Stress path effects on the shearing behaviour of root-reinforced soils

Plant roots increase soil shear strength. The increase primarily depends on the relative direction of the root orientation and the principal strains/stresses of the rooted soils. Most of the published work focused on the direct-shear behaviour of rooted soil, of which both the magnitude and direction of the principal stresses could not be controlled nor measured. Indeed, in the scenario of slopes, the stress path experienced by direct-shear soil samples and the associated shear strength parameters (e.g. cohesion and friction angle) derived are only relevant to the soil elements that are sheared in the direction parallel to the slope. The shearing behaviour of rooted soil following other stress paths, such as triaxial compression (near slope crest) and triaxial extension (near slope toe), have rarely been investigated. In this study, we conduct a comprehensive laboratory test campaign to study the effects of stress paths on the shearing behaviour including stress-strain (hardening and softening) on coarse-grained soils reinforced by the roots of vetiver grass (Chrysopogon zizanioides). Root-reinforced soil samples prepared to different root volume ratios (RVR; defined as the ratio of total root volume to total specimen volume) were subjected to undrained triaxial compression and extension stress paths at different confining stresses. We will present key experimental evidence to demonstrate how the different stress paths and RVRs affect the stress–strain behaviour of the soil. We will also present the effects of stress path on cohesion and friction angle and discuss the strength anisotropy of the rooted soils. The new test results will shed light on the selection of plants of desirable root architecture at different slope locations (i.e. crest, mid-slope, toe) to maximise the root reinforcement effects to shallow soils.