Browsing by Author "Mitchell, G P"

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    An investigation of lateral support systems by the finite element method
    (1993) Clark, Troy Cedric; Scheele, F; Mitchell, G P
    The design of lateral support systems, in the context of surface excavations, are usually done using conventional (classical) methods of analysis. For these design procedures limit state assumptions are made concerning the lateral earth pressures acting on the structure to determine the support system characteristics. No information with regard to the deformation of the soil adjacent to the structure can be provided. The objective of this thesis is to examine the finite element method of analysis as an alternative design tool which is adaptable to a wide range of situations. Finite element models are developed to investigate the influence of the plastic flow rule, wall friction and the soil type on the behaviour of a cantilever support system. Subsequently, the effect of wall stiffness, prop stiffness and the application of prop loads on the performance of a multiple level support system is examined. The results from these studies focus on wall displacements, lateral earth pressures, bending moments, plastic strain patterns and surface settlements behind the wall. The investigation provides extensive information about the entire soil-structure interaction of the system. This potential of the finite element method can be used in the optimization of support system design.
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    A study of brittle powder compaction using a combined discrete finite element approach
    (1999) Parker, I; Mitchell, G P; Pretorius, T S
    The Discrete Element Method (DEM) is a collection of numerical techniques based on a discontinuum idealisation of a physical system. Discrete Element Methods have been successfully applied to the simulation of the dynamic behaviour of granular media. Using the Finite Element Method to model the behaviour of each discrete element has resulted in the Combined Finite Discrete Element Method. This combination allows researchers to investigate the behaviour of assemblies of particles in which each particle need not behave in an entirely elastic mamrer. More importantly, discrete crack constitutive models may now be applied to each discrete element. The purpose of this thesis is to simulate the macroscopic behaviour of an assembly of brittle particles, using the Combined Finite Discrete Element Method. Brittle behaviour is incorporated via the use of a Mode I and Mode II brittle failure constitutive models. The Mode I and Mode II failure models used are the Rankine and Tresca failure models, respectively. The development of a pre-processor to generate the initial configuration of the particle assembly also formed a major component of the thesis. Algorithms to fill a plane with randomly shaped polygons were developed and implemented for the pre-processor. The cold compaction process is idealised as the compaction of a granular medium, in which each particle is a deformable polygon. Two algorithms are proposed and implemented to generate randomly shaped polygons. A further algorithm was developed to fill a simple bounding polygon with smaller polygons. A_commercial CFDEM code, ELFENExplicit, was used to investigate the effect of particle constitutive model and particle interaction on the macroscopic behaviour of the granular assembly. The built-in linear elastic and Mode I Rankine failure models were used in the initial simulations while a Tresca failure model was implemented to investigate the effect of a Mode II failure model.