Laboratory tests on ridging and rafting of ice sheets

Abstract

The mechanical deformation of a sea ice cover takes place through ridging and rafting. These processes have been studied in an ice basin by pushing two identical ice sheets together. Nonuniform ice sheets consisting of floes of thickness t1 and thin ice of thickness t2 connecting the floes were used. The major thickness t1 and the thickness ratio t2/t1 were varied. Ice sheets of uniform thickness (t2/t1 = 1) never formed ridges; they only rafted. However, when ice sheets of nonuniform thickness were used, initial rafting transformed into ridging. In general, high values of t1 and low values for t2/t1 favored ridging, while low values of t1 and high values for t2/t1 favored rafting. The forces during the tests were measured. During the initial rafting stage the force increased linearly with displacement. The experiments also suggest that the ridging force has a maximum value. This limit can be related to horizontal growth of the ridge or onset of ridging in another site. The relation between force and ice sheet thickness has also been analyzed. Further, from the force and the measured ridge profiles it was possible to estimate the ratio of work to change in potential energy. This ratio was about 15 for ridging and about 35 for rafting.