The utilization of dredged material in dike construction as a substitute for traditionally used materials is considered as an option to preserve natural resources such as marsh sediments. As a prerequisite for this application, the equivalency with respect to soil physical and mechanical properties of the materials must be assessed. Previous investigations have shown pronounced differences in shrinkage behavior and desiccation cracking between sediments and dredged material. The key objective of the study was to assess whether shrinkage of processed dredged material can be reduced by further processing, i.e., dewatering, which can be referred to as ripening.
Materials and methods
To compare the shrinkage behavior of the materials, three different methods of different scales were applied. Small-scale methods conducted were the standard procedure for the determination of the shrinkage limit and the determination of the coefficient of linear extensibility (COLErod). Large-scale shrink-swell experiments were carried out in a specially constructed test system with 90 l capacity for a period of up to 385 days. Here the materials were ripened, i.e., air-dried, until shrinkage almost ceased, and a rewetting-air-drying cycle was conducted. Shrinkage and swelling were determined during the processes by measuring the changes in volume. On the ripened materials, COLErod was determined.
Results and discussion
The experiments show that the shrinkage behavior of processed dredged material can be ameliorated by ripening. COLErod of the ripened materials were about 20–80% lower than COLErod of the un-ripened materials. The large-scale shrink-swell experiments showed that shrinkage in the second drying cycle amounted to less volume than in the first drying cycle and that shrinkage behavior in contrast to the first drying cycle, where pronounced proportional shrinkage was observed, was dominated by structural and residual shrinkage in this cycle.
Conclusions
Ripening of processed dredged material is considered a useful pre-treatment option to ameliorate the shrink-swell behavior of processed dredged material and to obtain a better functional equivalency with traditionally used dike construction materials such as fine-grained marsh sediments.
The utilization of dredged material in dike construction as a substitute for traditionally used aged marsh sediment is considered an advisable option with respect to ecological as well as economic aspects. As a prerequisite to the application, the equivalency with respect to soil physical and mechanical properties of the materials has to be verified. Previous investigations on the compactibility of dredged materials used for dike construction had shown that the bulk densities of these materials were considerably lower than bulk densities of aged marsh sediments. The aim of the investigations presented in this paper was to analyze whether the compactibility of the processed dredged material could be improved by enhanced dewatering of the material prior to construction. It was hypothesized that a decreased water content of the material would allow higher bulk densities to be achieved during construction and hence the soil physical properties would become more comparable to those of the aged marsh sediments.
Materials and methods
To examine whether the compactibility of dredged material can be enhanced by pre-drying, Proctor tests were carried out at different initial water contents. Moreover, it was examined whether the temperature of oven-drying at 30 and 105 °C affects the compactibility of these materials and whether ripening, i.e., the repeated drying and wetting of the dredged material under natural and laboratory conditions, can improve their compactibility.
Results and discussion
The investigations on the effect of the various further processing methods showed that the compactibility and therefore the suitability of processed dredged material for dike construction can be improved by air-drying. A linear relationship between dehydration and Proctor density was found. Air-drying to water contents of 10% dry weight (DW) resulted in an improvement of the Proctor density of up to 11%. However, the tests on the effect of the drying temperature on the compactibility showed that oven-drying had no additional effect on the compactibility of the dredged materials. Ripening under laboratory and natural conditions did not lead to statistically significant changes in the compactibility of the processed dredged material either.
Conclusions
Air-drying of processed dredged material to water contents less than 10% DW is considered to be a useful pre-treatment option to improve the compaction behavior of processed dredged material and to obtain a better functional equivalency with traditionally used dike construction materials such as fine-grained aged marsh sediments.
