### Browsing by Author "Scheele, Friedrich"

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- ItemOpen AccessAn anisotropic damage model for rock(1994) Sellers, E J; Scheele, FriedrichAn anisotropic damage model is proposed for the constitutive description of microcracking processes in brittle rock under a general loading path. Experimental data and micromechanical models are reviewed to quantify the effect of microcracking on the material stiffness and the mechanisms of microcrack formation in brittle rocks under compression are discussed. The sliding crack concept is adopted as the micromechanical basis of the anisotropic damage model. Undamaged material is represented with a linear elastic constitutive equation. Damage initiation is defined by a Coulomb friction law, which excludes damage at low deviatoric stress levels. The formulation of the directional damage extends the arguments of continuum damage models for tension cracking to general, tension and compression, stress states. This is achieved by the definition of damage in a subdomain of the total strain and the characterisation of the directional microcracking by a fourth order tensor internal variable, the damaged secant stiffness of the 'crack' strain subdomain. Induced anisotropy results from the reduction of components of the initial stiffness tensor in the direction of the positive principal 'crack' strains. Evolution of the damage magnitude is determined by the principle of maximum damage dissipation in terms of the undamaged energy norm of the positive part of the 'crack' strain tensor. Versatile evolution functions, based on the Weibull probability density function, are proposed for compression and extension damage modes. Unloading and reloading criteria are developed which are consistent with the sliding crack concept and introduce hysteretic behaviour. A numerical solution scheme is presented and the model is implemented in a nonlinear finite element program.
- ItemOpen AccessComputer program development for the analysis of inelastic beam and soil behaviour in geotechnical design(1992) Howie, C T; Scheele, FriedrichComputer-aided engineering requires the correct implementation of design methods in computer programs so as to play a beneficial role in engineering practice. This thesis describes the development of a computer program to analyse geotechnical engineering problems based on the principles of beam-soil interaction where the beam is supported by a single or two-layer soil system. In 1867, a foundation model was proposed by Winkler in which the elastic foundation beneath a horizontal beam could be viewed as a series of independent springs. Foundation reaction to beam deflection is, therefore, linear. A stiffness matrix, for use in matrix methods of structural analysis, has been developed to define this beam-soil interaction, and such a method can be incorporated into a computer program. Furthermore, an iterative technique was created to allow for inelastic soil response when using the elastic stiffness matrix. However, such a technique did not consider realistic soil behaviour, and has limitations is used for practical design. This research' work describes how use can be made of the pressure-displacement response relationship for a soil to bring greater realism to beam-soil modelling and analysis. Such a relationship is commonly determined in geotechnical design procedures through a plate load test in the field. In addition, the iterative technique is extended to include non-linear beam behaviour as well, and plastic hinging of the beam material is incorporated to enable limitation of inelastic response. While previous research has only considered foundations of a single soil only, a procedure to model a two-layered system is developed. Two-layered foundations are required for proper modelling of soldier pile support systems, an area of structural design in geotechnics chosen to demonstrate realistic design potential for the computer program. The two-layered principle is based on the derivation of a control parameter to differentiate between response from just the upper soil layer, and a combined response from both soil layers. The procedure is relatively simple, and no extra information is required other than the two pressure displacement relationships for the individual soil layers. A desktop computer program is described which incorporates the inelastic analysis features, as well as the two-layered soil system. The program makes use of a graphical user interface to offer the user an easy, interactive environment for analysing beam-on-soil foundation problems. As such, the program can be used directly, or for further research into beam-soil interaction. The program is applied in the analysis of both field and laboratory tests to ascertain its accuracy in predicting beam-soil interaction. The laboratory test measures the deflection of a horizontal beam on a single soil foundation medium, where the beam is loaded by a single jack at approximately mid-span. Computer predictions for such a test were in very close agreement with the laboratory observations, despite the small magnitude of beam displacements, and the fact the beam-soil system suffered a bearing capacity failure which affected the beam deflection. The field test was performed to investigate the performance of a flexible soldier pile under high anchor loading. Results of the computer analyses again show the program's predictions to be in very close agreement with the field measurements. Currently, the program does not include the facility to model soil layers behind a soldier pile, but the method developed in this thesis can easily incorporate multiple pressure-displacement curves for different soils. Final conclusions drawn express a need for more research into soldier pile systems before the techniques of this work can be used for routine design. Nevertheless, the development of the program has made a significant contribution to advancing the use of computer-aided design in this field of geotechnical engineering.
