Effect of partıcle shape on the shear strength of sand–EPS, concrete, and tile ınterfaces

The grain size distribution, particle shape, and interface material properties play a critical role in governing interfacial interaction and shear strength. In this study, the interface behaviour and shear strength characteristics of four sands with similar grain size distributions but different particle shapes were investigated using direct shear tests against smooth concrete, rough tile, and EPS geofoam. Image processing techniques were employed to quantify particle fractal dimension and roundness of sands, the surface roughness parameter of the tile and concrete, and the penetration characteristics of the EPS geofoam surface. The results show that increases in fractal dimension and particle angularity lead to higher interface shear strength and adhesion due to enhanced interlocking and particle penetration, particularly for rough and deformable interfaces. A novel image-based three-dimensional penetration measurement method was also developed to evaluate sand particle penetration into EPS geofoam under different normal stresses. This approach enabled the identification of the transition in interface behaviour from friction-dominated to adhesion-controlled behaviour. The findings reveal that interface behaviour is governed by the combined effects of particle morphology, interface stiffness, and surface roughness, highlighting the effectiveness of the proposed methodology for advancing the understanding and modelling of soil–structure interaction.