2.0 NEW ENGLAND CAPPING 

 OPERATIONS 



NED capping operations have evolved 

 since the initiation of the program. 

 Following the proposal of the first capped 

 mound project, Stamford-New Haven, the 

 SAC issued a report expressing particular 

 concerns on the following points: the 

 adequacy of the technology available to 

 point-dump and successfully cover 

 relatively small volumes of material; the 

 amount of capping material needed to 

 cover the mound to ensure adequate 

 containment; the difficulties associated 

 with discriminating physically or 

 chemically between cap and covered 

 material (in order to judge the 

 effectiveness of the cap); and the 

 probability that on impact cap material 

 might displace or mix with the sediments 

 to be covered. These issues were 

 addressed by investigating the effects of 

 various dredge and disposal methods and 

 by comparing different quantities and types 

 of mound and cap sediments. 



Point-dumping was accomplished 

 during creation of the very first 

 experimental capped mound (Stamford- 

 New Haven) by requiring the barges to 

 open the hopper doors only after pulling 

 alongside the marker buoy. Taut- wired 

 buoys were used to reduce the area of the 

 center of the mound. Previously, barges 

 released dredged material while steaming 

 across a dumpsite (Bokuniewicz 1989). 

 Data from the experimental program 

 suggested that point-dumping should be 

 utilized in order to restrict the spread of 



mound material, while cap material should 

 be spread laterally. 



Nationally, the most commonly used 

 dredging technique has been hydraulic; this 

 method fluidizes the sediments into a 

 slurry with > 80-90% fluids, thereby 

 reducing or destroying sediment cohesion 

 (Bohlen 1990). Of the currently utilized 

 dredge methods, the mechanical clamshell 

 bucket was found to be the most effective 

 at maintaining sediment coherence. The 

 importance of this fact was recognized 

 later in identifying dredged material in 

 cores (Section 4.0). In addition, storage 

 of dredged material in a hopper barge 

 allows some dewatering and consolidation 

 before disposal, so that material loss in the 

 water column is minimized. 



Initially, attention was focused on 

 cap: mound ratios in order to determine the 

 quantity of capping material which would 

 ensure complete mound coverage. The 

 success of a capping operation was later 

 found to depend on many factors more 

 relevant than cap: mound ratios, even when 

 estimates of the volume of dredged 

 material were uncertain. However, the 

 following discussion includes dredged 

 material volume estimates and cap: mound 

 ratios to provide this information in a 

 historical context. 



Both sand and silt were used for 

 capping material in early operations to test 

 the effects of variable grain size and water 

 content on potential mixing of cap and 

 mound material. Early monitoring 

 demonstrated that both sand and silt could 

 be effective at isolating contaminated 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



