67 



Geotechnical cores collected over the NHAV 93 mound within the past two years 

 have attempted to document the development and subsequent consolidation of the NHAV 

 93 mound. Despite the use of precision navigation and consistently revisiting stations, the 

 observed heterogeneity within the UDM and historic dredged material layers of the NHAV 



93 mound tends to lessen the ability to track sediment layers through the five-member 

 time-series data set. A certain degree of repeatability within the collected sediments was 

 required to follow individual sediment strata throughout the project, providing a baseline 

 used to quantify changes in layer thickness. However, cross-sections of a dredged material 

 mound have been proven to be valuable as "snapshot" data as well as ground-truth data for 

 comparison with subbottom profiling (Morris and Tufts 1997). 



Geotechnical coring as an investigative technique could be improved by acquiring 

 longer cores to obtain a sample of the ambient bottom throughout the time-series data set. 

 The gravity coring device utilized during the New Haven Capping Project had difficulty 

 penetrating the consolidated center of the 2.5 m high NHAV 93 mound, resulting in partial 

 recovery. The use of a pneumatic vibrocore equipped with a 5 m steel core barrel would 

 ensure complete penetration into the basement material to provide a baseline for 

 consolidation measurements. In addition, the use of chemical sampling of the recovered 

 sediment could provide valuable information on the origins of the various strata. 

 Determination of the relative concentrations of various contaminants would allow for the 

 differentiation of basement, historic, UDM, and CDM layers in either ubiquitous or 

 heterogeneous samples. 



The use of repetitive bathymetric surveys during the New Haven Capping Project 

 was proven to be an invaluable tool in observing the usually hidden dynamics of dredged 

 material mound construction (Morris et al. 1996). The same technique was employed 

 during the post processing of the CLIS 94 mound bathymetric survey data. A total of four 

 bathymetric survey data sets were used to follow the construction of the CLIS 94 mound 

 and expose the accumulation and consolidation of the bottom feature. By utilizing SAIC's 

 July 1994 and September 1995 surveys in conjunction with OSI's December 1994 and 

 April 1995 data sets, the events leading up to the final capped mound could be tracked and 

 volumes of material calculated. 



In the past, efforts have been made to account for differences in the volume of 

 material reported in disposal barge logs to the volumes of material detected acoustically. 

 The issue of mass balance has become clouded by large volumes of undetectable mound 

 apron material, over-estimation of barge volume by on-site inspectors, and compaction of 

 dredged material on the seafloor (Tavolaro 1984). The repetitive surveys over the CLIS 



94 mound have found central mound consolidation during disposal and capping activity to 



Monitoring Cruise at the Central Long Island Sound Disposal Site, September 1995 



