Subaqueous capping of very soft contaminated sediments

Abstract

A proposed 100 m × 100 m site for a subaqueous capping pilot-scale project in Hamilton Harbour is located in a water depth of about 15 m where the capping material will be in a relatively low-energy environment with little potential for erosion of the cap. The cap will consist of a 0.5 mm diameter clean sand that will be placed on the bottom with a tremie pipe discharge system mounted on a spudded barge. The design thickness of 0.5 m will provide an effective isolation of contaminants from the overlying water and biota. Consolidation following the placement of the cap is estimated to occur within the period of about 10–50 days. The settlement of the cap surface will range from about 14 to 21 cm. The results of a laboratory capping test, as well as theoretical results, suggest that the selected-placement technique will not disturb very soft contaminated sediments. The effectiveness of the cap will be tested by physical, chemical, and biological monitoring to ensure that the cap is placed as intended and that it prevents contaminant migration to the aquatic environment. Key words : subaqueous capping, cohesive sediments, contaminant isolation, in situ, consolidation, placement technique.