A ring torsion apparatus was used to evaluate friction between-dry sand and a steel surface over wide ranges of surface roughness and sand density. The deformation of sand near the steel surface was observed by X-radiography. The apparatus is suitable for evaluating skin friction under well-defined test conditions. The test results show that frictional resistance is primarily governed by the roughness of the steel surface, irrespective of the density of the sand.
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Yoshimi et al. (1981) studied this question.
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