Abstract
The presence of forests on hillslopes significantly reduces the slopes susceptibility to rain-fall-triggered shallow landsliding. This is due to the reinforcement of the hillslope soil by the forests tree-roots which increase theshear-strength of the soil, and in some instances, anchor the soil mantle to the underlying bedrock by deeply penetrating roots. Quantifying the reinforcing effects of tree-roots within soils and the evaluation of the stability of a hillslope using geomechanical and numerical models relies on realistic representation of the characteristics of the tree-root distribution within the hillslope and the mechanical strength of those tree-roots. The variety of experimental methods that have been developed since the 1960s and are used to generate this root-strength and rooted-soil shear-strength data are reviewed. The majority of these studies have focused on determining thetensile strength of individual roots by loading the root in a pulling device until it breaks and/or determining the shear-strength of rooted soil in comparison to root-free soil in a Coulomb-type shear-box test. These studieshave also generally either examined mature root systems in the field or relatively young plants grown in special containers specifically for tensile tests or laboratory shear-box tests. A particular difficulty that all studies have encountered is fixing or securing the ends of roots in the attachment device of the testing apparatus (so-called root-pullers or tensile strength testing machines) as the various styles of clamping employed can easily damage the root which reduces the measured strength or otherwise result in an unrealistic test result. Laboratory shear-box tests encounter a similar difficulty in that the roots are not generally fixed or constrained at the base of the shear-box; field shear-box tests tend to avoid this problem as the roots present their natural anchoring characteristics in the soil and rock substrate. A result universally reported in rooted-soil shear-box test studies is that the peak shear-strength of rooted soil significantly exceeds the peak shear strength of that soil in a root-free condition and that the rooted-soil peak strength is typically recorded at shear-displacement distance several times that of the root-free soil a result that fundamentally explains the reduced susceptibility of forested hillslopes to shallow landsliding. The variety of solutions developed to deal with the difficulties that root and rooted-soil tests present to practitioners are outlined. A set of suggested protocols for conducting root tensile-strength tests and field pullout tests so that a greater degree of uniformity in procedure is achieved are also presented. It is intended that the adoption of these protocols by practitioners in this field will enable more effective and direct comparison of test results and more confident interpretation with respect to the similarities and differences between test results generated from different plants and different field sites