Relationship between Fractal Characteristics and Shear Strength of Highway Spoil Site Materials

Highway spoil features a wide range of particle sizes and poor gradation, and their strength characteristics serve as key factors affecting the stability of spoil slopes. To explore the relationship between fractal characteristics of particle size and strength, this study examined 21 spoil sites along the Guizhou section of the Duyun‒Shangri-La Expressway. Based on the field investigation, particle size analysis tests, and laboratory shear tests, the particle composition and shear strength parameters of spoil were obtained. Based on fractal theory, the fractal characteristics of particle size under different gradation scaling methods were analyzed, and the relationship between shear strength parameters and the fractal dimensions of particle size was discussed. A formula for estimating the shear strength parameters of spoil was established. The results show that the particle composition of spoil varies significantly depending on their source and degree of weathering. Spoil from roadbeds and strongly weathered spoil generally exhibit unimodal distributions, while spoil from tunnels and with moderate weathering tend to exhibit bimodal distributions. The fractal dimension shows a strong correlation with both the internal friction angle and the fine particle content (mass percentage of particles smaller than 5 mm). When the fine particle content is ≤20%, the fractal dimension increases with the increasing of fine particle content, while the internal friction angle decreases with increasing fractal dimension. When the fine particle content exceeds 20%, the fractal dimension decreases with increasing fine particle content, and the internal friction angle increases with increasing fractal dimension. The proposed estimation formula for shear strength parameters was validated through case studies, and the strength parameters can be estimated based on particle composition, providing a reference for the stability assessment of spoil sites.