Quantcast Figure 8. Comparison of pre-dredged and post-dredged vane shear strengths

 

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Figure 7. Cumulative grain size distribution (by weight) curves for dredged material samples taken prior to dredging (Mobile Bay Channel) and post-dredged (Hopper Dredge)
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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 8. Comparison of pre-dredged and post-dredged vane shear strengths (to convert psf
to kPa, multiply psf by 0.048)
The pre- and post-dredged vane shear strength curves shown in Figure 8 are similar. Since the
difference in shear strength for a given water content is only approximately 2 psf (0.1 kPa), and
both curves approach a similar shear strength value at the liquid limit, the shear strength vari-
ability is negligible.
Two major differences are evident in the pre- and post-dredged materials. Hydraulic dredging
increased the water content from 212 percent to 546 percent, an increase by an approximate fac-
tor of 2.6. The other major difference is in the grain size distribution for the finest particles.
Although the two samples had a similar sand percentage (3 percent), similar silt percentage
(27 percent), and similar clay percentage (70 percent), the post-dredged sample had a 16-percent
drop in the amount of fine clay in the size range between 0.003 and 0.001 mm. The hydraulic
dredging operation may have removed the finest clay particles, or other factors such as sampling
locations, normal grain size variability, and/or limited sampling data may have influenced the
particle size discrepancy.
Mechanical Dredging. To observe the pre- and post-dredged sediment property changes due
to mechanical dredging, a composite sample from the pre-dredged Mystic River was compared
to a composite sample from the dump scow which was filled with clamshelled material from a
downstream dredging location. Although the spatial variability for these samples was discussed
above, and there is a strong likelihood that a composite sample is not a representative sample, the
two composite samples are compared in Figure 9, which is the grain size cumulative distribution.
A paired t-test (a statistical method useful for examining changes which occur before and after
an experiment) indicates that there is a statistically significant difference between the two
samples. The post-dredged material is coarser-grained and has less fines than the pre-dredged
material, which may be due to the dredging method, spatial variation, or other influences.
8

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