Effect of root volume density on the mechanical behaviour of saturated sand under drained and undrained conditions

Abstract

To determine the effects of root volume density on the mechanical behaviour of sand, drained and undrained triaxial compression tests were conducted on sand with root volume densities of 0.8%, 1.2%, 1.6%, 2.0%, and 2.4% under different confining pressures. Higher root content formed a denser and more uniform root network in the soil, enabling more roots to mobilize tensile stress, share external loads, and limit volumetric deformation. This enhanced the root-soil composite strength, reduced volumetric strain under drained conditions, and decreased excess pore water pressure under undrained conditions. The roots made a more pronounced contribution to the soil shear strength under lower confining pressures and undrained conditions. Specifically, with increasing confining pressure, the increment in the inherent soil strength far exceeded that in the additional strength provided by the roots. Under undrained conditions, the roots enhanced the soil strength by bearing part of the external loads and preventing the development of excess pore water pressure. Furthermore, the critical state line of a root-soil composite depended on the stress path. Since roots are non-granular materials and their mechanical reinforcement effect varies under different stress paths. Additionally, the roots enhanced liquefaction resistance of the sand by raising the initial effective stress required for triggering static liquefaction and the critical state effective stress. The greater the root volume density was, the stronger the liquefaction resistance of the sand.