The growing demand for sustainable construction materials has prompted interest in reusing dredged sediments as an alternative to traditional raw materials in dike construction. Before they can be reused, sediments must undergo the lengthy ripening process, during which they are transformed into stable soil. Accelerating this transformation could significantly improve the feasibility of sediment reuse. This study investigates whether biochar amendment can enhance the biophysicochemical ripening of dredged sediments, focusing on the influence of biochar application rate and particle size.

Dredged material from the port of Hamburg, Germany, that was dewatered and processed at the METHA plant, was mixed with biochar produced by Bio Energy Netherlands from the gasification of wood waste at 800-1000°C for 90-120 minutes. The mixtures contained biochar with varying application rates (2%, 4%, 6%) and particle sizes (<2 mm, 2-5 mm, >5 mm). Over the course of 15 weeks of field ripening, the sediment-biochar mixtures were exposed to natural weather conditions and turned weekly. Biochar amendment introduced additional porosity which increased water holding capacity by 33-72% compared to the control after 15 weeks of ripening, resulting in values of 24-72% DW. The oven-dried COLE, ranged from of 2.2 to 5.4% which represents a decrease of up to 54% relative to the unamended sediments. This improvement can be attributed to the non-plastic behavior of biochar and explains the decreasing shrinkage observed with an increasing application rate. Increasing particle size was correlated to decreasing shrinkage (p <0.05) which could be due to the interrupting effect of coarse biochar particles on tensile load propagation in the rods. A qualitative assessment of the structure development of the experimental variants suggests an acceleration of structure formation with higher biochar application rate and larger particle size when combined with weekly turning. This resulted in a faster breakdown of the dense and platy METHA material into smaller and more aerated aggregates. Overall, the physical ripening of the dredged material was improved with the addition of biochar at increasing application rates and particle size, which promoted a faster stabilization of sediment aggregates and enhanced physical properties beneficial for construction applications.

The occurrence of sulfur oxidation, the main chemical ripening reaction, was evidenced by a loss in the total sulfur content of samples and an increasing electrical conductivity during dry periods. The pH was expected to decrease as a result of the release of protons from this reaction, however this was not observed. Instead, increasing biochar application rates was correlated to a higher pH (p <0.05) and was evidence of the material's buffering capacity which can be attributed to its high functional group and mineral content. The total sulfur content reduced on average by 5% and 22% in the amended samples and the control, and this smaller decrease compared to the control could be explained either by a slower chemical ripening in amended sediments or by measurement limitations. Furthermore, the evolution of electrical conductivity over the 15 weeks of field ripening evidenced the accumulation of chemical reaction products in dry periods.

The influence of biochar on sediment physical and chemical properties, including the increased pore structure, water holding capacity, aeration and buffering capacity, all contributed to creating conditions favorable to microbial activity. A priming effect of biochar application could be observed in the first six weeks of ripening, with high respiration rates, high decomposition rates, and decreasing stabilization of organic matter. In this period, total organic carbon content decreased on average by 30\% in amended samples, compared to only 6% in the control. At the same time, nitrogen content decreased on average by 13% in the samples with biochar, further confirming the high microbial activity. This was followed by a period of decreasing microbial activity until the end of the experiment, which was marked by 14-32% lower respiratory carbon release of the amended samples compared to the control, decreasing decomposition rates and increasing stabilization of organic matter. Thus, biochar application accelerated the decomposition of labile carbon and enhanced the biological stabilization of organic matter in sediments.

These findings suggest that biochar amendment can significantly improve sediment ripening processes and can result in a material with properties desirable for dike construction.

Rising sea levels, combined with land subsidence, have heightened the concerns for safeguarding the coast and waterways of Europe. This effort will require a large amount of strained resources, such as marsh soil, which is becoming increasingly difficult to source. A promising alternative is utilizing the vast amount of dredged sediments extracted by ports each year. Studies have already been conducted on the feasibility of using such a material in dike construction and reinforcements, and guidelines have been developed. Treatment of the material is recommended before application, usually through stockpiling. Although this is widely accepted as a suitable method to treat sediments, the potential for optimizing it has been a subject of few studies. Therefore, focusing on physical ripening, this thesis set out to investigate the evolution of tensile strength and crack formation over time under different stockpile management practices. This research was motivated by the observation that unripened material tends to develop shrinkage cracks when applied in a dike, which creates preferential channels for water to flow through. An important factor influencing the formation of cracks is the material’s tensile strength. Other aspects investigated were the Coefficient of Linear Extensibility (COLE) , which characterizes the shrinkage behavior, and the Atterberg limits. The latter is an important parameter to determine the suitability of a material to be used in dikes. This thesis investigated mechanically de-watered dredged sediments processed in the METHA plant in Hamburg. After processing, the sediments were stockpiled (1,000-2,200 m3) with varying turning rates (none, 2x per year, and 4x per year) and vegetation management (removed before turning or not). Samples were collected over approximately two years, covering ripening times from six months to two and a half years. The results showed that tensile strength increased significantly after one turning event to around 105% of the original material, likely due to aeration improving structural stability. However, subsequent turning events led to a drop in tensile strength. SP7 and SP9 (4x turning per year) exhibited tensile strength 30 % lower than the original material after two years of ripening, suggesting that the repeated mechanical breakdown degraded soil structure. At the end of the investigated period, tensile strength was found to be highest in the control stockpile, where no turning was applied and vegetation was allowed to grow. Higher turning frequency, however, greatly benefited the compactability of the samples. When tensile strength tests were carried out compacting the samples to 95 % of their Proctor density, an increase in tensile strength of 430 % was found in the sample with the highest Proctor density (SP-9). In comparison, SP-4 (control) exhibited an increase of 280 % at 95 % Proctor density. COLE values stabilized after two years to values approximately half of the original material. This indicates reduced shrinkage potential and, therefore, a lower tendency for crack formation in ripened sediments. Crack formation experiments exhibited no consistent pattern across different stockpiling methods and ripening periods. Instead, the most influential factor appeared to be the reduction in shrinkage due to ripening. Crack Intensity Factor (CIF) and average crack width results were consistently lower in the stockpiled material than in the unripened original material. This was likely caused by the reduced shrinkage behavior observed in COLE tests. Finally, the results of the Atterberg limits determination showed that the material is unsuitable for use in the top layer of a dike, but is still appropriate for use as core material. Overall, the findings highlight that limited turning preserves tensile strength, while higher turning frequencies improve compactability. Under field conditions, however, the improved compactability also greatly benefits tensile strength, outweighing the structural instability caused by mechanical breakdown. Ripening also significantly reduces shrinkage potential, thereby decreasing crack formation behavior. These insights suggest a higher turning frequency is the best method for managing dredged sediment, when only tensile strength and crack formation are concerned.