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Slope research roots for vegetation

A five year research project investigating bioengineering stabilisation of a clay slope on the M20 in Kent is in its final stages. The project, run by CIRIA, has shown that vegetation can be used to help stabilise a cut clay slope.

The study has looked at the performance of a Gault Clay slope on the M20 near Maidstone which was steepened to 1:3 from the original 1:6, and then divided into six plots and planted with three different vegetation regimes. Groupings of willows/ alders, broom/gorse and grass mix were each planted on both drained and undrained parts of the slope. The vegetation, especially on the willow plots, has become well established during the project.

Plots were instrumented and geotechnical properties monitor-ed on a seasonal basis. Soil moisture was monitored using tensiometers and standpipes and Mackintosh probing was used to give an indication of shear strength. Initial analysis showed a significant difference between summer and winter readings, with vegetation type having some influence.

Further extensive testing and observation is now planned with a view for its application on the construction of the Channel Tunnel Rail Link. The final test phase will investigate root morphology and distribution, associated soil interaction along with more detailed investigation of the shear strength and hydrological properties of the soil. Nottingham Trent University will also carry out additional tests on the root reinforced soil.

Research, with the support of the DETR, TRL and the Highways Agency, is by a team comprising Nottingham Trent University, Silsoe College, Cranfield Univ-ersity and Wardell Armstrong.

The project steering group is chaired by Rodney Chartres of Bullen Consultants.

CIRIA is running a free half-day site visit and workshop on July 23. Contact David Churcher at CIRIA, tel: 0171 222 8891; fax: 0171 222 1708; email: david. churcher@ciria.org.uk.

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