- ItemOpen AccessExperimental and numerical investigation into the Sand/Geotextile shear interaction behaviour(1997) Wise, Chistopher Charles; Scheele, FriedrichGeotextiles are planar polymeric textile materials which are utilised in geotechnical engineering in various applications, including the reinforcement of soils which is achieved by laying geotextile sheets horizontally to carry the induced horizontal stresses. The behaviour of such reinforced soil structures (e.g. retaining walls) is determined primarily by the shear interaction between the soil and individual geotextile sheets. This dissertation presents an investigation into the shear interaction behaviour between a locally manufactured non-woven geotextile and Cape Flats sand. The literature review exposed a certain lack in understanding of the displacement and shear stress mechanisms involved in the pull-out of geotextiles from sand. Also, the prediction of either rupture or slippage failure was unclear. The shear stress at slippage failure has not previously been determined for confinements greater than 1OOkPa. The applicability of using direct shear tests (specified by the BS8006) to determine the friction parameters for design, is uncertain. Numerical techniques have been shown to be an adequate tool to analyse the pull-out mechanism of a geotextile in soil.
- ItemOpen AccessA new method of meshing in discontinuous deformation analysis (DDA)(1997) Clatworthy, David Elland; Scheele, FriedrichDiscontinuous Deformation Analysis (DDA) is a discrete element method developed by Shi [1988] specifically for modelling blocky rock masses. The DDA method is based on the assumption that deformation and failure of such rock masses is primarily due to differential movements of rock blocks, rather than strain and fracture of intact rock material. Strains and stresses are assumed to be constant over the area of each rock block. Contact between blocks is modelled using penalty functions, with Coulomb's friction law controlling sliding along block boundaries. Tests show that while DDA is not well suited to dynamic simulations where the velocities of blocks become large, it can model rock masses to a reasonable degree of accuracy in static analyses. There are various analysis control parameters which have a marked effect on the solution, however, and the user should take care in choosing suitable values for these parameters. A method is proposed here, in which certain blocks can be sub-divided into Finite Element meshes in order to obtain a more accurate description of their deformation. The method takes advantage of the fact that both DDA and the Finite Element Method (FEM) use the principle of stationary potential energy to obtain the solution equations for block equilibrium. Both DDA blocks and FEM elements can therefore initially be treated as DDA blocks, using the standard DDA formulation, and then the solution equations for the FEM elements are converted into Finite Element format by a simple transformation procedure before solution. First and second order DDA blocks are considered in this report, along with their equivalents in FEM, the C0-linear and C0-quadratic triangular elements. The C0-linear elements are found to be too stiff in modelling bending deformation, due to the assumption of constant strain throughout the element. The C0-quadratic elements are able to accurately model bending, however. It is shown through tests that the performance of these FEM elements, formulated within the DDA method, is identical to that obtained using the corresponding elements in conventional Finite Element programs. The sub-meshing method therefore allows mixed-formulation analyses, with DDA blocks and FEM meshes interacting within a single system, while remaining efficient, and reasonably simple to incorporate into existing DDA program codes. It would also be possible to model material non-linearity and fracture using this method.
- ItemOpen AccessPermeability and swell testing of selected South African bentonites(1997) Orsmond, Wyatt; Scheele, FriedrichBentonite, a natural clay composed predominately of montmorillonite has become a very useful product in Civil Engineering due to its extremely low permeability and high swell potential. Bentonite has been used extensively in countries like the United States of America, Canada and Europe as an impervious liner in dams, landfill sites, retention ponds etc. where it is used in its pure form and as a composite material mixed with soil. Bentonite liners are rapidly gaining interest in many other countries including Southern Africa. Bentonite is found world wide, and in South Africa (RSA) there are three deposits presently being mined namely Heidelburg (Western Cape), Koppies (Orange Free State) and Plettenberg Bay (Eastern Cape). The bentonite from each of these deposits are called Culseal I Culbond, Koppies and Plett respectively. They all vary with regard to chemical composition and little is known about there engineering properties. The aim of this research is to provide information both general and with regard to Civil Engineering on the bentonites presently being mined in South Africa to facilitate design and future research in this field. The triple layer crystalline sheets which make up a bentonite particle are held together by weak forces and exchangeable cations (which dictate the bentonite type). When brought into contact with water, the water is drawn onto the sheets to balance the charges forcing the layers apart resulting in swelling of up to 15 times the original particle thickness. This swollen, hydrated bentonite is almost impervious.