Mechanical Behavior of Compacted Arctic Ice Floes

Abstract

Coon, M.C., AIDJEX, U. of Washington Introduction The canopy of ice that covers the Arctic Ocean varies greatly in extent and distribution during a year and from year to year. The ice in the polar basin is not one solid mass; cracks and leads are present in all seasons as a result of variable stress acting on the pack ice. The spatial and temporal distributions of the ice cover are being continually mapped (see, for example, Refs. 1 and 2). The ice of the northern seas can be grouped into two main categories: water ice, which is 1 year's growth or less, and polar ice, which is of more than 1 year's growth. Winter ice, which varies from zero to about 2 1/2 m, forms in openings (polynyas, leads, and cracks) throughout the polar pack as well as in the open water that appears in the arctic offshore at the end of the summer. The thickness of polar ice is about 3 to 4 m at the end of winter and decreases to about 2 to 3 m at the end of summer. To imagine the arctic ice cover as a flat, featureless expanse is as erroneous as to believe it static, for the same winds and currents that cause cracks and leads cause hummocks and ridges. These extend several meters above the ice surface and several tens of meters below the surface (e.g., Ref. 3). Pressure ridges are linear accumulations of ice caused by the compression and shearing interactions of ice floes. The area affected by these ridged structures can be some 20 percent as reported by Wittmann and Schule and Kirillov. This short description of the arctic ice cover is given to indicate how varied the conditions of the ice may be. For example, the northern Alaskan coast has open water during August and September; however, for almost 8 months of the year, this area is covered by fast ice (sea ice that forms and remains fast along the coast). At least twice each year, as the ice breaks up in the spring and freezes in the fall, there can be various states of compacted and rarified floes. Even though the ice conditions on the Alaskan offshore are not easily typified, at least three conditions can be identified: large continuous ice sheets (fast ice); isolated floes moving under the action of the winds, tides, and currents; and small floes continuous with each other and at times with the main arctic pack. It is the stresses and deformations associated with the third condition that will be investigated here. This condition of small, continuous ice floes is shown in Figure 1. This photograph was taken by NASA remote-sensing aircraft under Ice-Dynamics project USGS from about 5,000 ft on March 16, project USGS from about 5,000 ft on March 16, 1971, at about 70 deg. North, 140 deg. West. It is apparent from Fig. 1 that the ice cover in this configuration can resist no tensile stress, for it would simply pull apart and produce areas of open water. However, the amount of compressive or shearing stress that this configuration could withstand is not so clear. The amount of stress that pack ice can transmit and its mode of deformation are of particular importance to the designer of offshore structures to be used in the arctic because it is this ice stress that will be transmitted to the structures. JPT p. 466