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DEM Modelling of Unbound Granular Materials for Transport Infrastructures: On soil fabric and rockfill embankments
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0001-9091-8963
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Unbound granular materials (UGM) are widely used as load-bearing layers and for embankment construction within transport infrastructures. These play a significant role on operation and maintenance of transportation systems. However, pavement and railway engineering still today rely heavily on empirical models based on macroscopic observations. This approach results in limited knowledge on the fundamentals at particle scale dictating the macroscopic response of the material. In this sense, the discrete element method (DEM) presents a numerical alternative to study the behaviour of discrete systems with explicit consideration of processes at particulate level. Additionally, it allows obtaining information at particulate level in a way that cannot be matched by traditional laboratory testing. All of this, in turn, can result in greater micromechanical insight.This thesis aims at contributing to the body of knowledge of the fundamentals of granular matter. UGM for transport infrastructures are studied by means of DEM in order to gain insight on their response under cyclic loading. Two main issues are considered: (1) soil fabric and its effect on the performance of coarse-fine mixtures and (2) modelling of high rockfill railway embankments. Among the main contributions of this research there is the establishing of a unified soil fabric classification system based exclusively on force transmission considerations that furthermore correlates with performance. In particular, fabrics characterized by a strong interaction between the coarse and fine fractions resulted in improved performance. A soil fabric type with a potential for instability was also identified. Regarding embankments, DEM modelling shows that traffic induced settlements accumulate on the top layers and therefore seem to be unaffected by embankment height above a certain value. A marked influence of degradation, even considering its nearly negligible magnitude, was observed, largely resulting in increased settlements.

Abstract [sv]

Grus i form av krossat bergmaterial används i stor utsträckning som obundna bär- och förstärkningslager inom transportinfrastrukturen och spelar där en viktig roll för drift och underhåll. Områden såsom väg- och järnvägsbyggnad bygger emellertid fortfarande väsentligen på empiriskt baserade modeller till stor del grundlagda på makroskopiska observationer. Denna metod resulterar i begränsad kunskap om de fundamentala mekanismerna på partikelnivå (d.v.s. enskilda gruskorn) som styr det makroskopiska verkningssättet. 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. Dessutom gör DEM det möjligt att få information på partikelnivå på ett sätt som inte kan matchas med traditionella laboratorieförsök. Allt detta kan i sin tur resultera i större mikromekanisk insikt.

Denna avhandling syftar till att bidra till kunskapen om grunderna för grusmaterialets verkningssätt. Obundna grusmaterial studeras med hjälp av DEM-modellering för att belysa verkningssätt under cyklisk belastning. Två huvudämnen beaktas: (1) skelettsstruktur och dess påverkan på verkningssättet för blandningar av fina och grova partiklar (2) DEMmodellering av höga järnvägsbankar. Bland de huvudsakliga forskningsbidragen är upprättande av ett enhetligt klassificeringssystem vad gäller skelettstruktur i grusmaterialet med enbart hänsynstagande till kraftöverföring som dessutom överensstämmer med grusmaterialets verkningssätt. I synnerhet observerades att skelettstrukturer som kännetecknas av en stark interaktion mellan grova och fina fraktioner resulterade i högre styvhet och mindre permanenta deformationer. Dessutom identifierades en typ av skelettstruktur med potential för instabilitet. Vad gäller järnvägsbankar visar DEM-modellering att trafikorsakade sättningar utvecklas främst på det översta lagret och därför inte påverkas av bankhöjden över ett visst värde. En väsentlig påverkan av nedbrytning, även med tanke på dess nästan försumbar storlek, observerades, vilket i hög grad resulterade i större sättningar.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 89
Series
TRITA-ABE-DLT ; 205
Keywords [en]
Discrete element method, granular materials, particle-scale behaviour, particle degradation, permanent deformation, resilient modulus, rockfill embankment, soil fabric
Keywords [sv]
Diskreta elementmetoden, grusmaterial, verkningssätt på partikelnivå, nedbrytning, permanent deformation, styvhet, stenfylld bank, skelettstruktur
National Category
Geotechnical Engineering
Research subject
Transport Science, Transport Infrastructure
Identifiers
URN: urn:nbn:se:kth:diva-273323ISBN: 978-91-7873-509-9 (print)OAI: oai:DiVA.org:kth-273323DiVA, id: diva2:1430090
Public defence
2020-08-18, Videolänk kommer / Video link is forthcoming, Du som saknar dator/datorvana kan kontakta jsilfwer@kth.se / Use the e-mail address if you need technical assistance, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Transport AdministrationSvenska Byggbranschens Utvecklingsfond (SBUF)
Note

