Rooting for sustainable performance

More frequent intense rainfall events, associated with climate change, increase the likelihood of shallow slope failures that lead to costly disruption of road and rail journeys, with risk to life and property. There have been recent slope failures adjacent to transport corridors in the UK, sometimes disrupting important road and rail routes for days. Vegetation has a stabilising effect on slopes: Plant root systems interlock with the soil, increasing its stiffness and strength. Uptake of water by root systems dries the soil profile, again increasing soil stiffness and strength. However, engineers need to be able to predict the combined root reinforcement and soil drying effects on slope stability, so that vegetation management can be used proactively to decrease the probability of slope failure. Vegetation has numerous benefits over conventional hard-engineering solutions, in terms of burying carbon in the soil, enhancing biodiversity, and improving the aesthetic quality of the environment for society.

This project will develop and test a quantitative coupled hydro-mechanical model for the in-service and ultimate-failure performance of slopes planted with vegetation. Rooted-soil represents an innovative sustainable construction material, with distinct mechanical and hydrological properties, that can be used in geotechnical systems. The model will be applicable to both slopes covered with natural vegetation and slopes where vegetation and soil have been chosen and managed according to engineering principles. The validated model will provide a clear framework for assessment and remediation of slopes with potential for reducing economic and carbon costs.

The model will be developed within a multi-scale continuum modelling framework. It will build on knowledge of the elemental components of the system, working from individual soil-root interaction, to continuum soil-root system, and to complete slope, incorporating spatial variability of materials. Modelling will be informed by X-ray CT imaging of the 3-D deformation of rooted soil undergoing shear, using the micro-VIS facility at the University of Southampton, and by field data from slopes containing established vegetation. Predictions of slope performance will be validated against scaled-slopes in the Dundee geotechnical centrifuge under different rainfall regimes. The geotechnical centrifuge enables the testing and monitoring of small-scale slopes containing roots at realistic stresses, which can be manipulated until the slopes ultimately fail.

Template guidelines will be produced for a manual of combinations of plant species, soils and management schemes for optimum performance of designed soil-plant systems suited to emerging climatic conditions. Example data will also be included to allow cost-benefit analyses when designing for slope improvement using vegetation. The potential to translate research findings into related areas will be investigated (e.g. river banks, sand dunes, flood embankments, agricultural and amenity systems). We will engage with an important group of Project Partners, representing key industrial sectors and infrastructure owners, to facilitate the rapid adoption of research findings.

It is well recognised that vegetation generally improves the stability of soil slopes, but it is difficult currently for engineers to quantify the likely reinforcement. This project will increase substantially our ability to predict the effects of vegetation on the mechanical properties of the root-soil system, in relation both to direct mechanical reinforcement offered by roots, and to associated change in the soil water regime. Vegetation can be financially more cost-effective than hard engineered facings and soil nailing, whilst environmentally enhancing biodiversity, sequestering carbon, and mitigating local flooding risks. Vegetation can be both aesthetically pleasing and important in stabilising soil in agricultural and forestry systems, as sometimes illustrated dramatically when vegetation clearance results in landslides and soil erosion, occasionally resulting in injury and loss of life.

The proposed research therefore has the potential to impact a wide spectrum of the civil engineering, forestry and agricultural industries, and both private and government owners of land and major infrastructure. This includes:
* Engineering consultants and contractors through better guidance on how to incorporate the effects of vegetation into geotechnical calculations and analyses, and advice on long-term management of vegetation to optimise performance.
* Infrastructure and land owners, through better managed and maintained slopes.
* Insurance industry, who assess risk associated with the integrity of engineered or natural slopes.
* The general public, through fewer disruptions to transport and other services, and other positive environmental benefits such as the aesthetic appearance of slopes, biodiversity and carbon fixation.

At the core of this project is a group of stakeholders (EPSRC Project Partners) composed of major UK industries and academics with an interest in the mechanical and hydrological interaction between vegetation and soil. The Partners include infrastructure owners (London Underground, Network Rail, Highways Agency), their engineering service providers (TRL Scotland, Mott MacDonald, CH2MHILL), and research and development organisations (Rail Safety and Standards Board, Forest Research). They will provide a client's and practitioner's view of the benefits of the research, thereby helping to direct its development and dissemination.

All of the Partners have provided Statements of Support that explain how they will contribute to the research, the benefits that it will bring to their organisation, and how they will help to ensure impact. The major inputs into the project by the Partners will be: attendance at and hosting steering meetings and workshops; reviewing detailed research findings; dissemination within partner organisations, including via presentations and meetings; authoring of joint publications, with particular involvement in the project guidance report; providing access to monitored sites and other site data.

Potential beneficiaries will have access to the results of the research through a programme of dissemination that will span both UK and international industry and government. Dissemination of scientific results will be carried out via publications in top international geotechnical and plant science journals, and articles will be published in wider readership journals and magazines. A final project guidance manual will be produced, outlining the scope and design of ideal soil-root systems.

Two workshops will be held, one at the start of the project, particularly to engage with new potential end users of the research, and the wider international community, and another towards the end of the project to disseminate results. Regular research updates and key outputs will be added to the project website, incorporating a RSS feed, and linked from appropriate academic websites and scientific/social media.