- Title
- Numerical simulation of dynamic compaction within the framework of unsaturated porous media
- Creator
- Ghorbani, Javad
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Dynamic Compaction (DC) is conducted by dropping heavy weights on the ground surface. It has the advantage of providing rapid improvement of the geotechnical properties of soil at relatively large depths; hence, it can reduce the overall time of the soil improvement process dramatically. Furthermore, this method does not usually need any off-site disposal of excess materials. Despite the wide use of DC as an economically attractive ground improvement technique and the abundance of experimental data for dynamic compaction, few detailed predictive methods for DC are available. There is a demand for a reliable theory to predict and explain the complicated nature of DC. As previous numerical simulations were mostly unable to accurately study the problem of dynamic compaction on unsaturated soils, the primary aim of the provided thesis is presenting and implementing a general finite element framework to investigate the response of partially saturated soils subjected to dynamic compaction. This would enhance the understanding of this mechanism together with providing a useful tool for studying the determinative parameters of this problem. Partially saturated soils are characterised by the simultaneous deformation of a porous soil, and its pore water and pore air and usually exhibit more complex behaviour than some other porous materials, as they often experience non-linear plastic (irrecoverable) deformation. This makes their computational modelling less straightforward than for some other coupled problems. In recent decades numerical modelling the response of partially saturated soils to external loadings within the so-called mixture theory has received increasing attention. In such analyses the mixture-theory (with or without some simplifying assumptions, e.g., considering an iso-thermal environment and immiscibility of fluid phases) has formed the basis for analysing thermo-hydro-mechanical and even chemical coupling in partially saturated soils. A challenge in modelling unsaturated soil as a three-phase material arises in solving the global equations of motion, where the presence of different phases, together with the existence of inertia forces in each phase, makes the solution of the coupled dynamic system computationally demanding. The selection of a consistent constitutive model within the theory of mixtures, that can incorporate suction forces into the description of stress, provides a further complication. The necessity of such incorporation has frequently been reported in experimental studies of unsaturated soils. Moreover, in cases of large deformations analyses, the infinitesimal strain theory should be replaced by an algorithm which can explain the occurrence of large deformations in the numerical framework. These require the adoption of a unique strategy for integration of the constitutive model for an unsaturated soil. This thesis will address the aforementioned challenges. The generalised-𝛼 method is introduced and implemented for solving the global equations of motions and an explicit integration strategy for large deformations analyses of partially saturated media is also presented for performing the numerical integration of the mechanical constitutive model. A series of numerical examples are given throughout the thesis for the purpose of validation and showing the ability of the developed model to capture various aspects of the response of partially saturated soils. Among others, the changes in pore water pressure, saturation degree, pore air pressure, and the bearing capacity of soil at different water contents are considered. Moreover, a viscous boundary was suggested and was implemented for damping reflected waves from the surrounding boundaries during dynamic compaction and a generalisation of ALE method is given for this type of analysis. The numerical study on dynamic compaction provided at the end of this thesis indicates that the ground settlement during the impact has a direct relationship with the slope of loading line and the applied energy. An inverse relationship also is seen between the slope of the critical state line, Poisson’s ratio and pre-consolidation pressure with the ground settlement. The role of determinative parameters involved in this problem were further studied particularly on the variation of void ratio with respect to the changes in the material parameters. Moreover, it is illustrated that drying the soils decreases the ground settlement and the generated excess pore pressure beneath the centre of the impact area. The relationship between impact energy and the dictated changes in suction is also shown to be inverse; that is applying higher energies further decreases the suction. In addition, the procedure of change in the permeability of water phase in response to the external dynamic loads is also illustrated. It is shown that upon the impact and the consequent reduction of suction, the permeability of water phase will increase. The massive energy release accompanied by the application of dynamic compaction makes it almost impossible to install some measurement instruments below the impact areas. Moreover, the traditional numerical approaches of modelling dynamic compaction mostly neglect the state of partially saturated soils during the analysis. Therefore, the presented numerical model can be advantageous over these methods in capturing some aspects of the response of partially saturated soils subjected to dynamic compaction which cannot be measured in the field studies or by the traditional numerical approaches. Although in this thesis the simplifying passive air pressure assumption in the analysis of partially saturated is avoided, a numerical study on the generated excess air pressure is performed which indicates that the excess air pressure generated during the impact can be considered to be fairly well below the atmospheric pressure. This suggests the possibility of performing the simplified analysis with the passive air pressure assumption for this particular problem with an acceptable accuracy. The finding can be beneficial for future analyses of this problem as using the simplified analysis can be highly advantageous due to reduction of the number of nodal degrees of freedoms, computational storage, the time of the analysis and the complexity involved in the problem.
- Subject
- dynamic analysis; porous media; unsaturated soils; dynamic compaction; finite element
- Identifier
- http://hdl.handle.net/1959.13/1317938
- Identifier
- uon:23543
- Rights
- Copyright 2016 Javad Ghorbani
- Language
- eng
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| View Details Download | ATTACHMENT01 | Thesis | 2 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 508 KB | Adobe Acrobat PDF | View Details Download |