Vegetation keeps trains on tracks
by Geraldine Hinter
Railway
tracks built on expansive clay soils and sites with poor drainage
often change their shape due to swelling, shrinkage or softening of
clays beneath the tracks, forcing trains to slow down to avoid
derailment and making them less efficient.
Researchers at the University of South Australia have found that planting trees and native grasses could be a cheap and effective solution for stabilising rail tracks built on poorly draining expansive soils.
Current methods used to improve the performance of rail tracks on these soils offer only a temporary solution for a continuing problem, according to senior lecturer in the School of Natural and Built Environments, Dr Don Cameron.
"Rail tracks are realigned and reshaped by lifting the rails and adding more crushed rock (ballast) beneath the tracks. This creates a deeper layer of rocks that attract more water, which drains to the clay soils below, further softening the soil base," Dr Cameron said.
"While drainage is the obvious answer to water logged subsoils, it is difficult to achieve, particularly on flat clay alluvial plains, because water has nowhere to drain.
"The solution is to find something that takes out the moisture and engineers have observed that in areas where vegetation was present, the track appeared more stable than at adjacent sites with no vegetation."
This finding has been confirmed by UniSA masters graduate, Wayne Potter, whose thesis demonstrated that trees were effective in drying the soil.
"Trees absorb and remove excess moisture to great depth and lateral extent using their roots, which act deeply and around the railway track. This leaves the soil stronger, stiffer and less likely to settle, apart from some initial drying settlement. In the long-term the subsoil should stay relatively dry," Dr Cameron said.
Potter’s measures of stiffness under traffic loading showed improvements of three to five times in the stiffness of soils when compared with sites having similar soils and no trees.
"While planting trees is a fairly costly exercise, stabilising the track this way is a lot cheaper in the long run than having to pay for continual track maintenance," Dr Cameron said.
In addition to trees, lecturer in the School of Natural and Built Environments, Joan Gibbs said there were big advantages in growing native grasses along rail corridors.
"Our research team has shown that native grasses dry the soil by encouraging the growth of microbes that make the soil into clods, which is a biological process that opens up the soil so that water doesn’t pond on the surface, but percolates between the clods to where it belongs, eliminating water logging problems that cause the track foundation to go soft and sink," Gibbs said.
"Native grasses are slow growing and spread to cover surface areas, suppressing weeds and reducing the risk of fire, which is a big issue on rail corridors. Weeds present a major fire risk, with a high fuel load that rages when set alight, while native grasses have a low fuel load that doesn’t burn easily.
"Trees and native grasses might be equally important in managing rail corridors and a combination of the two might be the answer in some areas. We can have trees that send roots out to absorb moisture or use native grasses, which allow water to percolate in and around the broader area," Gibbs said.
"Trees work deeper and will keep the surface fairly dry, but they can’t compete with native grasses if there’s heavy rain.
"Throwing poison on weeds creates a worse situation. Sprays kill surface weed growth but their roots remain in the soil and form a layer or mat that creates an impenetrable barrier. The weeds feed themselves and kill the microbes in the soil, which then compacts into heavy clay that’s impervious to water," she said.
"We are trapped by the weed spray cycle."
Research into the feasibility of revegetating the rail corridor was funded by the Rail Cooperative Research Centre, of which UniSA is a core partner, supported by the Australian Rail Track Corporation and Queensland Rail.