Figure 9. Cumulative grain size distribution (by weight) curves for pre- and post-dredged material samples after mechanical dredging

ERDC TN-DOER-D2

September 2004

Figure 9.

Cumulative grain size distribution (by weight) curves for pre- and post-dredged material sam-

ples after mechanical dredging

VARIABILITY DUE TO DREDGED MATERIAL PLACEMENT: In addition to the influence

of spatial variability and dredging operations, the properties of dredged material may change

depending on their final placement or disposal. Variability will be encountered in the placement

process due to the methods and equipment used. For example, sediments transported via pipeline

are layered differently than those placed by clamshell bucket. Also, the placed sediment likely

mixes with native sediment, which changes its properties and subsequent engineering behavior.

Open Ocean Placement. To make a general comparison between properties of pre-dredged

and post-placed sediments deposited at an open (unconfined) ocean site, pre-dredged material

sampled from the Gulf Coast's Mobile Bay was compared to previous data where similar mate-

rial was placed at an open ocean site near the entrance to Mobile Bay (Davis et al. 1999). The

fine-grained material was dredged with a clamshell bucket dredge, transported via split-hull

barge scows, and dumped through an approximate 30-ft (10-m) water column onto a sandy ocean

floor, which had a mean grain size (by weight) of 0.21 mm. Tables 5 and 6 list some of the geo-

technical properties of the pre-dredged and post-placed samples. Figure 10 shows the grain size

distributions.

Table 5

Median and Mean Grain Size (by weight) for Ocean-Placed Sediment Samples

Sample

Median, mm

Mean, mm

Pre-dredged (Mobile Bay)

0.001

0.013

Post-placed (open ocean)

0.015

0.077