The sliding interaction between sand and structural materials is involved in many geotechnical applications and is particularly important for the derivation of the shaft capacity of piles. Such interaction develops principally at the interface between the sand mass and the structural surface, and the comprehension of such interaction can be analysed through soil–structure interface tests. In particular, by using a modified version of the interface direct shear apparatus, that is, the constant normal stiffness direct shear apparatus, the friction characteristics of the interface and the role of the soil deformability on the experimental results can be studied. This paper focuses on the stress degradation occurring in these types of tests when cyclic loading is applied on sand–steel interfaces. Also, the post-cyclic response is analysed and compared to the response under monotonic conditions.Key words: interface, sand, shear stress, cyclic loading, stress degradation.

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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 interface friction angles derived from model pile tests and interface direct shear tests performed at constant normal load (CNL). In the laboratory the basic 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 dilatancy can be examined. The aim of this paper is to formulate 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 experimental results obtained from CNL and CNS tests on different interfaces.

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