Micro-scale modelling of granular filters
Micro-scale modelling of granular filters
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颗粒过滤器的微尺度建模
DOI：
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发表时间：
2014
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通讯作者：
T. Shire
中科院分区：
文献类型：
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作者：
T. Shire
颗粒过滤器被认为是堤坝最安全的关键元件之一。人们对此类滤波器的行为知之甚少，这反映在用于滤波器设计的经验导出规则中，这些规则已被证明是保守的，并且在某些情况下相互矛盾。在本论文中，粒子尺度数值分析用于提高对内部稳定性（粒状滤波器的要求）的理解，并评估一些常用经验设计规则的基本依据。内部稳定性描述了宽级配或间隙级配无粘性土的粗粒部分防止渗流下细粒部分侵蚀的能力。内部不稳定的两个条件是：（i）细颗粒比粗颗粒具有相对较低的应力（流体力学条件）和（ii）细颗粒应足够小以穿过粗颗粒之间的空隙收缩（几何条件）。依次评估每个条件。根据经验标准，使用离散元建模 (DEM) 来分析不同内部稳定性的土壤中的结构和有效应力分布，从而评估水力力学条件。特别是探索了 Skempton 和 Brogan (1994) 的假设，即内部不稳定的先决条件是减少细颗粒中的有效应力。结果表明，细粉传递的应力与土壤结构有关，特别是颗粒之间接触的数量和强度。这反过来又受到材料的粒度分布 (PSD)、细粉含量和相对密度的影响。引入了描述这种行为的概念框架。几何条件与空隙空间内收缩的大小密切相关。使用两种方法对具有不同 PSD 和相对密度的 DEM 样本内的缢缩尺寸分布 (CSD) 进行量化：加权 Delaunay 方法（Reboul 等人，2010）和最大球方法（Dong 和 Blunt，2009）。 CSD 曲线显示出具有相似的形状，可以使用特征过滤器颗粒直径对其进行有效归一化。结果显示与 Kenney 等人的实验工作非常一致。 (1985)，并为过滤器设计中特征粒径的使用提供科学支持。对具有应力引起的各向异性的 DEM 样本中的收缩进行的分析表明，较大和较小的收缩分别与主应力和次主应力一致。提出了描述空隙空间的随机游走网络模型来模拟多分散精细材料通过颗粒过滤器的运动。该模型对于识别有效和无效的基础/过滤器组合非常有用。
Granular filters are considered to be among the most safety critical elements of embankment dams. The behaviour of such filters is poorly understood, which is reflected in the empirically derived rules used for filter design, which have been shown to be conservative and to contradict each other in some cases. In this thesis particle-scale numerical analysis is used to improve the understanding of internal stability, a requirement for granular filters, and to assess the fundamental basis for some commonly used empirical design rules. Internal stability describes the ability of the coarse fraction of a broadly or gap-graded cohesionless soil to prevent the erosion of the finer fraction under seepage. Two conditions for internal instability are that: (i) the fine particles carry relatively lower stress than the coarse particles (hydromechanical condition) and (ii) the fine particles should be small enough to pass through the void constrictions between the coarse particles (geometric condition). Each of these conditions is assessed in turn. The hydromechanical condition is assessed by using discrete element modelling (DEM) to analyse the fabric and effective stress distribution within soils of varying internal stability according to empirical criteria. In particular the hypothesis of Skempton and Brogan (1994) that a prerequisite for internal instability is a reduction of the effective stress in the finer fraction is explored. The results show that the stress transferred by the fines is related to the soil fabric, in particular the number and strength of contacts between particles. This is in turn shown to be influenced by the particle size distribution (PSD), fines content and relative density of the material. A conceptual framework to describe this behaviour is introduced. The geometric condition is intimately linked to the size of the constrictions within the void space. The constriction size distribution (CSD) within DEM samples with differing PSDs and relative densities is quantified using two approaches: the Weighted Delaunay method (Reboul et al., 2010) and the Maximal Ball method (Dong and Blunt, 2009). CSD curves are shown to have similar shapes which can be usefully normalised using characteristic filter particle diameters. The results show very good qualitative agreement with the experimental work of Kenney et al. (1985), and lend scientific support to the use of characteristic particle diameters in filter design. An analysis of constrictions within DEM samples with stress-induced anisotropy shows that larger and smaller constrictions align with the major and minor principal stresses respectively. A random walk network model describing void space is proposed to simulate the movement of a polydisperse fine material through a granular filter. This model is useful for identifying effective and ineffective base/filter combinations.