Stability of the Gunneklev Fjord sediments

Abstract

The seabed of the Gunneklev Fjord, with exceptionally soft contaminated sediment, is to be capped. The sediment, containing, among others, mercury and dioxins mixed with natural mud, has an undrained shear strength less than 1 kPa and thickness up to 2.5 m. To reduce the potential for leaching of mercury and dioxin from these sediments, Hydro Energy AS developed a remediation plan for Gunneklev Fjord which includes capping of the contaminated sediment with either sand or a mixture of sand and active charcoal. This paper presents an assessment of the risk associated with the cap placement over the contaminated sediment. The potential for the cap causing sliding of the sediment, which could re-expose even more severely contaminated layers and spread the contaminated material, was analysed deterministically and probabilistically. The sediment's undrained shear strength and thickness and the cap thickness were modelled as random variables. The deterministic analyses gave a safety factor over 3. The probabilistic analyses indicated, however, that the probability of sliding can be high for areas where the seabed inclination is steeper than 1:50, and if the average undrained shear strength is less than 0.4 kPa. The analyses highlight the importance of careful cap placement on the sloping seabed of the fjord, and the need to control the cap thickness. The analyses were used to support decision-making on the design of the cap and the placement method.