The reinstatement of shallow slope failures can be costly and the potential for slips may be reduced by the use of vegetation planted to provide reinforcement, through the plant root system, and a reduction in moisture content and pore water pressure. This report describes trials carried out to assess the suitability of different tree and shrub species. An initial literature survey determined a range of tree, shrub and herbaceous species that had the potential to grow in the environmental conditions alongside highways. A screening trial with 25 species was set up to determine whether the selected species could establish and grow in two different over-consolidated clays. The five species that showed the most successful establishment and growth in the screening trial were then used in a three-year experiment (1997-1999) to measure shoot and root growth. The successful exploitation of the clay mass by the roots of the different species suggests that there could be positive effects on slope stability, as a result of increased soil shear strength and reduction in soil moisture content. The economic costs associated with different vegetation options were also established and provided typical costs for the planting, establishment and ongoing 5-year management of the six vegetation options. The information obtained is useful for allowing a relative comparison between the species options and also to provide absolute values in order to compare vegetation options, in general, to other forms of technical solution where soil stability is an issue on motorway verges. (A)

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