For the design and performance analyses of geosynthetic-reinforced soil structures under repeated loading, such as those induced by compaction, traffic and earthquakes, the understanding of cyclic soil–geosynthetic interface behaviour is of great interest. Nevertheless, experimental data concerning this type of behaviour are very scarce. A laboratory study was carried out and is described in this paper. This paper presents the behaviour of an interface between a silica sand and a high-strength geotextile under monotonic and cyclic loading conditions. A large-scale direct shear test device able to perform load- or displacement-controlled cyclic tests was used. The results obtained are presented and discussed, especially in terms of interface shear stiffness and damping ratio. Monotonic direct shear tests indicated that the coefficients of interaction for the sand–geotextile interface depend on the confining pressure. Cyclic direct shear tests indicated that the interface stiffness tends to increase during the first loading cycles, exhibiting slight variation after 10 cycles. Slightly higher values of shear stiffness and damping ratio were reached with displacement-controlled cyclic direct shear tests. The cyclic loading of the interface did not lead to the degradation of the post-cyclic peak shear strength; however, the post-cyclic shear strength for large displacements exhibited an important decrease.

Soil–geosynthetic interface shear strength is an essential parameter for the design and stability analysis of geosynthetic-reinforced soil structures. Economic and environmental reasons have led to increasing use of locally available residual soils with a significant percentage of fines and lower draining capacity, when compared with the traditional good-quality backfill materials. This paper describes an extensive laboratory study carried out using a large-scale direct shear test device, in which the influence of soil moisture content, soil density and geosynthetic type on the direct shear behaviour of the soil–geosynthetic interface was evaluated. The study involved a locally available granite residual soil and four geosynthetics: two geogrids (one uniaxial and the other biaxial), one geocomposite reinforcement (high-strength geotextile) and one geotextile. Test results have revealed that the increase in soil moisture content can measurably reduce the soil–geosynthetic interface shear strength. Regardless of soil moisture content, soil density proved to have a remarkable influence on interface shear strength, particularly when geogrids were used. Among the different geosynthetics tested, the biaxial geogrid was found to be the most effective reinforcement for this particular type of soil, concerning the direct shear mechanism. For soil–geogrid interfaces, the coefficients of interaction ranged from 0.71 to 0.99. For soil–geotextile interfaces, the coefficients of interaction varied from 0.54 to 0.85.