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Granular Materials for Transport Infrastructures: Mechanical performance of coarse–fine mixtures for unbound layers through DEM analysis
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0001-9091-8963
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Granular materials are widely used as unbound layers within the infrastructure system playing a significant role on performance and maintenance. However, fields like pavement and railway engineering still heavily rely on empirically-based models owing to the complex behaviour of these materials, which partly stems from their discrete nature. In this sense, the discrete element method (DEM) presents a numerical alternative to study the behaviour of discrete systems with explicit consideration of the processes at particulate level governing the macroscopic response.

 This thesis aims at providing micromechanical insight into the effect of different particle sizes on the load-bearing structure of granular materials and its influence on the resilient modulus and permanent deformation response, both of which are greatly influenced by the stress level. In order to accomplish this, binary mixtures of elastic spheres under axisymmetric stress are studied using DEM as the simplest expression for gap-graded materials, which in turn also can be seen as a simplification of more complex mixtures.

First, the effect of the fines content on the force transmission at contact level was studied. Results were used to define a soil fabric classification system where the roles of the coarse and fine fractions were defined and quantified in terms of force transmission.

A behavioural correspondence between numerical mixtures and granular materials was established, where the mixtures were able to reproduce some of the most significant features regarding the resilient modulus and permanent strain dependency on stress level for granular materials.

A good correlation between soil fabric and performance was also found. Generally, higher resilient modulus and lower deformation values were observed for interactive fabrics, whereas the opposite held for instable fabrics.

Mixtures of elastic spheres are far from granular materials, where numerous additional factors should be considered. Nevertheless, it is the author’s belief that this work provides insight into the soil fabric structure and its effect on the macroscopic response of granular materials.

Abstract [sv]

Grus i form av krossat bergmaterial används i stor utsträckning som obundna bär- och förstärkningslager inom tranportinfrastrukturen och spelar där en viktig roll för verkningsätt, drift och underhåll. Det finns emellertid begränsad kunskap om de fundamentala mekanismerna på partikelnivå (d.v.s. enskilda gruskorn), mekanismer som styr det makromekaniska verkningssättet. Områden såsom väg- och järnvägsbyggnad bygger fortfarande väsentligen på empiriskta baserade modeller p.g.a. dessa materials komplexa uppträdande under belastning. Denna komplexitet beror delvis på den diskreta naturen hos problemet vilket innebär att traditionell matematisk modellering som vore materialen homogena och kontinuerliga, blir inadekvat. Mot denna bakgrund utgör den s.k. diskreta elementmetoden (DEM) ett numeriskt alternativ för att studera verkningssätt hos diskreta system där man explicit beaktar mekanismerna på partikelnivå.

Denna avhandling, som baseras på tre vetenskapliga bidrag, syftar till att ge mikromekaniska insikter vad gäller effekten av olika partikelstorlekar på bärförmågan hos grusmateral och dess inverkan på styvhet och motstånd mot permanenta deformationer. Båda dessa parametrar påverkas kraftigt av spänningsnivån och kan studeras genom triaxialförsök. För att undersöka detta studerades med hjälp av DEM binära blandningar av elastiska kulor – den enklaste modellen av grusmaterial med språng i fördelningskurvan – som utsattes för axialsymmetrisk belastning. Denna modell kan i sin tur ses som en förenkling av mer komplexa blandningar.

Inledningsvis studerades effekten av finpartikelinnehållet på partikelkontakternas kraftöverföring. Resultaten användes för att klassificera olika typer av skelettstrukturer i grusmaterialet där den finare och den grövre fraktionens roller kvantifierades med utgångspunkt från kraftöverföringen i stället för från det makromekaniska verkningssättet.

Resultaten visade en korrelation vad gäller verkningssättet mellan numeriska blandningar och grusmaterial, där de numeriska blandningarna kunde reproducera några av grusmaterials viktigaste kännetecken vad gäller spänningsberoendet för styvheten vid avlastning och motståndet mot permanent deformation. Vidare visades att styvheten kunde bestämmas ur första belastningscykeln vilket underlättar att övervinna de begränsningar avseende beräkningstid som annars förknippas med DEM.

God överensstämmelse mellan grusmaterialets skelettstruktur och verkningssätt kunde också observeras. Generellt observerades högre styvhet och mindre permanenta deformationer för interaktiva skelettstrukturer medan det motsatta gällde för instabila strukturer.

Numeriska blandningar av elastiska kulor är långt från verkliga grusmaterial, för vilka ett stort antal ytterligare faktorer måste beaktas. Icke desto mindre är det författarens övertygelse att detta arbete ger insikter i grusmaterialets skelettstruktur och dess effekter på det makromekaniska verkningssättet hos grusmaterial.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 42 p.
Series
TRITA-JOB. LIC, ISSN 1650-951X ; 2032
Keyword [en]
discrete element method, force distribution, gap-graded mixtures, granular materials, particle-scale behaviour, permanent deformation, resilient modulus, soil fabric
Keyword [sv]
binära blandningar, diskreta elementmetoden, grusmaterial, kraftöverföring, verkningssätt på partikelnivå, permanent deformation, skelettstruktur, styvhet
National Category
Infrastructure Engineering Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-195598ISBN: 978-91-7729-199-2OAI: oai:DiVA.org:kth-195598DiVA: diva2:1046841
Presentation
2016-12-16, B3, Brinellvägen 23, Stockholm, 10:05 (English)
Opponent
Supervisors
Note

