Experimental investigation and numerical modelling of biomechanical variation of decomposed root: an application of vegetated slope stability

Abstract

Plant root reinforcement in soil bioengineering has gained increasing interest as a means of sustainable and environmentally friendly soil reinforcement and stabilization. In addition, quantifying evolutions of the biomechanical properties and mechanical root reinforcement to soil with the duration of root decomposition is important to agricultural land-use strategy and to soil stabilization purpose. However, the variations of these properties of the roots of herbaceous species, especially following herbicide application, have rarely been studied. This study aims to comprehensively measure the root morphological traits, root biomechanical properties, and root reinforcement of two contrasting vetiver species (Chrysopogon nemoralis and Chrysopogon zizanioides). In addition, the effects of root decomposition due to herbicide on the root biomechanical properties and root reinforcement of these two vetiver roots were investigated. A series of experiments, including root observation with a rhizobox system, uniaxial tensile test, and direct shear test, was performed. The herbicide (i.e., Propanil) was applied to four treatments of each species, considering four different durations of decomposition (7-, 28-, 56- and 112-days since herbicide application). In addition, the variation of root reinforcement was estimated using the existing root reinforcement models (i.e., Wu’s model and the extended root bundle model). The estimated root reinforcement then was used as an input parameter in numerical modelling to evaluate the effect of root decomposition on vegetated slope stability. The results showed that C. nemoralis could be an alternative to C. zizanioides in soil bioengineering applications. In addition, root decomposition highlighted the significant influence on root biomechanical properties (i.e., tensile strength, secant modulus, and breakage strain) and root reinforcement (i.e., root cohesion and maximum dilatancy) of two vetiver species. Consequently, root decomposition resulted in the deterioration of the protective function of two vetiver species on vegetated slope stability.