A little cohesion goes a long way: Estimating appropriate values of additional root cohesion for evaluating slope stability in the Eastern Australian highlands

The reinforcement of soils by tree roots has long been recognised to contribute significantly to maintaining the stability of hill slopes throughout Eastern Australia. Australian geotechnical engineers recognise this fact by recommending that hill slope trees should not be cleared in landslide risk management guidelines. Despite this recommended practice, values for additional root cohesion due to root reinforcement of the soil are rarely included in geotechnical assessments of the stability of Australian hillslopes. Estimates of additional root cohesion values appropriate for evaluating the stability of hill slope soils typically encountered in the Eastern Australian highlands are presented. These estimated values are based on those developed for stability assessment of eastern Australian coastal river banks. Three case studies from the Sydney area are investigated as exemplars and are used to demonstrate how the roots developed by mature hillslope forests reduce the likelihood of their mass failure by sliding or slumping.