The behaviour of deep-sea sediments under the impact of nodule mining processes

The complex processes of the suspension, resettling and consolidation behaviour of deep-sea sediments caused by deep-ocean mining were investigated on board the German RV SONNE and in the laboratory using sediment from the uppermost semi-liquid layer of the Peru Basin. On-board tests were accomplished directly with undisturbed sediment samples recovered by the maxicorer, a sediment sampling device especially developed for such studies. The impact was simulated by the use of a propeller-generated water jet. The results were used to prepare a substitute for the semi-liquid top layer for large-scale laboratory tests to simulate the sea-floor conditions to be expected during manganese nodule mining. In line with the design of most nodule miners, these tests were performed with the use of water jets. The amounts of displaced and suspended sediment were determined as was the resettling behaviour. The tests revealed that a mean value of 16% of the sediment was suspended by the water jets. The remainder was remoulded and laterally displaced for a short distance, but not suspended into the water column. The results of the on-board tests were confirmed by the laboratory tests and have great significance in the prediction of sediment clouds generated by ocean-mining activities. Additionally, effects on the resettling behaviour and formation of aggregates due to temperature and particle concentration were investigated. It was demonstrated that high particle concentration and high temperature within the suspension accelerate the aggregation process and thus lead to enhanced resettlement of sediment particles. Finally, the fabric of the resettled sediment was evaluated by its void ratio determined by the consolidation process. It is estimated that it will take at least 20 years for the sediment to regain its original density. (C) 2001 Elsevier Science Ltd. All rights reserved.