1,721,038 research outputs found
A hyperplasticity model for clay behaviour: an application to Bangkok clay
Full text linkThe main purpose of this thesis is the development of a new constitutive soil model emphasising the use of thermodynamic principles. This new approach to plasticity modelling, termed ‘hyperplasticity’, was first developed by Collins and Houlsby (1997) and Houlsby and Puzrin (2000). This idea has been further extended to continuous hyperplasticity in which smooth transitions between elastic and plastic behaviour can be modelled (Puzrin and Houlsby, 2001b). Applying hyperplasticity to this research, a kinematic hardening model specified by means of two scalar functionals is used to accommodate the effect of stress history on stiffness. A rate-dependent calculation for an approximation of the incremental stress-strain response is introduced. The model developed in the research is named ‘kinematic hardening modified Cam-clay (KHMCC) model’ and requires eight parameters (plus an extra parameter for rate-dependent analysis). Triaxial test results from the Asian Institute of Technology (AIT) and cyclic undrained triaxial data from Chulalongkorn University are employed to establish the soil parameters for the new model. The model is initially developed in terms of triaxial stress-strain parameters for the purpose of comparison with the experimental data on Bangkok clay. The model is expressed in FORTRAN code for implementation into the OXFEM finite element program. Two examples of real geotechnical projects in Bangkok (a road embankment and tunnelling in soft ground) are analysed under plane strain conditions. Comparisons of the numerical analysis results with field data are made. In addition, factors affecting the results of the analysis such as stress history and K0, are investigated
Properties of foam/sand mixtures for tunnelling applications
Full text linkThis thesis presents experimental work on foam/sand mixtures carried out in the Civil Engineering Laboratory at Oxford University, as well as the findings associated with it. This research represents the preliminary stage of a research project on Soil Conditioning Agents in Pipe Jacking and Mechanised Tunnelling, sponsored by the Pipe Jacking Association (PJA) and three water companies. The experimental work was carried out in order to evaluate the fundamental soil properties of foamed sand, in particular its compressibility, permeability and shear strength. The first chapter deals with the basic aspects of soil conditioning agents and their application to tunnelling, and provides an understanding of the fundamentals of foam behaviour. An introduction to the problems encountered in tunnelling is given. A brief study of foams and their properties is presented and the role of foams as soil conditioning agents is described. In the second chapter, a description of the foam generator used is provided. Two types of sand (fine and coarse), four types of foaming agents and a specific polymer mixture were employed for the testing. In some cases, sodium bentonite was used alone or in combination with foam and polymer. The sample preparation method, together with the results from measuring the reduction of the power input required to mix sand with foam, are presented. The third chapter presents the compression tests performed in a 75 mm Rowe cell. Results are presented as volume changed variation with the applied vertical stress. Quality control was carried out on the test results in order to evaluate the likely inconsistencies during the preparation and measurements. The most notable outcome was that for fine sand even at high pressure the final voids ratio of foam/sand mixtures after the compression remained higher than the maximum voids ratio of dry sand. Some measurements of the foam/sand mixtures permeability were also performed in the Rowe cell. Tests were carried out using the constant head principle with a "Marriotte bottle". Permeability values determined from testing are compared with indirect evaluation from the compression tests. In the fourth chapter direst shear tests in a standard shear-box are presented. Fine and coarse sand mixed with foam were test in shear under seven different vertical loads. Results are presented as plots of shear strength against horizontal deformation. Very low values of shear strength for foamed sand tests are recorded. The shear strength is plotted against the relative density index along with the experimental results from Bolton's correlation. Foamed sand shear strength values are scattered within a range below that of Bolton's Correlation. Finally, the conclusions form the test results are discussed
Assessment of design procedures for structural glass beams
Full text linkIn structural design, ductile materials are usually preferred, because failure is rarely sudden and catastrophic. Once the material has yielded, large observable strains will occur before total collapse of a structure. This thesis is about the structural use of glass. Glass is probably the most perfectly brittle material that exists. It demonstrates linear elastic behaviour right up to the point of failure. When a piece of glass fails, this always happens suddenly, by the high-speed propagation of a crack through the entire element. If glass is so different from the most common primary structural materials namely steel and reinforced concrete, then should we be using the same basic design approach for both? This thesis reviews current design methods tracing their development through this century. Current code formers are keen to bring all materials under the umbrella of Limit State Design. This philosophy has been developed for ductile materials and is shown to be somewhat inappropriate for materials where the main design criterion is not ultimate strength. A further chapter looks as the statistical behaviour of multi-ply beams. This was highlighted, at the recent Glass in Buildings conference, as being an important area for further research. It has been shown that statistically two or more beams are always better than one of equivalent thickness. A method of reducing the thickness of a glass beam, without affecting its probability of failure, has been described. This could have significant cost implications in the design of glass beams. It should be noted that this thesis is not about face-loaded glass elements such as windowpanes or structural glazing, where designs are governed by deflection and generally executed using rules of thumb, but about edge-loaded elements where strength governs and more rigorous design rules are required
