Effects of fractal dimension and water content on the shear strength of red soil in the hilly granitic region of southern China

Abstract

Deep residual soil, formed by the long-term weathering of large granite under the impact of high temperature and abundant rainfall in Southern China, is prone to instability and deformation, which has caused a large number of collapsing gullies. In this study, five different soil grading ratios and five water contents were employed to systematically investigate the effects of the soil fractal dimension and water content on the red soil shear strength. The soil fractal dimension increased with soil fine particle content (clay and silt); the soil saturated hydraulic conductivity (Ks) decreased with the soil fractal dimension; whereas the soil air entry value and residual water content (θr) showed a positive relationship with the fractal dimension. Shearing tests showed that the soil shear strength increased with normal stress but decreased with the soil water content. This reduction in the soil shear strength caused by the increasing water content was more notable for the soil with a lower fractal dimension. Analyses of the main effect tests showed that the water content presented greater effects on the soil cohesion and friction angle than the soil fractal dimension. A stepwise regression analysis showed that a linear relationship occurred between soil matric suction and soil cohesion (average R = 0.981 and P = .004) or friction angle (average R = 0.951 and P = .016). For the same matric suction, the soil cohesion and friction angles decreased with the soil fractal dimension, and the relationship between the soil fractal dimension with the soil cohesion (R = 0.951 and P = .013) and friction angles (R = 0.922 and P = .026) could be described by logarithmic functions. Soil fractal dimensions influenced both the soil hydraulic properties and shear strength parameters, and the overall effects of fine particles loss would decrease the shear strength of gully red soil.