POST-DISPOSAL BEHAVIOR OF SANDY DREDGED MATERIAL AT AN OPEN-WATER, INNER SHELF DISPOSAL SITE

Geotechnical properties of dredged material placed in an open-water di sposal site were monitored over a six-month period immediately followi ng disposal to determine the consolidation behaviour of the spoil moun d and the impact of dredged material on sediment transport behaviour. The dredged material consisted largely (> 90%) of fine to medium sand, with a significant proportion of gravel-sized pumice and shell fragme nts (1-10%), and minimal (< 0.5%) mud. This distribution was compatibl e with the natural sediment at the dump ground. Immediately after plac ement, mean grain size and settling velocity of surficial sediments in creased compared with pre-disposal values, and sorting reduced. Dredge d material bulk density and static friction angle were lower than thos e of pre-existing pre-existing dump ground sediments, yet moisture con tent was comparable. These changes are attributed to the dredging and injection processes. Due to the low mud content, the dredged material consolidated immediately upon deposition, attaining normal consolidati on with respect to ambient stresses within a short period. Reduced sta tic friction angles resulted in an enhancement of the potential for be dload transport relative to pre-disposal rates. Increased settling vel ocities resulted in decreased potential suspended sediment transport. Lower shear strength of the dredged material resulted in larger bedfor ms on the dump ground compared with surrounding areas. With time, prop erties of the materials reverted towards pre-disposal conditions as a result of increased sorting and packing of grains. However, after six months, the properties had not completely returned to previous conditi ons.