A large displacement finite element analysis of a reinforced unpaved road
Full text linkA series of finite element predictions of the behaviour of a reinforced unpaved road consisting of a layer of fill compacted on top of a clay subgrade with rough, thin reinforcement placed at the interface, is described in this thesis. These numerical solutions are obtained using a large strain finite element formulation that is based on the displacement method, and are restricted to the case of plane strain, monotonic loading. Separate elements are used to model the soil and reinforcement. In the finite element formulation, an Eulerian description of deformation is adopted and the Jaumann stress rate is used in the soil constitutive equations. Elastic perfectly-plastic soil models are used which are based on the von Mises yield function for cohesive soil and the Matsuoka criterion for frictional material. Emphasis is placed on obtaining new closed form solutions to parts of calculations that are performed numerically in many existing finite element formulations. The solution algorithm is based on a "Modified Euler Scheme". The computer implementation of the formulation is checked against an extensive series of test problems with known closed form solutions. These include the analysis of finite deformation of a single element of material and the calculation of small strain collapse loads. Finite cavity expansion is also studied. This numerical formulation is used to perform back analyses of a series of reinforced unpaved road model tests. The reinforcement tensions, and the stresses at the interface with the surrounding soil, are calculated using the numerical model and discussed with a view to identifying the mechanisms of reinforcement. Two existing analytical design models of the reinforced unpaved road are described and critically reviewed in the light of the finite element results
A thermodynamic approach to constitutive modelling of concrete using damage mechanics and plasticity theory
Full text linkRecent advances in computational mechanics have opened the potential of carrying out the analysis and design of concrete structures in a realistic manner with the use of nonlinear concrete models. This encourages the development of more capable and realistic constitutive models, based on a rigorous approach, for the analysis and design of concrete structures. This research focuses on the development of a thermodynamic approach to constitutive modelling of concrete, with emphasis on the rigour and consistency both in the formulation of constitutive models, and in the identification of model parameters based on experimental tests. The key feature of the thermodynamic framework used in this study is that all behaviour of the defined model can be derived from two specified energy potentials. In addition, the derivation of a constitutive model within this framework merely follows procedures established beforehand. The proposed constitutive model here is based on continuum damage mechanics, in combination with plasticity theory, hence enabling the macroscopic material behaviour observed in experiments to be appropriately modelled. Damage-induced softening is the cause of many problems in numerical failure simulations based on conventional continuum mechanics. The resolution of these problems requires an appropriate special treatment for the constitutive modelling which, in this study, is based on nonlocal theory, and realized through the nonlocality of energy terms in the damage loading functions. For practical applications in structural analysis, the model requires a minimum number of parameters, which can be identified from experimental tests. All the above features of the model have been incorporated in a unified and consistent thermodynamic approach, which also distinguish the approach from existing ones. Numerical implementation and application are important parts of the study. A suitable implicit scheme is adapted here for the integration of the nonlocal rate constitutive equations. For the solution of systems of nonlinear algebraic equations in finite element analysis, the arc-length method in combination with local constraint equations employing dominant displacements is implemented, and proves its reliability in this study. Application of the proposed constitutive models in the analysis and design of concrete structures is straightforward, with several numerical examples showing the practical aspects of the proposed modelling
The analysis of offshore foundations subjected to combined loading
