Geotechnical aspects of seabed pits in the Grand Banks area

Ice-created seabed pits found on the Grand Banks of Newfoundland can be analysed in terms of probable range of soil strength properties and failure mechanisms to determine the ice loads transmitted to the seabed. Almost all pits are less than 3 m deep; their existence can be explained by indentation failures caused by rolling icebergs. About 3.5% of the pits are more than 5 m deep and cannot be explained by iceberg impact. This paper presents a possible mechanism for their formation consisting of two actions: (1) the hard grounding of an iceberg, such that the keel becomes embedded in the seabed, and (2) the passive failure of the soil after the grounding event, predominantly caused by the loads associated with wave forces on the iceberg at the time of grounding or shortly thereafter. A 10 m deep pit located in the Hibernia area is analysed with respect to its configuration and soil parameters. Calculations have shown that, though waves in the Grand Banks area cause forces on the iceberg of a sufficient magnitude to create pits of the order of 10 m deep or more in the stiff Grand Banks soils, the controlling factor of pit depth is the ability of the iceberg keel to deliver these loads to the soil. Key words: iceberg pits, iceberg scour, pit formation, ice–seabed interaction.