No abstract

Mechanical root reinforcement is an important parameter to evaluate for stability analysis of rooted slopes. The contribution of roots is however difficult to quantify in situ without time-consuming methods or heavy equipment. Here we report field testing using the newly developed “corkscrew” method at two different sites with plantings of conifers and blackcurrant. In both sites we found positive correlations between root quantity and root reinforcement in surface layers where many roots were found. Below 125 mm depth, no correlations could be found, probably due to variability in soil stress and gravel content. Roots were shown not only to increase the soil peak strength, but also to add ductility to the soil, i.e., adding strength over much larger displacement ranges. Measured reinforcement, although similar to other experimental studies, was smaller than predicted using existing models. This may be attributed to the distinct difference in shear displacement required to mobilize the strength of rooted soil as compared with fallow soil. At displacements sufficient to mobilize root strength, the soil strength component has reduced from peak to a much smaller residual strength. The corkscrew method proved a promising tool to quantify root reinforcement in field conditions due to its ease of use and short test duration.

Mechanical root reinforcement is one of the mechanisms by which vegetation enhances slope stability. Common approaches to quantify this effect include either in situ shear box testing or destructive root sampling combined with a theoretical model to estimate reinforcement parameters. Both approaches, however, are time consuming. Here four new in situ techniques are evaluated to quantify mechanical root reinforcement and then compared under laboratory conditions. All four methods yield distinct results in soils reinforced with woody root analogues (acrylonitrile butadiene styrene rods), fine root analogues (polypropylene fibres) or stones. Two methods (adaptations of penetrometer testing, dubbed ‘blade penetrometer’ and ‘pull-up’) are suitable for spatially locating rooted zones and individual roots, while the other two (‘pin vane’ and ‘corkscrew’ extraction) demonstrate potential for directly quantifying the rooted soil stress–strain behaviour. These simple methods are suitable for use on difficult-to-access terrain where many measurements are needed to quantify spatial and temporal variability of root-zone properties for geotechnical calculations. The techniques are quicker to use than conventional methods and so should improve the reliability of slope stability predictions.