Determination of the shearing behaviour of root-permeated soils with a large-scale direct shear apparatus

A Yildiz,F Graf,C Rickli,S.M Springman

doi:10.1016/j.catena.2018.03.022

Abstract

Soils with roots or root-like inclusions have often been tested in direct shear to quantify the effects of vegetation on the shear strength of soil, and in turn, the stability of slopes. However, a straightforward evaluation of root reinforcement is challenging due to the complex nature of roots, and the dependency of soil behaviour on many factors. An Inclinable Large-scale Direct Shear Apparatus (ILDSA) was built to study the shearing behaviour of root-permeated soils. Planted specimens, consisting of two different sets of species, were prepared with a moraine, sampled from a recent landslide location, and tested in direct shear subsequent to saturation. Relationships of peak stress ratio with dry weight of roots, maximum dilatancy angle and void ratio were investigated to evaluate the behaviour of root-permeated soil. The combined approach, of taking both presence of roots and dilatant behaviour of soil into consideration, results in a more realistic understanding and quantification of the effects of root reinforcement, at least, for laboratory testing of root-permeated soils.

Highlights

Soil bioengineering methods, the use of vegetation to prevent surficial erosion or shallow mass movement (Gray and Sotir, 1995), serve as a promising alternative to traditional civil engineering applications to stabilise either man-made or natural slopes against superficial failure
If the dry weight of Salix appendiculata is excluded from the total above ground biomass (AGB), the average dry weight of total AGB reduces to 27.9 g for HHP6
The shearing behaviour of root-permeated soil was explained via the relationships of peak stress ratio, maximum dilatancy angle, root biomass, and void ratio

Summary

Introduction

The use of vegetation to prevent surficial erosion or shallow mass movement (Gray and Sotir, 1995), serve as a promising alternative to traditional civil engineering applications to stabilise either man-made or natural slopes against superficial failure. Roots improve the slope stability both mechanically, with roots crossing a potential failure surface (Waldron and Dakessian, 1982), and hydrologically by evapotranspiration resulting in increased suction in the ground (Blight, 2003; Springman et al, 2003), and to a lesser extent by altering the soil structure (Graf and Frei, 2013; Loades et al, 2010) Roots perform their mechanical reinforcement function by working as tension-carrying fibres that transfer the shear stresses in the soil matrix into tensile resistance via the interface friction along their surface (Gray and Barker, 2004). The intercept on the shear stress axis is denoted as “root cohesion” (cR) value

Methods

Results

Discussion

Conclusion