Turning sediments into soil

Abstract

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.