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Plant roots for engineering soil properties

Roots bind soil together as a natural biological slope reinforcement
"The use of soft or ecological engineering approaches is becoming of increasing importance about the many challenges facing society in relation to flood prevention, coastal erosion, and slope stabilisation.

Dr A.G. Bengough, James Hutton Institute and University of Dundee

Landslides are a very widespread natural hazard, ranging from small shallow-seated slides of relatively small volumes of soil to the dramatic collapse of whole hill or mountain sides. They result from the natural attrition of gravity winning the battle with the frictional and cohesive bonds between soil particles perched on slopes. Intense prolonged rainfall events frequently trigger failures by lubricating the interface between soil particles, decreasing the suction (effective) stresses that help to hold a slope together. In many natural systems, vegetation plays a key role in two particular respects. Firstly in binding the soil together mechanically with its roots, with the tensile strength of the root system mechanically strengthening the soil. Secondly in changing the water regime in the soil, with evapotranspiration and canopy interception increasing the suction in the soil, and hence effective stresses, friction, and cohesion between soil particles. 

Landslides can be particularly dangerous and damaging if they occur adjacent to transport corridors or human habitation. The UK the railway system experienced 271 shallow slope failures on Network Rail’s 9,300 km of embankments and cuttings in the period 2012 to 2014. In June 2012, heavy rain resulted in landslides that caused the simultaneous closure of both the East and West Coast Mainline routes, cutting rail links between England and Scotland. On the roads network, there have been repeated localised problems - most notably on the A83 “Rest and Be Thankful” route in Argyll. This short stretch of road was closed for the fourth time in six months in March 2014, with a total of 11 road closures during the period 2011-2014.  Although this trunk route has a much smaller traffic volume than many others, its closure had major implications for local communities because it necessitated a diversion of up to 50 miles.

The outlook for landslide frequency is uncertain with increasing probability of intense rainfall events in Europe being predicted in the IPCC 2013 report. More prolonged intense rainfall events are likely to result in more slope movements and greater disruption to transport infrastructure, as noted by the Department of Transport and the UK Rail Safety and Standards Board. There are many ways of mitigating against landslide risk with widely varying cost implications and the likelihood of being effective. No single solution is suited to all situations. Conventional hard engineering solutions such as soil nailing offer guaranteed stability at considerable expense per metre of slope stabilised, whereas improvements to drainage and appropriate selection and management of vegetation offer a cheaper but less-certain alternative to stabilising slopes in less critical locations.

The use of soft or ecological engineering approaches is becoming of increasing importance about the many challenges facing society in relation to flood prevention, coastal erosion, and slope stabilisation. Appropriate use of vegetation to manipulate the soil is key to many aspects of the systems studied, and fundamental to this is the role of plant roots systems and their physical interactions with the soil around them. The use of plants to engineer the soil environment may be more cost effective than conventional engineering alternatives, more aesthetically appealing, and provide a much better and more diverse habitat for humans and wildlife alike. At present there is still much uncertainty as to the best species to employ in particular situations, and our ability to make quantitative predictions of plant performance for a range of applications is still in its infancy as compared with our ability to predict the behaviour of concrete and steel.

The challenge of appropriate vegetation management alongside our railway corridors has been particularly newsworthy with respect to the issue of leaves on the line, where this sometimes results in speed restrictions and delays to commuter trains. Choice of appropriate species management here is again crucial, as large trees overhanging railway lines may obscure visibility, shed many leaves in autumn, and add to the total loading of a slope. The relative importance of mechanical and soil-drying reinforcement of slopes by roots differs between species, and we must improve our ability to predict the multiple effects of vegetation on both soil mechanical and hydrological behaviour, so that engineering contractors can be confident in the effects of vegetation on the factor of safety of a slope. Given the enormous potential cost, ecological, aesthetic and carbon-saving benefits of ecological engineering approaches, this is a key area for future research.

This article was originally published in the Summer 2015 issue of The Geographer, the magazine of the Royal Scottish Geographical Society, and has been reproduced by kind permission of the RSGS.

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The James Hutton Research Institute is the result of the merger in April 2011 of MLURI and SCRI. This merger formed a new powerhouse for research into food, land use, and climate change.