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Slopes are all around us in the urban environment, and the soil on some of these slopes may be inherently unstable. Old, natural slopes in rural and forest areas have often developed a degree of stability over time. But artificial slopes within urban areas that are part of developments, or that are adjacent to infrastructure such as roads and railways, can be less stable, and may require stabilisation using engineering, bioengineering or ecoengineering methods.
Movement of soil and rock down unstable slopes due to gravity is called mass wasting; surface movement resulting from the effects of wind and water is called erosion. Both processes can affect the safety of people living or working on or near slopes, and on the quality of water for people living in the wider area. The expansion of urban areas, and associated deforestation and construction activities, are increasing the area of unstable or vulnerable slopes.
Decisions on suitable methods for slope stabilisation first require an evaluation of the hazard. Deep mass movements (deep landslides) are difficult to control and require engineering solutions. Shallower mass movement (shallow landslides) and erosion processes are more suitable for control using bioengineering or ecoengineering methods that will have the added benefit of enhancing the urban greenspace.
Methods include the use of stone, steel, concrete and geosynthetics to stabilise or shore up slopes that are liable to landslides. These methods should, where possible, be combined with use of plants and trees to enhance the urban greenspace.
Bioengineering techniques combine engineering methods with natural or living materials to protect or restore slopes and reduce erosion. Methods include using brush mattressing to minimise erosion, and the planting of shrubs, plants and trees to stabilise the soil.
Ecoengineering is defined as a long-term ecological strategy to manage a site with regard to natural or man-made hazards (Stokes et al. 2007). Such hazards include landslides, rockfall and erosion.
If used appropriately, each of these methods can have an added benefit of enhancing the urban greenspace. A range of bioengineering and ecoengineering methods for slope stabilisation are described by Norris et al. (2008).
Research conducted by Forest Research into soils and the development of tree roots has been used in developing models and decision-support systems for using vegetation to help stabilise soils.
Coarse and fine roots form a dense network that binds soil together on slopes, and play a major role in minimising soil loss in both rural and urban areas. Wherever possible, the use of vegetation should be encouraged as part of the strategy to minimise soil loss.
ECOSLOPES (Eco-engineering and conservation of slopes) was a multidisciplinary EC-funded project that was completed in 2004. The project has produced techniques and tools to improve slope stability and reduce erosion. Across Europe, the effects of forest vegetation on soil stabilisation, rockfall and erosion were examined on a variety of reference sites. The removal of plantation trees, and the impact of repeated forest fires on slope degradation were evaluated, along with the recovery of the ecosystem and consequences for erosion.
As the uprooting of trees in damaging storms can contribute to a considerable increase in the loss of soil from steep sloping sites, Forest Research staff compared the stability and root architecture of trees growing on sloping and horizontal ground.
Digitising 3D root architecture data allows us to analyse the symmetry and density of tree roots that have grown on slopes. Data gathered from tree-anchorage field experiments conducted in West Scotland, and from 3D digitising of large root systems, were used in developing the ECOSLOPES models of tree architecture, slope stability and tree stability.
As soil loss will be vastly accelerated following windthrow of trees on steep slopes, it is essential that trees growing on slopes must be carefully managed to minimise wind risk.
BS 1377 (1990) Methods of Testing Soils for Civil Engineering Purposes. British Standards Institution, London.
Coutts, M.P., Nielsen, C.C.N. and Nicoll, B.C. (1999). The development of symmetry, rigidity and anchorage in the structural root system of conifers. Plant and Soil 217: 1–15.
Nicoll, B.C., Achim, A., Mochan, S. and Gardiner, B.A. (2005). Does steep terrain influence tree stability? A field investigation. Canadian Journal of Forest Research 35: 2360–2367.
Nicoll, B.C., Berthier, S., Achim, A., Gouskou, K., Danjon, F. and van Beek, L.P.H. (2006). The architecture of Picea sitchensis structural root systems on horizontal and sloping terrain. Trees – Structure and Function 20: 701–712.
Norris, J., Stokes, A., Mickovski, S.B., Cammeraat, E., van Beek, L.P.H., Nicoll, B.C. and Achim, A. (2008). Slope Stability and Erosion Control: Ecotechnological Solutions. Springer, Dordrecht, the Netherlands.
Stokes, A., Spanos, I., Norris, J.E. and Cammeraat, L.H. (eds) (2007). Eco- and Ground Bio-Engineering: The Use of Vegetation to Improve Slope Stability. Proceedings of the First International Conference on Eco-engineering, 13–17 September 2004, Thessaloniki, Greece. Developments in Plant and Soil Sciences Vol. 103. Springer, Dordrecht, the Netherlands.
Tobin, B., Čermák, J., Chiatante, D., Danjon, F., Di Iorio, A., Dupuy, L., Eshel, A., Jourdan, C., Kalliokoski, T., Laiho, R., Nadezhdina, N., Nicoll, B., Pagès, L., Silva, J. and Spanos, I. (2007). Towards developmental modelling of tree root systems. Plant Biosystems 141: 481–501.
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