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

气候变化意味着许多岩土结构在未来将面临比其设计者先前考虑的更恶劣的条件。具体来说，干旱、热浪、洪水或大雨都会损害用于保护采矿或城市废物储存地点的土壤覆盖物，因为材料要么开裂，要么被水浸透。这种失败的环境和经济后果是灾难性的，而且将是灾难性的。拒水（疏水）砂可能能够抵抗这种变化，因此是一种新的和及时的替代目前的覆盖层材料。然而，这些材料是新材料，我们还不知道水是如何通过它们或将粒子粘在一起的。了解这一点将大大有助于它们所支撑的结构的工程设计。我们知道土壤的行为是由其孔隙中所含的水量决定的。在正常的土壤中，水被困在土壤颗粒之间形成凹形的“桥梁”，通过一种被称为“吸力”的现象将颗粒粘在一起。沙滩沙堡就是一个典型的例子：如果沙子完全是湿的或干的，它就会倒塌，但如果沙子是湿的，它就会站立。另一方面，疏水材料是一种水会在其表面形成珠子而不是散开的材料。在干旱地区，当颗粒被植物油覆盖或暴露在非常高的温度下（例如在森林火灾期间），自然形成疏水性土壤。如果在实验室用受污染的水或化学物质处理，土壤也会变得疏水。被困在疏水表面之间的水与正常土壤中的水形成非常不同的结构；水不是通常的凹形状，而是在颗粒之间形成凸“球”。这种形状表明，水的作用是迫使颗粒分开：与吸力相反。已经做了一些工作来检验这种可能性，但是，到目前为止，工程师们还没有一种方法来预测如果水在这种情况下土壤会如何表现。这个项目将重置我们对水如何与疏水土壤相互作用的理解。首先，我们将使用最先进的显微镜技术来观察水的凝结和生长，以了解它如何与单个土壤颗粒及其周围的土壤颗粒相互作用。这些知识将告诉我们水结构中存在多大的压力，以及粒子是被拉到一起还是被迫分开。利用这些知识，我们将开发测试来循环含水量，以将土壤含水量与干燥和润湿期间的压力联系起来；这是一种关键现象，用于预测水在暴雨期间如何通过材料。使用x射线断层扫描生成的三维重建将告诉我们，压力的变化是否会改变粒子的排列，这可能会改变水通过材料的方式。最后，我们将开发方法来测试土壤的强度如何受到这些水压的影响以及它如何随这些水压的变化而变化。了解强度如何变化是允许工程师使用这些新材料设计结构的关键。

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.