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Soil and roots are the foundations of our development

Soil and roots are the foundations of our development (c) James Hutton
"Plants play multiple roles in stabilising soil such as removing water and also binding soil together through both their physical presence and also due to substances their roots produce.

Dr Kenneth Loades, Environmental and Biochemical Sciences, James Hutton Institute; Dr Gráinne El Mountassir, Civil and Environmental Engineering, University of Strathclyde

2015 is the International Year of Soils, highlighting how much we owe to soils. Looking at soils from an engineering perspective is one area that is easy to overlook but, like soil for food production, it is literally the foundations. We rely on soils to provide us with the platform on which to build our towns and cities and the infrastructure connecting them. Initial studies by specialised engineers assess the suitability of the ground for the proposed development. These engineers, geotechnical engineers, often recommend a strategy of ground improvement to improve its engineering properties, for instance how much weight the soil can hold and how stable the soil will be long term.

One of the most common materials used historically in ground improvement is cement due to its predictable long-term stability and its ability to tackle many different types of problems. Significant issues are associated with the production and use of cement due to its production accounting for 5% of annual global carbon dioxide emissions, resulting in a need to develop alternative approaches to stabilising soils. Researchers at the James Hutton Institute and the University of Strathclyde are developing alternative technologies for improving the engineering performance of soils using natural approaches for reducing associated environmental issues.

There is a growing area of research into the use of plant roots to stabilise soils at different scales, from mountains to roadside embankments. Plants play multiple roles in stabilising soil such as removing water and also binding soil together through both their physical presence and also due to substances their roots produce. The difficulty is in understanding what makes a good root for performing a variety of roles. Plants are incredibly diverse, similarly to what you see above the ground, diversity below the ground is equally great. Some plant roots are particularly good at going deep into soil but may not produce a large quantity of roots, for example trees. Alternatively some produce a large number of shallow, fine roots which colonise a large proportion of surface soil, e.g. grass.

Understanding variability over the seasons is also critical as a root system changes through the development of new roots and the death of others. Such variability concerns engineers as they need to know how strong a particular approach will make the soil for long term stability. This is a complex problem. Not only do you have huge variability in plant root systems but also changes to roots dependent on the soil environment they are growing in.

Plants however are very clever and can adapt to different soil conditions. If a soil is particularly hard, compact, roots can make themselves fatter letting them to go into soils better. Changes in roots are not always that obvious. In some species when a root starts to suffer due to reduced oxygen in soil, for example resulting from waterlogging, they can alter their internal structures to make it easier for oxygen to reach the growing part of roots, the tips. Another strategy adopted by plants is to make the root more water resistant through the production of lignin. Although we know what happens we must also understand the effect on the mechanical properties, such as how resistant they are to breaking and also how elastic they are. Some roots very stretchy, like rubber, with others being very stiff and more like wire. Understanding what drives these differences between roots will allow optimization of the root system of different plants depending on what the application requires.

Another new and promising area of research for stabilising soils is the use of bacteria. Some bacteria commonly found in soils, can be used to trigger a chemical process, producing a biogrout. This biogrout can bind soil particles together, making the soil stiffer, stronger and also make it more difficult for water to pass through it. Essentially starting from loose sand it is possible to create sandstone, in this way what takes much longer to occur in nature we can engineer to take place in days, making it a useful technology for preparing the ground before construction.

Some of the challenges of this technology include controlling where the biogrout forms, and how even the level of grouting is throughout the soil, so that you end up with similar engineering properties across the volume of soil treated. At present the process relies on injecting a single type of bacteria into the ground to produce the biogrout, however in the future the hope is that we could make use of the bacteria already present in the ground.

There is so much that ground engineers can learn from nature; this is only the beginning. Hopefully by making use of natural processes we can develop more efficient engineering solutions with reduced environmental impact.

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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.