Removing or cultivating vegetation on slopes to promote stability is not a clear-cut issue.
There have been a number of high-profile cases recently where tree removal from earthworks has led to very vocal and well-organised campaigns to stop works being carried out. This focus led the Geotechnical Asset Owners Forum (GAOF) to review the topic to try to find a common view on the matter.
Infrastructure owners should limit the amount of vegetation clearance to that which is absolutely necessary and ensure it is undertaken at the right time of the year - generally in winter - and the habitats these plants provide should also be taken into account. It is recognised that vegetation can provide a screen between the transportation corridor and the public which neighbours generally want kept.
But vegetation has a number of impacts on a slope. Roots may reinforce slopes, but only provide small benefits to overall stability at very shallow depth. Vegetation will take up and intercept water, resulting in increasing suctions (at least seasonally) in the slope. Serviceability movements from settlement and heave are induced predominantly by the hydrological action of vegetation. They cause deformations to the slope, cracking of the slope surface in the summer, leading to infiltration pathways available for the autumn and also cause strain softening of clay material, which may lead to progressive failure of the slope.
“As the removal of all vegetation from plastic clay embankments can cause failure, it is imperative that removal of vegetation should be carried out with an understanding of the risks involved.”
Academic studies for London Underground and Network Rail show that slopes with high water demand trees, such as oak, poplar and willow, can shrink as much as 60mm in hot, dry summers but are unlikely to heave back to the same levels.
As the removal of all vegetation from plastic clay embankments can cause failure, it is imperative that removal of vegetation should be carried out with an understanding of the risks involved. If the site is considered to be high risk, vegetation should not be removed unless the capability of the embankment to withstand increased pore water pressures is analysed.
Current research suggests that removal of trees from the upper two thirds of the slope achieves the necessary balance of serviceability improvements while maintaining soil suctions at the toe to maintain stability.
The GAOF found that vegetation management during and after remedial works is particularly problematic. Choice of solution, construction methodology and access should all be considered with a view to minimising tree removal.
It is essential that the public’s understanding of the work is managed from an early stage as in certain cases planting may not be possible.
What is clear, is that the role of vegetation and its effective management on transport infrastructure slopes is a complex issue.