Doctor of Philosophy
School of Civil, Mining, and Environmental Engineering
Water ingress in soils through infiltration can trigger instability leading to failures in slopes and embankments under drained conditions. Subsequent investigations on such failures have shown that the infiltration of water in soils can cause a reduction in effective stress thus triggering the instability. In addition, studies have indicated that the constant shear drained (CSD) triaxial test can depict an unstable phenomenon where effective stress is continuously reduced. In the current study, the first section of research focuses on the Discrete Element Method (DEM) modelling of the instability behaviour of granular materials during CSD conditions.
The CSD condition was modelled by decreasing the mean effective stress on an assembly of particles under strain-controlled loading. The instability condition was predicted at the particle scale level using particle second-order work increment. The DEM contact parameters have been calibrated to capture the macroscopic responses and the instability behaviour consistently with the laboratory experimental observations. The effect of different range of initial stress states at the beginning of CSD condition such as different initial mean effective stress, void ratio and deviatoric stress on the instability behaviour were analysed. In addition, the micromechanical parameters such as coordination number, anisotropic coefficients (geometric, mechanical) were extracted to assist in characterising the instability behaviour during CSD conditions. The stress state of the soil (i.e., at the onset of CSD) condition has shown a significant influence on the evolution of anisotropic coefficients, an evident behaviour change was noted once the CSD condition was imposed. Regardless of initial stress states and densities before the onset of the CSD condition, all samples have experienced instability during the shearing stage. However, the initial density of soil and stress state influence the time occurrence of instability.
The second section of the research focuses on the DEM modelling of cone penetration test (CPT) on dry granular material. The influence of state variables and different modelling parameters on CPT measurements were investigated. The analysis details the effect of isotropic confining stress, relative density, stress ratio of the soil, mean particle size, boundary conditions and particle shape during CPT testing. The representation of all the DEM - CPT results on the soil behaviour type (SBT) chart resulted as 'Clean Sands.' This response coincides with the particle size distribution considered in this study. The stress distribution within the vicinity of the cone is dominant at the cone tip position by a large magnitude. Contact force network plots supported the overall evolution of the stress profile in the chamber. The length and propagation of strong force chains within the chamber are dependent on the initial conditions. From the overall stress response observed in all the test conditions, a 'critical zone' was identified within the specimen, where the width and height of the critical zone varied between three to six times the radius of the cone depending on the initial conditions of the sample.
Using DEM, the mechanical behaviour of dry soil during CPT can be simulated effectively. However, the effect of pore water pressure in all the cases cannot be captured using just DEM numerical method. Hence, the Lattice Boltzmann Method (LBM) was introduced to model the fluid behaviour during CPT testing. A two-dimensional LBM – DEM coupled model was employed to simulate the penetration rate effect on saturated granular materials. Initially, the coupled numerical model was calibrated using a one-dimensional consolidation test. The results obtained from the 1D consolidation test simulation showed good agreement with the analytical equation proposed by Terzaghi. The model has captured a steady-state cone resistance and excess pore fluid pressure response for all penetration rate conditions. The penetration rate influence on the cone resistance was insignificant. However, the excess pore fluid pressure was found to increase with an increase in the penetration rate adopted, which was qualitatively consistent with the laboratory calibration chamber studies.
The displacement of particles during cone penetration developed low-pressure zones resulting in the pressure gradient in the fluid system. With the increase in the penetration rate, the particle displacements increased, thereby this led to large-fluid pressure gradient’s development across the chamber. As a result, maximum pore fluid pressure was generated in the high penetration rate and minimum in the low penetration rate conditions. The excess pore fluid pressure distribution plots have shown that maximum pore fluid pressure was found below the cone region and over the cone shoulder position. In addition, fluid force evolution during the test has shown that the magnitude of the fluid force generated in the chamber is directly proportional to the penetration rate. Furthermore, a consistent evolution pattern of fabric anisotropy has been observed throughout the depth in all the penetration rate conditions. The fabric components and have dominated around the cone region and at the boundary region, respectively. This indicates that the preferential orientation of contacts was vertical direction at the cone region and horizontal direction at the boundary region.
Allulakshmi, Krishna, A Coupled LBM – DEM Modelling of Cone Penetration Test in Granular Materials, Doctor of Philosophy thesis, School of Civil, Mining, and Environmental Engineering, University of Wollongong, 2022. https://ro.uow.edu.au/theses1/1497
FoR codes (2008)
090501 Civil Geotechnical Engineering
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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.