34 



was identified from the monitoring results; specifically, the time necessary to allow for 

 self- weight consolidation of dredged material. 



4.4 Consolidation of the Maintenance Material 



The overall depth of the cell appeared to increase by 1-4 ft during the 10- week 

 period between the bathymetric data collected immediately after capping in July 1997, and 

 in October 1997. The sequential bathymetric data were used to calculate an overall 

 consolidation of at least 2 ft in the far southwestern comer of the cell, and in the central 

 cell below the sand wave deposits. Conservatively, this resulted in an overall consolidation 

 of the dredged material from initial deposition to 10 weeks after capping of 20-40% . 

 Previous data collected on dredged material consolidation indicated that this value is 

 probably a minimum, considering volume reductions of up to 50% for fine-grained 

 materials have been attributed to consolidation (e.g., Poindexter-Rollings 1990). More 

 accurate estimates of the rate of consolidation within the CAD cells will be useful for the 

 second phase of the BHNIP (Section 5.0). 



The acoustic data consistently indicated the presence of a large block of material in 

 the southeast comer of the cell overlying the uncapped dredged material (Figure 3-8). 

 Although no samples were collected from the area of this topographic anomaly, the most 

 logical conclusion is that a large block of BBC fell from the cell wall and settled on top of 

 the dredged material. Although this is highly conjecmral, the inference can be drawn that 

 the bearing strength of the uncapped material had increased by the October survey to 

 support the overlying block of material. If this is the case, sometime during the 10-week 

 period between the completion of the dredging project and the October monitoring survey, 

 the material developed sufficient self- weight consolidation to optimize capping. 



4.5 Implications of Erosion from the Unsupported Cell Walls 



The side-scan sonar data from this report and from prior data collection efforts 

 (ENSR 1997a), as well as the bathymetric and subbottom data, suggested strongly that the 

 slopes of the unsupported cell walls have become less steep, and material has fallen from 

 the walls into the pit. The video capmred images of the surface of the cell showing blocks 

 of BBC on top of the CAD cell cap (Figure 3-9). Because the clamshell dredging process 

 that was used to create the cell walls left a sawtooth pattem along the cell walls, it is likely 

 that the dredging process weakened parts of the wall resulting in sloughing or calving from 

 the weakened walls. Video data collected in the cell confirmed that the steep walls are 

 scalloped, potentially increasing the chance for erosion of the cell walls (Figure 3-10). 



Boston Blue Clay has a high strength and is relatively firm (CDM 1991), indicating 

 that the erosional process will not continue indefinitely, and may be limited to areas that 

 were weakened during the dredging process by the clamshell bucket. In addition, spud 

 marks on the side-scan image were close to the edge of cell, potentially contributing to 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



