| Summary: | The interface behavior between a caisson and the surrounding soil plays an important role in the installation of suction caissons as foundations for offshore wind turbines. A series of shear tests were carried out using a modified direct shear apparatus to study the interface shear behavior between sand and concrete. Sand samples with three particle size ranges (0.63–1.25 mm, 1.25–2.5 mm, 2.5–5.0 mm) and concrete plates with different relative roughness were used to explore the influence of the relative roughness parameter (<i>R</i><sub>n</sub>) and mean particle size (<i>D</i><sub>50</sub>) on shear behavior. The responses from the pure sand shear test are also discussed for comparison. Test results show that the higher the relative roughness (<i>R</i><sub>n</sub>), the greater the maximum shear stress (<i>τ</i><sub>max</sub>) appeared. The interface shear stress was weaker than that of the pure sand test. Furthermore, the interface friction angle (<i>φ</i>) of sand–concrete was closely related to the relative roughness of the concrete surface. Under the same conditions, the interface friction angle (<i>φ</i>) increased with relative roughness due to the effect of sand particles breakage and redistribution. By contrast, the effect of the mean particle size (<i>D</i><sub>50</sub>) on the interface friction angle (<i>φ</i>) was less significant. However, for the pure sand shear test, the friction angle (<i>φ</i>′) obtained from the traditional shear test apparently increased with <i>D</i><sub>50</sub>, indicating that the friction angle was more affected by <i>D</i><sub>50</sub> in the pure sand test than in the interface shear test.
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