remedy surface-weighted average concentrations (SWACs), and a cost-benefit analysis were used to optimize the remedial design and reduce potential construction costs while ensuring the achievement of remedial action objectives (RAOs).

Approach/Activities:

The on-site inactive landfill was utilized for constructing a sediment dewatering gallery to support the geotextile tube dewatering and final on-site disposal of dredged sediment. Approximately 70,000 cubic yards of impacted sediment were hydraulically dredged, and an approximately six-acre subaqueous sediment cap was constructed over select areas in the creek system and along all shorelines. Post-dredge sediment results and a bathymetric survey were compared against the previously performed three-dimensional dredge modeling to evaluate the effectiveness of the impacted sediment removal and to modify the dredging approach to limit unnecessary over-dredging. The dredged material was pumped into geotextile tubes and staged at the adjacent landfill to support sediment dewatering and eventual landfill closure.

Following the completion of dredging and months of further dewatering, a geotechnical evaluation of the sediment contained in the geotextile tubes was conducted to reduce uncertainty associated with handling and incorporating the dewatered sediment into the final, closed landfill. The primary technical uncertainties included the moisture content, cohesion, and stability of the sediment during landfill closure grading and capping. Geotechnical data collected through sampling of the sediment within the geotextile tubes placed on the landfill were modeled to evaluate the stability of the sediment within the ultimate closed landfill. Based on bench-scale testing performed by Langan and the remedial contractors, amendment of the sediment contained in the geotextile bags was deemed necessary. Sediment amendment was essential to aid in material drying, general workability, and ensuring slope stability and material strength requirements were met. Portland cement, at a dosage of approximately 8% to 12% by mass, was used to achieve sediment drying and strength. The amended sediment and pre-existing landfill waste were subsequently incorporated to the landfill to achieve design grades.

Results/Lessons Learned:

Langan’s multidisciplinary expertise and innovative approach helped to solve complex challenges associated with the sediment dredging, subaqueous sediment capping, sediment dewatering/stabilization and landfill closure, as well as erosion and sediment pollution control design, land use permitting, and advocacy with both state and federal regulatory agencies. Optimizing and controlling the sediment dredge volume was critical in providing for on-site consolidation and management of dredged material, as well as avoiding costly off-site transport and disposal. Final sediment amendment and stabilization was critical to support landfill closure construction, grading, and final slope stability.

Steven Ueland

Affiliation

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Tidal Creek System Hydraulic Dredging, Subaqueous Capping, and Landfill Closure