It's soil, Jim, but not as we know it: unlocking the hydromechanical behaviour of hydrophobic sands

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Climate change means that many geotechnical structures will face harsher conditions in the future than their designers previously considered. Specifically, droughts, heat waves, flooding or heavy rain will all compromise soil covers used to protect mining or municipal waste storage sites, as material either cracks or becomes waterlogged. The environmental and economic consequences of such failures are and will be disastrous. Water repellent (hydrophobic) sands may be able to resist such changes and so be a novel and timely substitute for current cover layer materials. However, these materials are new and we do not yet know how water passes through them or acts to stick particles together. Understanding this will greatly help the engineering of structures they support.We know that soil behaviour is governed by the amount of water trapped within its pores. In normal soils, water trapped between soil particles forms concave 'bridges' which act to stick the particles together via a phenomenon known as "suction". A classic example of this is a beach sandcastle: if the sand is fully wet or dry it collapses but, if moist, it stands. On the other hand, a hydrophobic material is one where water will form beads on its surfaces, rather than spreading out. Hydrophobic soils naturally form in arid regions when particles are coated with plant oils or if exposed to very high temperatures, for example during forest fires. Soils can also become hydrophobic if treated with contaminated water or chemicals in the laboratory. Water trapped between hydrophobic surfaces forms very different structures to those in normal soils; instead of the usual concave shape, the water forms convex 'balls' between the particles. This shape suggests that the water acts to force the particles apart: the opposite of suction. Some work has been done to examine this possibility but, as yet, Engineers do not have a method to predict how the soil will behave if the water is in this condition.This project will reset our understanding of how water interacts with hydrophobic soils. Firstly, we will use state-of-the-art microscopy techniques to observe water as it condenses and grows, to understand how it interacts with the individual soil particles and those around them. This knowledge will tell us what pressures exist in the water structures and whether the particles are being drawn together or forced apart. Using this knowledge, we will develop tests to cycle the water content to relate the soil's water content to pressures during drying and wetting; a critical phenomenon when predicting how water will pass through the material during, for example, heavy rainfall. 3D reconstructions, generated using X-ray tomography, will tell us whether these changes in pressure change the particle arrangements, which may change how water passes through the material. Lastly, we will develop methods to test how the soil's strength is affected by and how it varies with changes in those water pressures. Understanding how strength varies is key to permitting Engineers to design structures using these new materials.