QC 20161116

Available from: 2016-11-16 Created: 2016-11-03 Last updated: 2016-11-16Bibliographically approved
List of papers
1. Force transmission and soil fabric of binary granular mixtures
Open this publication in new window or tab >>Force transmission and soil fabric of binary granular mixtures
2016 (English)In: Geotechnique, ISSN 0016-8505, E-ISSN 1751-7656, Vol. 66, no 7, 578-583 p.Article in journal (Refereed) Published
Abstract [en]

The effect of fines content on force transmission and fabric development of gap-graded mixtures under triaxial compression has been studied using the discrete-element method. Results were used to define load-bearing soil fabrics where the relative contributions of coarse and fine components are explicitly quantified in terms of force transmission. Comparison with previous findings suggests that lower particle size ratios result in higher interaction between components. A potential for instability was detected for underfilled fabrics in agreement with recent findings. It was also found that the threshold fines content provides an accurate macroscopic estimation of the transition between underfilled and overfilled fabrics.

Place, publisher, year, edition, pages
ICE Publishing, 2016
Keyword
discrete-element modelling, fabric/structure of soils, particle-scale behaviour
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-185171 (URN)10.1680/jgeot.14.P.199 (DOI)000377361700005 ()
Note

QC 20160721

Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2016-11-15Bibliographically approved
2. Resilient properties of binary granular mixtures: A numerical investigatio
Open this publication in new window or tab >>Resilient properties of binary granular mixtures: A numerical investigatio
2016 (English)In: Computers and geotechnics, ISSN 0266-352X, E-ISSN 1873-7633, Vol. 76, 222-233 p.Article in journal (Refereed) Published
Abstract [en]

The effect of stress level on the resilient modulus for binary mixtures of elastic spheres under triaxial loading is investigated using the discrete element method. The secant modulus during the first cycle of unloading is used as an estimate of the modulus after several load cycles due to computational time restrains. Later in the paper, its adequacy as an accurate and efficient estimator is shown. Numerical results are statistically compared with existing relations characterizing the stress dependency of the resilient modulus for real granular materials. It is concluded that the modulus prediction is significantly improved considering the effect of the deviator stress in addition to the confinement stress, obtaining a good correlation between the modulus and the confinement to deviator stress ratio for the numerical mixtures. The stress dependency of a recently proposed soil fabric classification system, based on force transmission considerations at particulate level, is also studied and its correlation with performance investigated. It is found that the relative load-bearing role of coarse and fine components is governed by the deviator to confinement stress ratio. However, the implemented fabric classification is fairly insensitive to changes in this ratio. Regarding resilient performance, interactive fabrics show the stiffest response whereas underfilled fabrics should be avoided due to a potential for instability.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Discrete element modelling, Fabric of soils, Granular mixtures, Resilient modulus
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-186145 (URN)10.1016/j.compgeo.2016.03.002 (DOI)2-s2.0-84963737153 (ScopusID)
Note

QC 20160511

Available from: 2016-05-03 Created: 2016-05-03 Last updated: 2016-11-15Bibliographically approved
3. A Numerical Study on the Permanent Deformation of Gap-Graded Granular Mixtures
Open this publication in new window or tab >>A Numerical Study on the Permanent Deformation of Gap-Graded Granular Mixtures
2016 (English)In: Proceedings of the Third International Conference on Railway Technology: Research, Development and Maintenance / [ed] J. Pombo, Stirlingshire, UK: Civil-Comp Press , 2016Conference paper (Refereed)
Abstract [en]

Permanent deformation accumulation of unbound granular layers under traffic plays a critical role in the performance and need for maintenance of pavements and railway structures. In this paper, the discrete element method is used to study the permanent strain behaviour of binary mixtures of elastic spheres, as an idealization of gap-graded mixtures, under triaxial monotonic loading. The effects of stress level and soil fabric structure, based on a recently proposed classification system founded on micromechanical considerations, are assessed by subjecting mixtures with varying fines contents to different stress levels. Additionally, mixtures are loaded to static failure to study the dependency of the permanent strains on the closeness of the applied stress to failure stress, in accordance with existing empirical models. Numerical results are also compared with the experimentally determined behaviour of granular materials. The findings indicate that numerical mixtures are able to reproduce some of the most significant features observed in laboratory tests on granular materials, further encouraging the use of numerical simulations to enhance the understanding of granular media behaviour. Additionally, a good correlation between fabric structure and performance is obtained, giving additional support to the use of the studied fabric classification system for performance characterization.

Place, publisher, year, edition, pages
Stirlingshire, UK: Civil-Comp Press, 2016
Series
, Civil-Comp Proceedings, ISSN 1759-3433
Keyword
granular mixtures, permanent deformation, discrete element method, fabric of soils, static strength
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-185172 (URN)10.4203/ccp.110.15 (DOI)2-s2.0-84964355508 (ScopusID)
Conference
Third International Conference on Railway Technology: Research, Development and Maintenance
Note

QC 20160419

Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2016-11-15Bibliographically approved

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