In this study oedometric compression tests of hydrocarbon coke, Fontainebleau sand and silica sand are simulated in three dimensions using breakable particles. The method adapts a rigorous breakage criterion for elasto-brittle spheres to represent failure of grains isolated between platens or within granular masses. The breakage criterion allows for the effect of particle bulk and contact properties to be treated separately. A discrete fragmentation multigenerational approach is applied as a spawning procedure. The number of particles quickly increases during the simulation, but is kept manageable by systematic fine exclusion and upscaling. Fine exclusion leads to mass losses between generations, but that loss is accounted for outside the mechanical model. Sensitivity analysis shows that it is enough to keep 53% of the crushed particle mass within the mechanical model to correctly reproduce experimental macroscopic behaviour. Practical upscaling rules are proposed for (a) contact stiffness, (b) breakage criteria and (c) grain size distribution, and validated simulating the same test, reducing by half the initial number of particles. The results are promising as both the mechanical and grading evolution are well captured with two orders of magnitude savings in computing efficiency.
A three-dimensional discrete element model is used to investigate the effect of grain crushing on the tip resistance measured by cone penetration tests (CPT) in calibration chambers. To do that a discrete analogue of pumice sand, a very crushable microporous granular material, is created. The particles of the discrete model are endowed with size-dependent internal porosity and crushing resistance. A simplified Hertz-Mindlin elasto-frictional model is used for contact interaction. The model has 6 material parameters that are calibrated using one oedometer test and analogies with similar geomaterials. The calibration is validated reproducing other element tests. To fill a calibration chamber capable of containing a realistic sized CPT the discrete analogue is up-scaled by a factor of 25. CPT is then performed at two different densities and three different confinement pressures. Cone tip resistance in the crushable material is practically insensitive to initial density, as had been observed in previous physical experiments. The same CPT series is repeated but now particle crushing is disabled. The ratios of cone tip resistance between the two types of simulation are in good agreement with previous experimental comparisons of hard and crushable soils. Microscale exploration of the models indicates that crushing disrupts the buttressing effect of chamber walls on the cone. A three-dimensional discrete element model is used to investigate the effect of grain 6 crushing on the tip resistance measured by cone penetration tests (CPT) in calibration 7 chambers. To do that a discrete analogue of pumice sand, a very crushable microporous 8 granular material, is created. The particles of the discrete model are endowed with size-9 dependent internal porosity and crushing resistance. A simplified Hertz-Mindlin elasto-10 frictional model is used for contact interaction. The model has 6 material parameters that are 11 calibrated using one oedometer test and analogies with similar geomaterials. The calibration 12 is validated reproducing other element tests. To fill a calibration chamber capable of 13 containing a realistic sized CPT the discrete analogue is up-scaled by a factor of 25. CPT is 14 then performed at two different densities and three different confinement pressures. Cone tip 15 resistance in the crushable material is practically insensitive to initial density, as had been 16 observed in previous physical experiments. The same CPT series is repeated but now particle 17 crushing is disabled. The ratios of cone tip resistance between the two types of simulation are 18 in good agreement with previous experimental comparisons of hard and crushable soils. 19Microscale exploration of the models indicates that crushing disrupts the buttressing effect of 20 chamber walls on the cone. 21 KEY WORDS: 22Discrete element method, pumice sand, cone penetration, particle crushing, double porosity 23 *Manuscript Click here to view linked References 2 2
Granular materials reach critical states upon shearing. The position and shape of a critical state line (CSL) in the compression plane are important for constitutive models, interpretation of in situ tests and liquefaction analyses. It is not fully clear how grain crushing may affect the identification and uniqueness of the CSL in granular soils. Discrete-element simulations are used here to establish the relation between breakage-induced grading evolution and the CSL position in the compression plane. An efficient model of particle breakage is applied to perform a large number of tests, in which grading evolution is continuously tracked using a grading index. Using both previous and new experimental results, the discrete-element model is calibrated and validated to represent Fontainebleau sand, a quartz sand. The results obtained show that, when breakage is present, the inclusion of a grading index in the description of critical states is advantageous. This can be simply done using the critical state plane (CSP) concept. A CSP is obtained for Fontainebleau sand.
