Climate-change effects on the performance of bioengineered clay fill embankments

Under the effect of climate change, increasingly intense rainfall has caused frequent failures of UK transport infrastructure slopes/embankments. These failures have severely disrupted the serviceability of the transport network (which are vital in supporting national economic growth) and consequently led to significant socio-economic losses. There have been challenges for the engineers, planners and stakeholders to devise environmentally-friendly stabilisation techniques to withstand the negative impact of irreversible environmental change, while at the same time protecting the natural environment/ecosystems, which underpin the economic prosperity, health and wellbeing of society. Various slope stabilisation methods have been developed, such as sprayed concrete cover and piling. However, these traditional "hard" engineering methods have high embodied CO2, resulting in greenhouse gas emissions that have been linked to further increased climate change. This emphasises the urgency to develop a low-carbon and more sustainable engineering solution that can increase resilience and protect vital transport embankments.

The slope bioengineering method (SBM) using stem cuttings (known as live poles) is an aesthetically-pleasing, environmentally- and ecologically-friendly alternative to the traditional "hard" engineering methods, as this technique provides additional environmental and societal benefits of carbon fixation, enhanced biodiversity and ecosystem restoration within the built environment. Plant roots provide direct mechanical stabilisation to embankments and also act as a "bio-pump" during transpiration to remove soil moisture, which in turn increases soil strength and, hence, embankment stability. However, seasonal variation of soil suction due to plant transpiration potentially results in ground surface settlement/heave, which disrupts the serviceability of embankments (e.g. train speed restriction and delay, poor railway track quality and maintenance). Such disruption is more prominent when embankments are made of clay material that is vulnerable to shrinkage/swelling upon soil moisture changes. An interesting question hence arises: Is SBM suitable to be applied to clay fill embankments, and is it capable of maintaining slope stability and preventing from excessive slope deformation simultaneously?

The project will evaluate critically the effectiveness of SBM to combat the influence of different climate-change scenarios on the performance of clay fill embankments. The work described in this proposal represents the first systematic physical model tests for small-scale model embankments (made of real soil) supported by novel water-uptake pole models within a geotechnical centrifuge. The pole models will be designed to have similar strength and stiffness to real poles, and will also be capable of simulating the effects of plant root-water uptake in the soil. Highly-instrumented centrifuge tests are designed to investigate holistically whether the change of the soil water regime due to root-water uptake in a bioengineered embankment magnifies the clay shrink-swell response, which in turn leads to seasonally-driven failure and ground surface settlement/heave. Different vegetation management schemes (through selection of plant types and arrangements) will also be examined to optimise the performance of embankments for minimising ground surface settlement, while enhancing embankment stability. The project will provide a unique test database that contains new knowledge for end users to develop increased confidence for wider deploying SBM in practice.

The new knowledge and insight derived from this project will not be limited only to transport infrastructure slopes/embankments, but extends also to wider engineering applications. These include enhancing the performance of earthworks for flood defence and landfill covers, which are critical elements of civil infrastructure that are vulnerable to the effects of climate change.

The project outcomes will have multiple economic and societal impacts to the following beneficiaries:

(i) Consulting engineers and owners of transport embankments:
The project findings will provide enhanced capabilities to this group of beneficiaries, who are responsible for designing a safer and more resilient built environment, including the two Project Partners Network Rail and Highways England. The datasets of physical model tests will provide useful insights for these beneficiaries to foresee the effectiveness of using SBM to combat climate-change effects on the performance of infrastructure embankments. In an ever-competitive civil engineering market, the new insights gained will assist in raising the international profile of UK research, design and consulting excellence for developing sustainable slope stabilisation techniques, aiding UK consultant firms in competing successfully tendering for work internationally. This will enhance the global influence of UK PLC.

(ii) Government:
Wide deployment of the slope bioengineering method to increase the resilient and environmental sustainability of the built environment will help assist the UK Government to get closer to meet the Department of Energy and Climate Change (DECC)'s 2050 ambitious target of legally binding reduction of greenhouse gas emission by 80% by 2050.

(iii) Society/public:
Enhanced performance of bioengineered embankments will bring direct benefit to the travelling public, who will experience reduced disruption. The low-carbon SBM also provides significant environmental and societal benefits. Integrating vegetation in engineering design (i) offsets the carbon use, (ii) creates pathways for carbon sequestration, (iii) offers ecological improvements to slope rehabilitation, plant biodiversity and recovery of ecosystem structures and functions, and (iv) provides a more aesthetically-pleasing appearance of transportation earthworks in the built environment.

(iv) University engineering students:
The new scientific knowledge derived in this project will be synthesised as part of the teaching materials in undergraduate and MSc courses at University of Dundee (Geoenvironmental Engineering). This will make new graduates able to recognise the negative impact of long-term climate change on civil infrastructure and, hence, the importance of devising sustainable and resilient stabilisation methods. Educating engineering students with the latest technological advancement of the SBM will encourage them to consider and deploy this low-carbon, environmentally- and ecologically-friendly technique in practice more widely.