Impact of Climate Change on Geostructures with Reference to Transport Infrastructure

Transport infrastructure is linear and therefore comprised of geostructures consisting of several soils of differing characteristics i.e. particle size, mineralogy and thus plasticity. Furthermore, many geostructures in the United Kingdom (UK) were constructed prior to the implementation of regulation and therefore not built to modern standards. For asset management, this is particularly important when considering predicted changes in climate patterns owing to climate change, which are expected to yield larger seasonal soil-atmosphere moisture fluctuations. This will likely result in increased drying-wetting amplitude of compacted soils following their initial compaction.

This doctoral thesis investigates the impact of drying-wetting due to climate change on the geomechnical behaviour of different compacted UK soils. This is important to determine transport infrastructure asset management strategies for future climate scenarios to ensure resilience of the rail and road networks.

To achieve this a novel drying-wetting methodology underpinned by soil water retention behaviour was developed which allows for comparison between drying-wetting of soils with different water retention characteristics. Drying-wetting boundaries were also controlled using gravimetric water content levels in order to mimic realistic field conditions typically observed at substructure level. Drying-wetting was conducted for 10 cycles on three soil mixtures representative of those incorporated into the UK transport network. This includes a low plasticity clayey sandy silt, representative of Devensian glacial tills, a high plasticity clayey silt (commercial kaolin) used to benchmark against experimental findings in the literature and a very high plasticity silty clay, similar to the London clay.

This study incorporates the use of ultrasonic pulse transmission testing validated against conventional bender element, the measurement of saturated drained stress-strain behaviour and micro X-ray computed tomography (CT) to characterise changes in microstructure at select water contents.

During drying-wetting the very high plasticity silty clay experienced the greatest reduction in small strain stiffness due to significant macroscopic cracking, while the low plasticity clayey sandy silt experienced small deviation from the compacted conditions. The results were also analysed by normalising changes in small strain stiffness and showed that an increased drying amplitude, i.e. from the transition zone to residual conditions, resulted in a reduction small strain stiffness. However, an increased wetting amplitude, from the compacted state to full saturation, resulted in a smaller comparative reduction in small strain stiffness. These findings were not surprising and were generally in line with the microstructural changes observed for the soils following drying-wetting processes.

The results from the saturated drained stress-strain behaviour showed that irrespective of the drying and wetting boundaries adopted all soils exhibited an increase in strain softening, initial tangent modulus (Ei), peak deviator stress (qpeak) and dilatancy ratio (dg). This was attributed to the increase in effective stress during drying-wetting captured by the current suction ratio (CSR).
From a practical standpoint, when results were plotted against modified plasticity index (Ip’) an inverse correlation was obtained following drying-wetting for small strain stiffness and changes in volume. In practice these findings indicate the potential for plasticity to be used as an indicator for the performance of soils, incorporated into both historic and modern infrastructure, to the impacts of climate change. Furthermore, the results highlight that soils with high Volume Change Potential (VCP) in southern England are vulnerable to post compacted changes in behaviour (e.g. cracking) when compared to Devensian glacial tills located in northern England. Indeed, UKCP18 future climate scenarios show that southern England will likely also experience increased deviations from mean rainfall and summer temperatures compared to the northern England which may exacerbate this contrast in soil behaviour.