Mechanical Characteristics of Gravel-Block Soil Considering Particle Fragmentation Fractals

To investigate the mechanical characteristics of gravel-block soils in the cold regions, four large direct shear tests were designed under different coarse particle contents and three dry density conditions. The stress variations during shearing and the particle fragmentation rate after shearing were measured. The experimental results indicate that when p5 (the proportion of particles larger than 5 mm) ≥ 40%, the samples exhibit strain hardening behavior, and the stress–strain curve does not exhibit a peak within the range of the tests. The rock fragment skeleton exhibits excellent deformation resistance. With increasing coarse particle content, the internal friction angle of the soil initially decreases and then increases, while the cohesion initially decreases and then increases. Moreover, with increasing initial dry density, both the cohesion and internal friction angle of the gravel-block soils gradually increase. The fractal dimension increases with the increase in the particle fragmentation rate, indicating that the fractal dimension can also represent the degree of particle fragmentation in the soil. The relative fractal dimension increases exponentially with the increase in coarse particle content, indicating that the coarse particle content has a significant impact on the degree of particle fragmentation of gravel-block soils. The higher the coarse particle content, the greater the degree of particle fragmentation of gravel-block soils. When the coarse particle content increases from 0% to 60%, the fractal dimension decreases from 2.825 to 2.555, and the shear strength of the gravel-block soils continuously improves. During the shear process, the gravel-block soils transition from poor grading to well grading, with coarse particles breaking and fine particles filling the gaps between the coarse particles, resulting in a reduction in soil porosity and an increase in particle fragmentation rate and fractal dimension. The research outcomes of this experimental study provide guidance for the study of debris-covered slope landslides in cold regions.