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Dem Simulation of Triaxial Tests on Coarse Aggregates Applicable for Base and Sub-Base of Roads and Railways

Hajian, Sadegh | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53709 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Pak, Ali
  7. Abstract:
  8. In this study, numerical modeling of the micromechanical behavior of gravel aggregates applicable for base and sub-base of roads and railways under triaxial compression tests is conducted. The material is a type of well-graded gravelly soil with a maximum particle size of 20 mm, whose application is recommended by the UK Highways Agency. The prominent role of this material in the performance and endurance of the roads shows the importance of studying its behavior to acquire a better understanding of its mechanical properties. A number of triaxial tests using monotonic and cyclic loadings have been performed in the laboratory to investigate the behavior of this type of material. In the present study, in particular, monotonic triaxial tests on cemented and uncemented specimens are of concern. Due to the discrete nature of the soil, the discrete element method (DEM) is used for numerical simulations, which can model the particulate nature of discontinuous media such as soil or fractured rock. By using DEM, the micromechanical behavior of well-graded gravel is investigated. This study shows formation and orientation of strong force chains in the samples in the direction of the maximum principal stress. It is demonstrated that the shear strength of the triaxial specimens depends on the creation and performance of these strong force chains inside the sample. The obtained failure mechanisms from numerical modelings were illustrated and compared with those recorded in the course of laboratory observations. By investigating the sample stress tensor, it was found that the stresses on the upper and lower boundaries of the triaxial sample (loading plates) were more than the internal stress of the sample and the fabric tensor mathematically showed that the triaxial test is axisymmetric. Then, based on the obtained micromechanical responses, the macro-mechanical behavior of the triaxial specimens was predicted and evaluated. Since the behavior of the triaxial specimens depends on their boundary conditions, in the modeling of the triaxial tests, two types of boundaries (rigid and flexible) were used to examine the influence of the latex membrane on the sample behavior. The results obtained from the simulations using these two boundaries were compared with each other and it was shown that using the flexible boundary does not greatly affect the shear strength and does not alter the sample fabric either, however, it reduces the sample dilation. Finally, the effects of the membrane Young's modulus and soil-membrane friction coefficient on the behavior of the samples were investigated. The present study, in addition to providing a better insight regarding the micromechanical mechanisms inside a triaxial specimen that affect the sample behavior, can be a good guide on how to simulate the triaxial tests using the discrete element method
  9. Keywords:
  10. Discrete Element Method ; Gravel ; Triaxial Test ; Numerical Modeling ; Railway ; Coarse Aggregates

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