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Title: Micro-scale modelling of granular filters
Author: Shire, Thomas
ISNI:       0000 0004 5353 3185
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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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.
Supervisor: O'Sullivan, Catherine Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available