Planning for Cap and Habitat Restoration Material Placement, Verification, and Long-Term Monitoring

摘要

Background/Objectives. Consistent with technical guidance from U.S. Army Corps of Engineers and U.S. Environmental Protection Agency, many sediment remediation projects involve hybrid remedies including dredging, capping, backfilling, and habitat restoration. Sediment caps may be placed as the primary remedy or following dredging to obtain a desired grade and/or contain residual contaminated sediments. Backfilling and habitat restoration often entail the placement of materials selected to accelerate natural recovery or promote targeted habitat species. Experience from a growing body of completed projects involving capping and backfill placement indicates that it is critical during the project planning and bidding stages to estimate the anticipated quantity of capping materials that will be necessary to achieve the project objectives. In addition, serious consideration must be given to methods that will be used to verify cap construction and monitor the caps over the long term based on the capping materials and site conditions. Approach/Activities. From a simplistic standpoint, the quantity (volume) of capping or habitat restoration materials can be estimated based on the area and thickness of planned placement. However, actual volumes required to achieve those dimensions will be larger than this theoretical “neatline” and will depend on a number of factors including but not limited to: 1.Project and contractual requirements 2.Over-placement payment allowances (and resulting buildup of cap) 3.Cap side slopes (i.e., losses associated with “runout”) 4.Losses associated with materials handling, transport, and placement 5.Uniformity of placement across target area(s) 6.Material type and gradation 7.Loss to water column via suspension and transport 8.Mixing with underlying sediment 9.Subgrade settlement Several of these factors also govern the selection of the appropriate method(s) to accurately verify the completion of construction and to monitor the caps into the future. It is critical that these methods be selected based on the site conditions and specifics of the cap to facilitate accurate post-cap physical and chemical monitoring.