Discrete Element Modelling of Transitional Soils

工程土的数学本构模型是基于洁净砂土或洁净粘土的室内试验数据建立的，目前主要用于设计分析和预测土的响应。这些土壤遵循一致的变形模式，称为临界状态框架。显然，为了优化岩土结构的设计，有必要了解自然土壤如何以及为什么可能与理想化的清洁坚硬的土壤表现不同，并表征这些差异。最近对巴西残留土壤、波河淤泥或冰岛冰川耕作的研究突出了其他类型土壤的存在，这些土壤不遵循关键国家框架，但以更戏剧性的方式存在。这些土既不是砂，也不是粘土，所以被称为过渡性土。可以归类为过渡性土壤的土壤的范围正在增加，但目前它们的行为模式不能以一致的方式统一。小颗粒(细粒)的高比例、粗粒土壤的粒度分布或可碎性是否对过渡行为的形成有重要影响尚不清楚，建议使用计算离散单元法(DEM)进行研究。DEM可以通过可视化在单个颗粒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，因为一方面DEM可以模拟与实验室测试数据相当的宏观变形数据，另一方面它可以在离散颗粒接触的水平上捕捉信息，并提供微观信息来辅助寻找用于本构土壤模型的物理合理参数。使用Itasca PFC3D程序，土壤颗粒可以由一组粘合的球来表示。通过在可破碎颗粒中加入不同比例的超细土颗粒，可以数值研究颗粒的粒度分布和颗粒破碎的影响。粒子接触处发生的微观信息(例如破裂)将被计算出来，并与实验数据进行比较。然后，就有可能确定这种非常规行为发生的时间和原因，并帮助为过渡土壤的新范例奠定基础。

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.