Discrete Element Modelling of Transitional Soils

工程土的数学本构模型主要用于设计分析和预测土的响应，制定基于干净的砂或干净的粘土的实验室数据。这些土壤遵循一致的变形模式，称为临界状态框架。很明显，为了优化土工结构的设计，有必要了解天然土壤如何以及为什么会与理想化的干净的牢不可破的土壤表现不同，并消除这些差异。最近对巴西残积土、波河淤泥或冰岛冰碛物的研究强调了其他类型土壤的存在，这些土壤不遵循临界状态框架，但以更引人注目的方式存在。由于既不是砂也不是粘土，这些土被称为过渡土。可归类为过渡性土壤的土壤范围正在增加，但目前它们的行为模式不能以一致的方式统一起来。高比例的小颗粒（细粒）、颗粒大小分布或粗粒土的可压碎性是否对形成过渡特性有重要影响尚不清楚，并建议使用计算离散元法（DEM）对此进行研究.DEM可用于通过可视化在单个粒子相互作用水平上发生的事情来理解控制这些不同模式的隐藏物理因素。它是一个强大的工具，一方面，DEM可以模拟宏观变形数据，是可比的实验室测试数据，另一方面，它可以捕获的离散颗粒接触的水平上的信息，并提供微观信息，以协助搜索过程中的物理合理的参数，用于本构土模型。使用伊塔斯卡PFC3D程序，土壤颗粒可以由一组粘结球来表示。通过将不同比例的极细土颗粒加入到可破碎颗粒的集合体中，可以数值地研究颗粒尺寸分布和颗粒破碎的影响。将计算颗粒接触时发生的微观信息（例如破碎），并与实验数据进行比较。这样就有可能确定何时以及为什么会发生非常规行为，并有助于为过渡性土壤的新范式奠定基础。

The mathematical constitutive models for engineering soils that are principally used today in design analyses and in predicting the responses of soils are formulated based on laboratory data of either clean sand or clean clay. These soils follow consistent patterns of deformation called the Critical State framework. It is clear that to optimise the design of geotechnical structures it is necessary to understand how and why natural soils might behave differently to idealised clean unbreakable soils, and to characterise these differences. Recent work on residual soils from Brasil, silts from the Po River or a glacial till from Iceland has highlighted the existence of other types of soils that do not follow the Critical State framework, but in a more dramatic way. Being neither sands nor clays these soils have been called transitional soils. The range of soils that can be classified as transitional soils is increasing, but at present their patterns of behaviour cannot be unified in a consistent manner. Whether or not a high proportion of small-sized particles (fines), the particle size distribution or the crushability of coarse-grain soils are important effects to the formation of transitional behaviour is still unclear, and it is proposed to investigate this using the computational Discrete Element Method (DEM).DEM can be used to understand the hidden physical factors governing these different patterns by visualising what happens at the level of individual particle interactions. It is a powerful tool as on one hand the DEM can simulate macroscopic deformation data that are comparable to laboratory testing data, on the other hand it can capture information at the level of discrete particle contacts and provide microscopic information to assist the search process of physically plausible parameters to be used in constitutive soil models. With the Itasca PFC3D program, soil particles can be each represented by a group of bonded balls. By adding different proportions of very fine soil particles into an assembly of crushable particles, the effects of particle size distribution and particle breakage can be investigated numerically. Microscopic information that happens at particle contacts (for example breakage) will be calculated and compared with experimental data. It will then become possible to identify when and why the unconventional behaviour happens, and help form bases for a new paradigm for transitional soils.