Experimental Study on Direct Shear Strength of Sea Ice

When structures such as oil drilling rigs are constructed on or through the ice plate in coastal and offshore regions, the shear strength of sea ice must be estimated to determine the loading on these structures. Testing methods for shear strength must be established so that shear strength of sea ice in various conditions can be determined. The authors have been conducting, for five years, direct shear strength experiments using sea ice samples from the Okhotsk Sea coast. Physical characteristics of sea ice, including shear strength, depend to a great extent on such properties as salinity, porosity, grain size, etc; thus, there is variation in the test results of five years experimentation since the samples obtained varied from year to year. The following conclusions were drawn from this experiment: i) Under certain conditions the relation between shear strength and vertical stress can be represented by Coulomb’s equation of soil; ie, where T s : shear strength, C * : apparent cohesion (shear strength at σ v σ v = 0), φ * : angle of internal friction, σ v : vertical stress ii) The shear strength of sea ice increases, approaching a constant, with decreasing ice temperature. iii) The shear strength decreases with increasing ice shear area; an analogous relation exists in concrete. iv) The shear strength is not greatly dependent on either the shear velocity or stress rate. v) The shear strength is greater and generally increases more rapidly with decreasing ice temperatures in planes perpendicular to the ice growth direction than in planes parallel to it. vi) Two types of failure occurred in the sea ice samples. In the case of (sample diameter)/(sample length) less than 2 the failure was induced by shear only. With d/ l >2, the failure was not solely caused by shear since the existence of a small gap between the ice sample and shear box introduced a bending moment.