###### Abstract  
This study uses a ring shear apparatus to measure the interface shear stress between five types of sand and three surfaces: steel, PVC, and stone. Experiments were conducted under normal stresses of 25, 50, and 100 kPa at a constant shear rate of 0.5 mm/min. The research examines the impact of various sand properties, including particle size distribution, median particle size, particle shape, and initial density, as well as the surface roughness and hardness of continuum materials. The results show that interface shear strength is significantly influenced by the mechanical interlock between sand particles and surface asperities, which is affected by the normalized roughness and hardness of the materials. Machine learning models, including Multiple Linear Regression and Random Forest Regression, were used to predict peak and residual shear strengths, demonstrating high accuracy. Additionally, an empirical equation was generated using eight input parameters, considering the peak and residual interface shear strength as outputs. ring shear; interface shear; continuum surfaces; granular material; sand +
Footnote †: journal: Medical  
interface friction between Fiber Reinforced Polymer (FRP) composites and sand decreases as the D\({}_{50}\) value increases, implying that larger particles tend to reduce frictional resistance at the interface due to fewer contact points per unit area. [PERSON] [14] supported these findings using a ring shear apparatus, showing that larger particles have a lower friction angle than smaller particles of the same mineralogy when sliding on identical rough surfaces. Particle morphology, namely roundness and sphericity, significantly impacts interface shear strength. Angular particles, with their irregular shapes, often increase the friction angle and interface shear strength due to their ability to interlock and resist movement. In contrast, rounded particles facilitate sliding, resulting in lower shear strength. [PERSON] and [PERSON] [15] indicated that shear mechanisms at the sand-geomembrane interfaces are heavily influenced by particle shape, with angular sands providing higher friction coefficients compared to spherical glass beads, which exhibited the lowest interface friction angles. Their experiments used a simple shear apparatus to compare the effects of different particle shapes. Angular particles create more interlocking and higher resistance to movement, enhancing shear strength, while round particles roll over each other easily, reducing shear resistance. The initial density of sand is another crucial factor affecting its interface shear strength. Sand density influences particle packing and the number of contact points, thereby affecting the internal friction angle and shear resistance. [PERSON] [16], using a direct shear box apparatus, observed that the interface shear behavior between FRP composites and granular materials significantly depends on the initial density of the soil mass. In structural material, rough surfaces increase mechanical interlock between sand particles and structural materials, enhancing shear resistance. [PERSON] and [PERSON] [9] introduced the normalized roughness parameter R\({}_{\text{n}}\) to quantify this effect. Their research, utilizing a direct shear box apparatus, showed that higher R\({}_{\text{n}}\) values correlate with increased peak interface friction. A study conducted by [PERSON] et al. [17] examined the coupling effects of normalized roughness and hardness on peak interface friction. Their research involved four different continuum counter-face surfaces (GFRP, copper, high carbon steel, and mild steel) interacting with glass beads and sand. The findings revealed that peak interface friction increased with higher R\({}_{\text{n}}\) and the extent of this increase was influenced by the hardness of the counter-face materials. [PERSON], [PERSON] and [PERSON] [17] suggested that the hardness of the counter-face material can significantly impact interface shear strength.