Due to the natural process of sedimentation dredging must be carried out worldwide to maintain channel depths in coastal regions to provide safe passage for ships. The disposal of dredged material has become a major problem for the Port of Brisbane’s modern ports, as regular maintenance dredging at a rate of 1 million m3/pa has to be carried out regularly. The available disposal sites are almost full. Consequently, alternative disposal techniques have to be taken into consideration to rectify the problem.
This report describes part of an ongoing investigation into the feasibility of modifying fine-grained dredged sediments for beneficial uses by heating to high temperature. In the present stage of investigation, the desiccation of fine-grained dredged material prior to heating and the influence of climate were examined. Nine dredged sediment samples from reclamation areas at Clunies Flats, Fisherman Island, Toondah Harbour and Weinam Creek were collected and tested in two separate phases. Two manufacture clays, kaolin and bentonite were also tested. The physical and engineering properties of the samples, such as their Atterberg limits, specific gravity, organic content, particle size distribution, moisture characteristic and moisture diffusivities were determined by laboratory testing.
Field testing was also undertaken. During this testing, soil strength profiles were obtained using a manual cone penetrometer at all six sites visited during the second phrase of testing. All penetrometer tests were performed under dry condition over two days.
An analytical model of evaporative drying of dredged sediments developed by Benson and Sill (1991) was used to determine the moisture content and diffusivity of the sediments. This involved matching Benson and Sill’s theoretical curve with the experimental desiccation curve. New empirical correlations for the diffusivity rate were derived using the new data.
Moisture characteristic curves were produced that were used later to convert the suction profiles into moisture content profiles for use in the theoretical analysis of the desiccation of dredged sediments disposed of on land.
Existing analytical solutions for the variation of suction profiles with time were available that suited the initial and boundary conditions for modelling the field sediments. The solutions were expressed in dimensionless forms in order to facilitate subsequent use.
Moisture deficits (average evaporation minus average precipitation) for all States and Territories in Australia were calculated using climate data obtained from the Commonwealth Bureau of Meteorology. Monthly and annual moisture deficit maps for all States and Territories of Australia have been produced using this data.
It is recommended that the desiccation test for the samples collected in the second phrase to be extended, as those samples were not completely dry at the end of the testing reported here. Humidity should be taken into consideration in future desiccation testing, as its effect significantly the overall diffusivity value obtained. In addition, the exact age and depth for the site where the sample is collected must be known in order to get a good result from the analytical model.