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Numerical Modeling of Non-Cemented and Cemented Ballast Aggregates under Triaxial Test with a Flexible Membrane Using Discrete Element Method (DEM)

Eshraghi, Hamid Reza | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56660 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Pak, Ali
  7. Abstract:
  8. This research deals with numerical simulation of triaxial test to determine behavior of non-cemented and cemented ballast aggregates used in base and subbase of roads and railroads. Ballast aggregates, thanks to their high strength and durability, are widely used in various fields including dam, river, road construction, and railway. One of the most widespread uses of this material is in the road body, and according to the recommendation of the British Highway Center, the materials used in the road body are well-graded gravel with the maximum size of 20 mm. Cement is also used as a corrective material to enhance the performance of road bodies. Therefore, knowing the characteristics of these materials is indispensable for designers and researchers, so far many tests have been performed on these materials to understand their behavior under static and dynamic loads (traffic), one of the most important of these tests is the dynamic or periodic triaxial test. which determines the macroscopic soil parameters such as stiffness, shear strength, compression, and expansion rate and, if more advanced equipment is used, lateral deformation and energy consumption in the sample will be determined as well. However, the triaxial test also has its own limitations, one of the most important of which is the lack of ability to determine soil properties at the microscopic and particle scale, and for this reason, in this research, the numerical method of discrete elements (DEM) which has not only the ability to model the discrete nature of the soil at the particle scale but also the ability to distinguish jointed stones, is used. These investigations were carried out to numerically study the behavior of aggregate materials under static loads and various all-round stresses, and by considering the flexible boundary, which simulates the flexible membrane of the triaxial test well. The obtained numerical results have been compared and validated with laboratory results. Then, by using different contact models and microparametric relationships, the microscopic behavior of this type of material has been interpreted based on phenomena such as the formation of contact chains, neighborhood number and rupture mechanism. In the following, the effect of cementation in these materials and the extraction of resistance and behavior results of 1% and 2% cement samples were investigated and finally, the microscopic results of cementless and cemented samples were compared with each other. The obtained results show that in addition to the use of suitable flexible membrane, the resistance and behavior of non-cement and cement samples are in good agreement with the laboratory results. Also, in the microscopic scale, the neighborhood number of non-cement samples is always decreasing. While with the addition of cement, the neighborhood number increases slightly and then decreases, and increasing the percentage of cement also leads to an increase in the neighborhood number. On the other hand, the dominant contact chains are formed almost vertically in the cutting phase of non-cement samples. While with the addition of cement, contact chains appear in horizontal directions, which justify the increase in shear strength of cement samples. As the amount of cement increases, the contact chains in the horizontal directions are more strongly defined. Also, the mechanism of rupture of non-cement and cement samples in medium all-round stresses is X-shaped, and the angle of rupture does not depend on the amount of cement
  9. Keywords:
  10. Finite Element Method ; Triaxial Test ; Aggregates ; Coarse Aggregates ; Cemented Sand ; Static Loads

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