Assessing the contribution of vegetation to slope stability

Many embankments and cuttings associated with the transportation infrastructure in the UK are only marginally stable. Engineering techniques such as soil nailing, geosynthetic reinforcement, improved drainage and ground improvement by stabilisation are available to improve stability, but the cost can be high. A lower-cost solution may be to utilise vegetation, either self-seeded or planted. The benefits and drawbacks associated with vegetation have been the subject of some debate. The problems caused by vegetation in relation to building foundations are well documented, and confirm that vegetation can have very significant influences on geotechnical parameters. Appropriate properly maintained vegetation can have the same significant influence to help provide additional stability to soil slopes. This paper considers the potential engineering influences of vegetation and how it can be characterised on site within a geotechnical framework for stability assessments. The direct reinforcement available from the roots of trees and shrubs is identified as providing one of the most significant contributions to slope stability. Case studies in the UK, Greece and Italy demonstrate how results from in-situ root pull-out tests may be used to estimate the potential reinforcement forces available from the roots. A scheme is presented to designate zones of influence within the soil according to the size and nature of the vegetation.