Existing guidance on the installation of screw piles suggest that they should be installed in a pitch-matched manner to avoid disturbance to the soil which may have a detrimental effect on the in-service performance of the pile. Recent insights from centrifuge modelling have shown that installing screw piles in this way requires large vertical compressive (or crowd) forces, which is inconsistent with the common assumption that screw piles pull themselves into the ground requiring minimal vertical compressive force. In this paper, through the use of the Discrete Element Method (DEM), the effects of advancement ratio, i.e. the ratio between the vertical displacement per rotation to the geometric pitch of the helix of the screw pile helix, on the installation resistance and in-service capacity of a screw pile is investigated. The findings are further used to assess the applicability of empirical torque capacity correlation factors for large diameter screw piles. The results of the investigation show that it is possible to reduce the required vertical compressive installation force by 96% by reducing the advancement ratio and that although over-flighting a screw pile can decrease the subsequent compressive capacity, it appears to increase the tensile capacity significantly.
Summary The mechanical behavior of granular materials is characterized by strong nonlinearity and irreversibility. These properties have been differently described by a variety of constitutive models. To test any constitutive model, experimental data relative to the nature of the incremental stress–strain response of the material is desirable. However, this type of laboratory data is scarce because of being expensive and difficult to obtain. The discrete element method has been used several times as an alternative to obtain incremental responses of granular materials. Crushable grains add one extra source of irreversibility to granular materials. Crushability has been variously incorporated into different constitutive models. Again, it will be helpful to obtain incremental responses of crushable granular materials to test these models, but the experimental difficulties are increased. Making use of a recently introduced crushing model for discrete element simulation, this paper presents a new procedure to obtain incremental responses in discrete analogs of granular crushable materials. The parallel probe approach, previously used for uncrushable discrete analogs, is here extended to account for the presence of crushable grains. The contribution of grain crushing to the incremental irreversible strain is identified and separately measured. Robustness of the proposed method is examined in detail, paying particular attention to aspects such as dynamic instability or crushing localization. The proposed procedure is later applied to map incremental responses of a discrete analog of Fontainebleau sand on the triaxial plane. The effect of stress ratio and granular state on plastic flow characteristics is highlighted. Copyright © 2016 John Wiley & Sons, Ltd.
This volume forms the proceedings of the 1st International Symposium on Screw Piles for Energy Applications (ISSPEA), held at the University of Dundee, 27-28 th May, 2019. This conference is the first such event organised at the University of Dundee and was originally designed to be a small event to disseminate the findings of the EPSRC sponsored Supergen WindHub Grand Challenges project: Screw piles for wind energy foundation systems. The impetus to expand the guest list and scope of this event came after discussion with Dr Alan Lutenegger of the University of Massachusetts Amherst who organised the successful 1st International Geotechnical Symposium on Helical Foundations. Unfortunately, the eagerly anticipated 2 nd symposium in this series did not occur as planned so it was decided to partially plug this gap in screw pile innovation reporting by expanding the scope and invitees of ISSPEA. This conference has been organised by the Geotechnical Engineering Research Group at the University of Dundee representing the Screw piles for wind energy foundation systems project partners with academic teams at Durham University and the University of Southampton. The first ISSPEA provides an excellent opportunity for academics, engineers, scientists, practitioners and students to present and exchange the latest developments, experience and findings in screw pile engineering for renewable energy applications. The proceedings contains 12 papers and 9 extended abstracts with the latter representing the presentations made at the event that were not supported by a full paper. The proceedings contain one invited keynote paper from Alan Lutenegger on the current state-of-understanding of the engineering behavior of screw piles and helical anchors. This paper presents an overview of historical applications of screw piles, with discussions on aspects of their design and behaviour which are both understood and in need of further research, using case studies as examples. Other papers in the proceedings look at a variety of topics including: installation requirements and effects; cyclic behaviour; advanced numerical modelling of screw piles, including the use of DEM and MPM to incorporate installation effects into the models; and screw piles used in industrial applications. It is hoped that this proceedings and symposium will lead to similar future meetings and serve as a useful indicator of the current state of innovation and deployment. It is also hoped the event and proceedings will act as the springboard for new lines of research and development and increased use of screw piles for a variety of applications. We are grateful to all the authors and reviewers for their efforts in the preparation of the papers. Finally, the Organisers would like to acknowledge the support and efforts of the Local Organising Committee, paper reviewers and the support of our industrial partners.
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