Depth-Dependent Variations in Fragmentation and Shear Strength of Gravelly Soil Under Shallow Overburden and Groundwater Conditions

The mechanical behavior of gravelly soils in shallow overburden environments is significantly modulated by groundwater fluctuations, which govern the in situ moisture distribution and, consequently, control particle fragmentation processes and the evolution of shear strength. Fragmentation was quantitatively characterized through particle size distribution, fractal dimension, mean particle size, and statistical parameters derived from Weibull and generalized extreme value distributions. Shear strength was evaluated via consolidated–drained triaxial tests. The results demonstrate a clear depth dependency of the soil’s physical and mechanical properties. As the sampling depth increased from 3.0 m to 26.5 m, the natural moisture content decreased from 2.89% to 0.77%. Fractal analysis reveals that the fractal dimension of the particles increased from 2.304 to 2.671, reflecting an intensified fragmentation process at greater depths. Triaxial test results indicate that with increasing depth, the internal friction angle increased from 28.63° to 40°, while the cohesion decreased from 18.59 kPa to 5.47 kPa. These quantitative variations suggest that the coupled effects of groundwater variation and overburden pressure collectively govern the evolution of fragmentation characteristics and the divergent trends observed in shear strength components.