Hiroshima Bay

pit was needed to compete the disposal of all expected contaminated mate-

rial. This procedure (Figure 37) was to be completed only once, and sub-

sequent maintenance would be completed using normal methods.

A suction dredge was converted to act as

the discharge vessel with the suction pipe

used as the discharge pipe. Conditions on

the suction dredge operation were (a) no

overflow, (b) no water jets in suction proc-

ess, (c) lower working speed, (d) must use

onboard pumping systems for contaminated

sediment discharge, (e) contaminated water

from silt and degassification must not be dis-

charged overboard, and (f) contaminated mix-

tures cannot be pumped overboard. The

discharge pipe was extendable to 30 m and

was equipped with a modified discharge

opening (diffuser). The diffuser directed the

discharge radially and reduced the exit veloc-

ity to between 0.3 and 0.4 m/sec. The

dredge was also equipped with a degassifica-

tion system. Contaminated material was

dredged with a modified stationary suction

dredge. Its suction mouth was equipped

such that only the upper layer of the dredged

material was touched, and the suction intake

had no moving parts or waterjets. The objec-

tive was to maintain the in situ density of the

dredged contaminated material throughout

the dredging, transporting, and discharging

Figure 37. Rotterdam Harbor CAD

operations. Pollution of the groundwater

project

through the bottom of the dredge pit was

also of concern. After researching this problem, it was decided to place a

layer of clay as a liner in the bottom of the dredge pit.

Hiroshima Bay

Hiroshima Bay in the Inland Sea of Japan was the site of bottom-

sediment improvement testing using a special barge unloader sand

spreader (Kikegawa 1983). The investigation demonstrated that the sand-

overlaying process was successful using a barge unloader sand spreader

(Figure 10), and the sand layer had only minor irregularities in thickness

with a mean thickness of 0.5 m. Coarse particle size (0.1 to 10 mm)

containing shells with silt content of 0.1 to 0.3 percent was used as the

overlaying material. The discharge sand quantity during the spreading op-

eration was estimated using the pump suction pressure. Bottom sediment

resuspension during discharge was measured with a portable turbidity in-

strument, which showed the resuspension of the bottom sediment was up

to 1.5 m above the seafloor. The depth of spreading did not cause any

noticeable differences in the spreading capability. The sand spreading

did result in turbulence in the bottom sediment, but contamination of the

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Chapter 10 Case Studies