Effect of Sustained Loading on the Direct Shear Behaviour of Recycled C&D Material–Geosynthetic Interfaces

Recycled construction and demolition (C&D) wastes have been pointed out as a feasible alternative to traditional backfill materials of geosynthetic-reinforced structures, but the current knowledge about the interface behaviour between these unconventional (recycled) materials and the reinforcement is still limited, particularly as far as the time-dependent response is concerned. In this study, a series of large-scale direct shear tests was conducted using an innovative multistage method to evaluate the influence of shear creep loading on the direct shear response of the interfaces between a fine-grained C&D material and two different geosynthetic reinforcements (high-strength geotextile and geogrid). The peak and large-displacement interface shear strength parameters obtained from tests involving sustained loading were compared with those from conventional interface tests. Test results have shown that the shear creep deformation of the interfaces increased with the magnitude of sustained loading. The test specimens experienced additional vertical contraction during the creep stage, which tended to increase with the applied normal stress. For the recycled C&D material–geotextile interface, the sustained loading induced a reduction in the apparent cohesion and a slight increase in the friction angle, when compared to the values estimated from conventional tests. In turn, for the geogrid interface, the apparent cohesion values increased, whereas the friction angle did not significantly change upon shear creep loading.