19_ 



within a reasonable range, as prior studies of dredged material have shown volume 

 reductions of up to 50% for fine-grained materials attributable to consolidation (e.g., 

 Poindexter-Rollings 1990). 



Viewing the data from a three dimensional perspective, the bathymetric data 

 suggested that the slope of the cell walls had become less steep over the 10 weeks between 

 surveys (Figure 3-3). Several areas appeared to show slumping of the wall itself (note far 

 northwestern comer). Sequential side-scan sonar datasets collected during the disposal 

 phase also suggested the outline of the cell wall changed through time (ENSR 1997a). 

 Because the clamshell dredge that was used to create the cell left a sawtooth pattern along 

 the cell walls, it is likely that the dredging process weakened parts of the wall and material 

 sloughed or calved from the walls into the cell. Because Boston Blue Clay is relatively 

 firm (e.g., CDM 1991), this process will likely not continue indefinitely, but until a stable 

 slope for BBC material has been reached. The force of ship propellers along the piers may 

 have also tended to weaken the unsupported walls, especially along the eastern wall of the 

 cell. 



An anomalous topographic peak in the southeastern corner of the cell was noted 

 several feet higher than was measured in the previous postcap survey in July (Figure 3-2). 

 The location of the peak was consistent with the hard reflector seen in the side-scan sonar 

 data, and was also seen in the subbottom results (Section 3.4). As discussed below, this 

 deposit may be material that ended up overlying the uncapped dredged material, possibly a 

 remnant of material fallen from the cell wall. 



Two N-S transects of raw depth soundings from the October survey (not gridded 

 data) were extracted from the data to produce cross sections of bathymetry. Lanes 6 

 (central cell) and 7 (eastern cell) were selected (Figure 3-4). The data were plotted to 

 show the transition from the rough surface associated with the re-dredged sand cap to the 

 smooth surface noted in side-scan sonar data. The results showed that the rough, uneven 

 surface slopes down to the smooth surface, and the transition is approximately Va of the 

 distance from the southern end of the cell (Figure 3-5). Note that north and south are 

 reversed between Lanes 6 and 7, and are plotted relative to the direction of the boat transit. 

 The difference in topographic expression was investigated further with vibracore data. 



3.3 Vibracore Results 



A total of 12 cores at six stations were recovered, with core lengths ranging from 

 45-241 cm (Table 2-1; Figure 3-4). In all, a total of 14 cores were attempted; two core 

 liners became smck (Cores CAD-6C, 7A; Figure 3-4). Cores collected from CAD-2A and 

 2B appeared to be outside of the cell. The recovery of 72-105 cm of apparent dredged 

 material, however, suggested that the cores were acmally collected in the cell. One of the 

 CAD-2 cores showed evidence of BBC on the top drive motor, indicating the core was 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



