New in situ techniques for measuring the properties of root-reinforced soil: laboratory evaluation

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.