Soil-structure interaction problems are often involved in geotechnical applications. Sand-structure interface behaviour depends on several factors, but principally on the tendency of the interface soil to dilate or contract in a restraining medium. This feature explains the differences observed between the interface friction angles derived from model pile tests and interface direct shear tests performed at constant normal load (CNL). In the laboratory the elementary device that can reproduce this behaviour is the constant normal stiffness (CNS) direct shear apparatus. Through these tests, both the characteristics of the discontinuity surface and the role of the dilatancy can be examined. The aim of this paper is the formulation of an elastoplastic model for interfaces between sand and rigid inclusions. Based on some experimental tests performed with the CNS direct shear apparatus, a plastic potential, a yield surface and a hardening law are derived, which require only CNL test results in the parameter derivation. Finally, the predictions of the model have been compared with the experimental results obtained from CNL and CNS tests on different interfaces.
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