Discrete element modelling of clay

这个项目的目的是用离散元方法来解释粘土力学行为的颗粒尺度来源。粘土和所有土壤一样，都是由固体颗粒和流体组成的颗粒状物质。然而，粘土表现出最复杂的行为，仍然是最不被理解的。近60年来，临界状态土壤力学框架一直被用来描述和预测土壤的一般行为，但这种行为的起源很少被研究。在这个指导框架内，所有土壤(即砂土和粘土)都表现出相同的一般行为模式。例如，当土壤被剪切时，它会膨胀或收缩，这取决于应力水平以及初始土壤的密度或松散程度。如果土壤在不断增加的各向同性应力下压缩，在达到足够高的应力后，体积就会永久减少。对于沙子来说，这是已知的颗粒粉碎所致。事实上，McDowell和De Bono(2013)最近已经证明，正常压缩线是土样在受到压缩时所遵循的体积-应力空间中的一条线，它仅仅是颗粒强度的函数(具体地说，平均颗粒压碎强度随着颗粒尺寸的减小而增加的速率)。因此，这为土壤行为的一个众所周知的基本特征提供了一个微观力学解释。另一方面，对于粘土来说，控制其整体行为的潜在机制仍然未知。这是因为粒子的尺寸很小，很难观察或测量它们之间的相互作用。粘土中的单个颗粒太小，肉眼看不见，而且如此之小，以至于这些颗粒之间的相互作用由分子力而不是机械力控制。与沙子相比，粘土颗粒的形状也更复杂，比如六角形的小片或圆柱形的管状颗粒。作用于粘土颗粒之间的颗粒间作用力高度依赖于环境条件(如pH、盐度等)。这些力可以是吸引的，也可以是排斥的，粘土颗粒的不同部分之间可能存在不同的力(例如‘边’和‘面’)。颗粒之间不同的力组合导致了真实粘土中颗粒的许多不同的几何排列。只是粒子在工程应用(即加载/卸载)中如何相互作用，以及它们的几何排列如何改变或控制宏观行为仍然是推测的，由于粒子尺寸较小，目前无法通过实验观察到粒子。因此，对于什么引起或导致观察到的现象，如承受增加的(例如，各向同性)应力时体积减小，或剪切过程中的体积变化，没有基本的理解。这些现象将通过使用离散元方法(DEM)来模拟和研究具有不同粒间力的粘土的行为来解释。离散元是一种计算大量离散粒子相互作用和运动的数值工具。默认情况下，大多数DEM模拟通常只涉及实体之间的机械接触，这很容易计算；但也可以实施任何数量的自定义、更复杂的相互作用定律。DEM模拟通常受到可用的计算硬件的限制，到目前为止，很少有人对粘土进行建模。这一雄心勃勃的项目将使用数字高程模型在进行各种应力路径测试时，对含有大量颗粒和真实颗粒相互作用的数值粘土样本进行“内部观察”，从而改变我们理解(和教授)粘土行为的方式。揭示粘土行为的根本原因将使工程师和研究人员能够开发更准确的模型，最终将导致地基和地下结构的更安全、更经济的设计。

The aim of this project is to use the Discrete Element Method to explain the particle-scale origins of the mechanical behaviour of clay. Clays, like all soils, are granular materials composed of solid particles and fluids. Yet clays exhibit the most complex behaviour and remain the least understood. The Critical State Soil Mechanics framework has been used to describe and predict the general behaviour of soils for around 60 years, but the origins of this behaviour have rarely been investigated. Within this guiding framework, all soils (i.e. both sands and clays) exhibit the same general patterns of behaviour. For example, when a soil is sheared, it will dilate or contract, depending on the stress level and how dense or loose the initial soil is. If a soil is compressed under increasing isotropic stress, after a high enough stress is reached, a permanent decrease in volume occurs. For sands, this is known to be due to particle crushing. In fact, the normal compression line, which is a line in volume-stress space, which a sample of soil follows when subject to compression, has been shown recently by McDowell and de Bono (2013) to be solely a function of the particle strengths (specifically, the rate at which the average particle crushing strength increases with decreasing particle size). This therefore provides a micro mechanical explanation for a well-known and fundamental feature of soil behaviour.For clays on the other hand, the underlying mechanisms which control the bulk behaviour remain unknown. This is due to difficulty in observing or measuring particle interactions due to their small size. The individual particles in clays are too small to be seen with the naked eye, and are so small that the interactions between these particles are controlled by molecular forces rather than mechanical forces. Clay particles also have more complicated shapes when compared with sand, such as hexagonal platelets or cylindrical tubes. The inter-particle forces acting between clay particles are highly dependent on the environmental conditions (e.g. pH, salinity, etc.). These forces can be attractive or repulsive, and different forces may exist between the different parts of clay particles (e.g. the 'edges' and 'faces'). The variety of different combinations of forces between particles leads to many different geometrical arrangements of particles in real clays. Just how the particles interact during engineering applications (i.e. loading/unloading) and how their geometrical arrangement changes or controls the macroscopic behaviour remain speculative, and the particles are presently impossible to observe experimentally due to the small size.There is therefore no fundamental understanding as to what causes or leads to observed phenomena such as a decrease in volume when subjected to increasing (e.g. isotropic) stress, or volume change during shearing. These phenomena will be explained by using the Discrete Element Method (DEM) to model and investigate the behaviour of clays with varying inter-particle forces. DEM is a numerical tool which computes the interactions and motion of a large number of discrete particles. By default, the majority of DEM simulations are typically only concerned with mechanical contacts between entities, which are easily calculated; yet it is possible to implement any number of custom, more complex interaction laws. DEM simulations have been typically limited by the computational hardware available, and to date clay has rarely been modelled. This ambitious project will use DEM to 'look inside' a numerical clay sample with a large number of particles and realistic particle interactions as it undergoes a variety of stress path tests, changing the way we understand (and teach) clay behaviour.Revealing the underlying origins of clay behaviour will allow engineers and researchers to develop more accurate models and ultimately will lead to safer, more economic designs of foundations and underground structures.