In order to provide a safe navigable water depth, the Port of Brisbane Corporation undertakes periodic maintenance dredging in the Moreton Bay area, which amounts to about 600,000 m3of sediments every year.
A recent change in disposal regulations gives preference to on-land disposal, thus forbidding sea-disposal as it was previously commonly done. However, suitable near-waterfront disposal land is in short supply and the dredge spoil disposal sites currently used by the corporation will be filled to capacity soon. As a consequence, new solutions that are both economically and environmentally viable must be found.
The investigation presented addresses the feasibility of improving the geotechnical properties of fine-grained dredged sediments by heating and pressing so as to make them suitable for beneficial uses in brick production or as high-quality aggregates. Due to their natural high plasticity and salinity, the use of these sediments in their natural state is limited.
Previous research projects investigated the effect of heating in reducing the natural high plasticity of dredged sediments. It has been shown that heating to temperatures up to 500°C can significantly decrease the plasticity. Mechanical pressing can be used to reduce markedly the salinity of the sediments. However, this has not been investigated in the past.
The present investigation extends the previous work on sediments heating, and addresses the effect of mechanical pressing on the salinity and geotechnical engineering properties of dredged sediments.
Dredged sediments were sampled from reclamation pond S3B (weir box and discharge point) at the Port of Brisbane on Fisherman Islands. Parts of these sediments were air dried, oven dried and heated in a kiln at temperatures up to 500ºC. Others were first mechanically pressed at various pressures up to 80Mpa, before being heated in a kiln at temperatures from 800ºC to 1100ºC.
Both sediments (weir box and discharge Point) were found to be fine-grained, discharge point sediments being somewhat coarser.
The natural organic content of both weir box and discharge point sediments was high. The dominant clay mineral of both sediments was kaolinite (15%), the geotechnically least problematic of the common clay minerals.
Results obtained concerning the heat treatment of sediments confirm that the plasticity decreases with increasing temperature and sediments become non plastic at about 500ºC. Heat treatment doesn’t have any effect on the salinity of the sediments. Concerning the effect of mechanical pressing on fine-grained sediments, the results were inconclusive. Qualitatively, it is obvious that pressing can markedly reduce the salt content of fine-grained sediments but no consistent results could have been obtained.
The investigation underlines the potential of heat treatment to reduce sediments plasticity but no conclusions can be inferred about the effect of mechanical pressing on sediments salinity. Further investigations have to be carried out to fully assess the feasibility of improving fine-grained dredged sediment properties by heating and pressing (optimum heat treatment and pressure).