QC 20200518

Available from: 2020-05-18 Created: 2020-05-13 Last updated: 2020-05-18Bibliographically 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, p. 578-583Article 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
Keywords
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 ()2-s2.0-84963728575 (Scopus ID)
Note

QC 20160721

Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2020-05-13Bibliographically 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, p. 222-233Article 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
Keywords
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)000376551100022 ()2-s2.0-84963737153 (Scopus ID)
Note

QC 20160511

Available from: 2016-05-03 Created: 2016-05-03 Last updated: 2020-05-13Bibliographically 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, Published 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
Keywords
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 (Scopus ID)
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: 2020-05-13Bibliographically approved
4. A discrete element material model including particle degradation suitable for rockfill embankments
Open this publication in new window or tab >>A discrete element material model including particle degradation suitable for rockfill embankments
2019 (English)In: Computers and geotechnics, ISSN 0266-352X, E-ISSN 1873-7633, Vol. 115, article id 103166Article in journal (Refereed) Published
Abstract [en]

A material model for future implementation in high rockfill embankments has been developed using discrete elements. Compared to previous modelling of railway ballast representing particles as clumps of spheres with bonded asperities, much simpler breakable clumps are used. This allows considering not only corner breakage but also particle splitting without a prohibitive computational time, something unique when modelling three-dimensional assemblies of particles. Moreover, breakage is controlled by values of contact forces and particle loading configuration, resulting in significantly fewer parameters and with a much clearer physical meaning. All in all, it results in a more computationally efficient and robust model suitable for implementation in rockfill embankments. Numerical monotonic and cyclic triaxial tests are performed under a range of low deviatoric to confinement stress ratios, as anticipated for railway embankments. A comparable degree of resemblance to empirical results as the previous modelling efforts with bonded asperities is observed when including degradation. Results at particle level proved useful to partially explain the observed macroscopic responses; however, these were substantially affected by breakage and none of the studied variables could, on its own, satisfactorily fully explain the observed behaviour. As a matter of fact, a complex interdependency of different factors, both at particle and macroscopic level, was identified that ultimately explained the macroscopic response. The key contribution is thus presenting an efficient and realistic material model specifically aimed at modelling high rockfill embankments including degradation, something not attempted to date.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Discrete element modeling, Granular materials, Rockfill embankment, Particle-scale behavior, Particle degradation
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering, Soil and Rock Mechanics
Identifiers
urn:nbn:se:kth:diva-263509 (URN)10.1016/j.compgeo.2019.103166 (DOI)000506718800008 ()2-s2.0-85071493258 (Scopus ID)
Note

QC 20191202

Available from: 2019-12-02 Created: 2019-12-02 Last updated: 2020-05-13Bibliographically approved
5. Discrete element modelling of rockfill railway embankments
Open this publication in new window or tab >>Discrete element modelling of rockfill railway embankments
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Uncertainty on the development of settlements due to traffic loading exists within railway embankments, playing a potentially critical role in maintenance for slab-track configurations. A previously developed model for large constructions of unbound stone-based materials has been implemented for modelling rockfill embankments. Particles are represented by simple breakable tetrahedral clumps of spheres with four asperities each. Both corner breakage and particle splitting are allowed and differentiated depending on contact geometry. Embankments with heights ranging between 2 to 10 m are generated by successive dumping and compaction of layers of clumps on top of each other, mimicking the construction of real embankments. Cyclic loading of the embankments representing railway traffic, for both breakable and unbreakable assemblies, was carried out. No clear influence of embankment height on settlements was observed, as these accumulate on the top layers independently of the number of layers below. A clear effect of degradation was observed, largely resulting in increased settlements. Regarding the resilient response, a linear stiffening effect of embankment height was observed with a minor influence of breakage. Results at particle level were proven useful in explaining the observed behaviour. The key contribution is showing that it is possible to model high rockfill embankments under a large number of loading cycles and furthermore including degradation, something not attempted to date.

Keywords
discrete elements; granular materials; railway embankment; rockfill; settlements; particle degradation
National Category
Geotechnical Engineering
Research subject
Transport Science, Transport Infrastructure; Civil and Architectural Engineering, Soil and Rock Mechanics
Identifiers
urn:nbn:se:kth:diva-273322 (URN)
Available from: 2020-05-13 Created: 2020-05-13 Last updated: 2020-05-13

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