Full text linkShallow offshore foundations, which achieve their stability through the foundation bearing on the seabed, can in most applications be idealised as large rigid circular footings subjected to vertical, horizontal and moment loading. A small strain linear-elastic perfectly-plastic three- dimensional finite element program was developed to analyse this combined loading problem. The selection of a suitable three-dimensional finite element for accurate and computationally efficient analysis was based on the element's ability to model incompressible soil conditions, using exact numerical integration. A new approach for evaluating element suitability is developed and is quantified in terms of the parameter free degrees-of-freedom (equal to the degrees-of-freedom minus the incompressibility constraints). The tetrahedron family of three- dimensional elements is found to be in general more suitable and computationally efficient than the Serendipity and Lagrangian cube families. The 20-node quadratic strain tetrahedron is adopted for all analyses presented in this thesis. For combined loading, most of the available elastic analytical solutions are for rigid circular footings placed at the surface. Finite element analyses reveal certain inadequacies in some of these solutions. The elastic numerical analyses also examine the effect of footing embedment for three cases of embedment geometry. This demonstrates that the increase in footing stiffness (reduction in displacement) due to embedment for horizontal and moment loading is developed at shallower depths, and has a greater magnitude, than for vertical loading. The stability of a rough rigid circular footing placed on the surface of an undrained clay is examined. Zero thickness interface elements, despite experiencing some numerical instability problems, were found to model the footing-soil interaction better than conventional continuum elements. However, application of interface elements to footings which can lose contact with the soil, under moment loading conditions, resulted in numerical instability of the solution. A simpler model was therefore used to define the shape of the three-dimensional failure envelope at high vertical load. Comparisons with the semi-empirical inclination factors of bearing capacity solutions are included
Development of the cone pressuremeter
Full text linkThe cone pressuremeter is an in situ testing device comprising a pressuremeter mounted behind a cone penetrometer of the same diameter. Previously reported tests had indicated that the cone pressuremeter can provide measurements of soil strength, stiffness and in situ stress. The study presented in this thesis is aimed at developing methods of interpretation of the cone pressuremeter that can be applied with confidence to a variety of soil types. Carbonate sands have been the cause of significant problems associated with the design of foundations for offshore structures. A programme of cone pressuremeter testing in a carbonate sand from the west coast of Ireland is presented. Tests were carried out in a calibration chamber where conditions of vertical and horizontal stress and relative density were controlled. The influence of these parameters on measured values of cone resistance and pressuremeter limit pressure is assessed. Similar tests were also carried out in a felspathic sand, and correlations have been presented for deriving horizontal stress and relative density that are applicable to most types of sand. The influence of creep strains and of overconsolidation were other features of carbonate sand that have been assessed with the cone pressuremeter. A numerical model which accounts for the crushing characteristics of carbonate sand is presented, and is shown to improve significantly predictions of limit pressure measured in the calibration chamber. Cone pressuremeter tests were carried out in soft clay at the Bothkennar test site in Scotland. An analysis based upon cavity expansion theory was shown to provide good estimates of undrained shear strength and stiffness compared with results from other in situ and laboratory tests. Estimates of the in situ horizontal stress were found to be unrealistically high. Shear modulus in both sand and clay has been measured from unload-reload cycles carried out during pressuremeter expansion. The stress levels and strain amplitudes of these cycles have been shown to influence the shear modulus greatly. In sand, a procedure for relating these moduli to those at an extremely small reference strain is presented. In clay, shear moduli are shown to give a remarkably close agreement to others reported from Bothkennar, when due account of the strain amplitude is made. Finally, a time/cost analysis between the cone pressuremeter, the cone penetrometer and the self-boring pressuremeter is presented. The cone pressuremeter is found to be a cost-effective device bearing in mind the amount and quality of information it can provide
Behaviour of footings for offshore structures under combined loads
Full text linkThe lack of knowledge about the behaviour of footings for jack-up rigs under storm loads poses a design problem which can be tackled by model testing. The areas of prime concern are the ultimate loads on footings under combined loading, which affects the safety of the rig, and the rotational stiffness, which affects the interaction between the foundation and the structure. A programme of loading tests was performed on model footings on clay, and was divided into two stages: monotonic loading and cyclic loading. The clay samples were obtained by consolidating Speswhite kaolin slurry in cylindrical tanks 450mm in diameter. The strength and compressibility characteristics of the samples were verified by means of standard laboratory tests. The model footings were 50mm and 100mm in diameter and several shapes were tested: circular flat plate, cones of various angles and model spud-cans. Loads and displacements were monitored using appropriate instrumentation and a data logger. A series of central vertical loading tests provided data for comparison with existing bearing capacity theories. Combined loading tests were performed applying a displacement controlled horizontal load at a fixed height above the footing which was also subjected to a fixed vertical