39 



top of the dredged material (additional dredged material, cap material, BBC), the strength 

 of the material will increase and improve the ultimate success of final capping. 



• Confirming Cap Coverage and Thickness 



The basic methods used for this survey can be successful in determining cap spatial 

 coverage and thickness in tandem with improved operational methods. Modifications to 

 these procedures are summarized for the individual types of monitoring. 



Bathymetry. Bathymetry proved to be a useful tool to monitor overall consolidation of the 

 project. The advantage of the Boston Harbor project is that the underlying BBC is 

 relatively incompressible, so that any difference in height of the material can be attributed 

 to consolidation of the dredged material itself. It is imperative, especially in small areas 

 like the CAD cells, to replicate survey parameters so that electronic depth differencing 

 between surveys can be conducted. Typically, consolidation will occur rapidly at first, and 

 then slow and become more gradual (e.g., Poindexter Rollings 1990). At the point the 

 consolidation curve begins to flatten is the optimum time to begin capping from a strength 

 perspective. Bathymetry can be used as a tool during dredging, therefore, to allow 

 qualitative analysis of the consolidation state of the material. 



Using bathymetry to estimate cap thickness is problematic, however, because of further 

 consolidation of the dredged material after cap placement. Use of subbottom (below) will 

 allow a more comprehensive and accurate acoustic assessment of cap thickness (Murray et 

 al. 1994a). 



Coring. Coring has the advantage of actual, visual evidence of the state of the cap. It is 

 limited, however, to measuring in only discrete points so that, with uneven coverage, it 

 may provide an inaccurate picmre of the overall cap. Coring can be used to evaluate the 

 boundary between cap and dredged material, so that the ability of subbottom (below) to 

 detect the overall coverage of the cap can be evaluated. Vibracores are required for this 

 type of operation. 



Subbottom. As discussed above, if the material is sufficiently consolidated to minimize 

 mixing between the sand and cap, subbottom is the most promising method to evaluate 

 both the spatial coverage and overall thickness of a sand cap. It becomes less useful if the 

 sand is unevenly placed so that sound is lost at the surface, and if the mixed interval 

 exceeds the depth resolution of the subbottom frequencies. 



Sidescan. Sidescan data were essential in Phase 1 because of the unique topography of the 

 sand waves. If the sand cap is uniformly placed throughout the cell area, sidescan will 

 show the uniform deposit but will need groundtruth data (cores or video) to document the 

 lithology. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUA TIC DISPOSAL CELL 



