| Reference : Discrete element analysis of granular materials |
| Dissertations and theses : Doctoral thesis | |||
| Engineering, computing & technology : Civil engineering | |||
| http://hdl.handle.net/10993/18412 | |||
| Discrete element analysis of granular materials | |
| English | |
Van Baars, Stefan  [University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Engineering Research Unit >] | |
| 1996 | |
| Delft University of Technology, Delft, Netherlands | |
| Doctor in Civil Engineering | |
| [en] geotechnics ; discrete elements ; shear band | |
| [en] Discrete Element Analysis of Granular Materials
During oil and gas production, several years after drilling a borehole, sand particles and small sandstone particles start to break away from the borehole surface. These particles can damage the transport pipes and other equipment in a short period of time. By simulating this borehole behaviour with the thick-walled cylinder test, four phenomena were found which cannot be explained by conventional continuum mechanics: 1. Despite the compressive stress, failure occurs on the micro level due to tension cracks. 2. These cracks are not diagonal to, but parallel to, the borehole surface. 3. These cracks cause two diametrically opposite breakouts. 4. The functional failure of the borehole starts at a higher radial pressure than predicted. In 1979, Cundall developed a computer model, based on the basic elements of granular materials, i.e. the grains themselves and their interactions, to describe the behaviour of these materials. Lindhout tried, in 1992, to use this model to describe the cylinder test. Due to compaction problems, stability problems and the large computational time, this could not be achieved. Therefore a new model was developed by the author, which does not use the equations of motion, but the equations of equilibrium, to calculate the new grain positions. This model can be used both for non-cohesive grains (sand) and for cohesive grains (sandstone). The results can generally be described by an advanced Mohr-Coulomb model. However, there are a few exceptions. Firstly, during loading of a granular structure, many contacts between the grains will collapse, not due to shear deformation as Coulomb suggests, but due to tension failure. Secondly, these micro cracks always occur in the direction of the major principal stress, which might be another direction than the observed failure surface. In this way, the axial micro cracks form a diagonal failure surface during a biaxial test, but the axial micro cracks in a cylinder test may form a failure surface parallel to the borehole surface. During the formation of natural sandstone, the difference between the horizontal and vertical stress causes anisotropy in the strength behaviour of this material. This or other anisotropies may explain the diametrically opposite breakouts. The conclusion that a borehole fails at a higher radial pressure than predicted, originates from the definition difference between local failure and functional failure and the large rest capacity of a thick-walled cylinder. | |
| http://hdl.handle.net/10993/18412 | |
| Discrete element analysis of granular materials
Stefan van Baars Proefschrift Technische Universiteit Delft met samenvatting in het Nederlands ISBN 90-9009494-6 NUGI 816 Trefwoorden: geotechnics, discrete elements, shear band, crack growth |
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