load. The main series of tests involved a parametric study of the relevant variables. Special tests allowed the assessment of the effect of embedment of the footing and the interaction of a flexible leg with the foundation. Cyclic loading tests were carried out using a load controlled system which applied a sinusoidal load simulating wave action. Effects of currents were investigated by introducing an offset to the loading cycle. The influence of amplitude and period of loading as well as the influence of vertical load were also investigated. Special tests were carried out to cover some peculiarities of real loading conditions. Fitting of a three-parameter hyperbola to the test results provided a systematic and accurate method of analysis of monotonic loading tests, leading to valuable information involving stiffness and ultimate loads. Analysis of cyclic loading tests yielded useful qualitative information regarding the progress of settlement and the variation of rotational stiffness and damping ratio with the number of cycles
Foam as a soil conditioner in tunnelling: physical and mechanical properties of conditioned sands
Full text linkEarth pressure balance (EPB) tunnelling machines are commonly used for the construction of tunnels in soft soils. These machines use the excavated soil in a pressurised head chamber to apply a support pressure to the tunnel face during excavation. How well an unstable face is supported in an EPB machine depends on effectively transferring a constant pressure from the support medium to the surface of the face. If the support pressure is not constant, but instead changes, the varying pressure inevitably leads to collapse of the face or heave on the surface ground. A machine may be designed to work in “Ideal ground” conditions. However, natural soils rarely have these properties, and conditioning of the soil is usually necessary to change its properties to suit the machine. Effective soil conditioning significantly improves the machine performance and control of the soil flow through the screw conveyor. However, for soil conditioning as commonly used in practice, the effects of different conditioning treatments on soil properties and the machine performance are not clearly understood, and problems with EPB machine operations related to the soil properties are often encountered. This thesis presents experimental investigations of direct shear box tests on conditioned sands, compressibility tests on conditioned sands and a model EPB screw conveyor operating with sandy soils. Index tests were performed to investigate effects of foam and polymer conditioning treatments on the plastic fluidity of different Leighton Buzzard (L.B.) sands and Thanet sand. The index tests allowed assessment of conditioning treatments for sandy soils, and optimum ranges of treatments for Leighton Buzzard sand and Thanet sand are suggested. In the series of shear box tests, performed on conditioned Thanet sand and conditioned L.B. sands, one of the most important findings was that the pore water pressure controls the strength of the sand foam mixtures. An increase in foam injection ratio (FIR) might produce an increase of pore water pressure and a decrease of shear stress. In the series of compressibility tests, performed on conditioned Thanet sand and conditioned L.B. fine sand, one of the most important findings was that increasing FIR does not increase maximum gas expelled. The FIR and the initial relative density of the specimen have to be related to the capability of the specimen to retain the gas and establish a coexistence between grains of soil, water and gas bubbles. Finally, from the series of model (1:10 scale), screw conveyor tests, performed on conditioned Thanet sand, conditioned L.B. fine sand and conditioned Garside sand, it can be concluded that the Oxford screw conveyor model can be used as a trial machine to study the effects of different operating conditions on conditioned sand specimens similar to those used on site. This can allow choice of conditioning methods to improve the performance of an EPB machine at a given site
The direct shear strength and dilatancy of sand-gravel mixtures
No full textA total of 87 direct shear tests in a large direct shear-box apparatus have been used to investigate the strength and dilatancy of sand-gravel mixtures. This paper focuses on the differences in behaviour between a silica sand (yellow Leighton Buzzard sand) and sand-gravel mixtures obtained by adding fractions of two kinds of gravel to the sand. The purpose is to find a relation between the grain-size characteristics of the materials and the shearing resistance. Experimental results are analysed in terms of the frictional and dilatant contributions to the strength of mixtures as a function of their relative density, and are compared with dilatancy theories and empirical equations. The addition of gravel to the mixtures, even at low fractions (less than 0.1 by volume), causes an increase in peak friction angle (φ′peak) which results both from higher dilatancy at failure (ψmax) and higher constant volume friction angle (φ′cv). Use of the minimum voids ratio (emin) of the materials allows the data for the two families of mixtures to be normalized and interpreted in terms of φ ′cv and the ratio (φ′peak -φ′cv/ψmax. The relationships between relative density (Dr), ψmax and φ′peak-φ′cv are only partly explained on a physical basis, so we develop empirical equations to predict the peak shear resistance of sand-gravel mixtures (up to gravel contents of 0.5) on the basis of easily measurable quantities. Such equations constitute a practical tool to overcome the problems arising from the impracticality of testing coarse material in the standard shear-box apparatus. © Springer 2006
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