The use of glass ballotini as a granular material provides the opportunity to simultaneously stud... more The use of glass ballotini as a granular material provides the opportunity to simultaneously study internal stress fields and internal fields of deformation as a sample is submitted to boundary perturbations. Digital image correlation makes use of the visible fabric of the material to deduce a field of displacements from one digital photographic image to the next. If the glass granules are immersed in a fluid having the same refractive index, then observation with polarized light exploits the photoelastic properties of the glass to reveal information about the stresses. Again, comparison of digital photographs enables changes in stress conditions from one image to the next to be discovered. Tests performed in a simple loading device which forces rotation of principal axes in parts of the granular mass are presented to demonstrate the unique potential of this dual experimental configuration.
When circularly polarised light is passed through a granular material under boundary stresses pat... more When circularly polarised light is passed through a granular material under boundary stresses patterns—‘light stripes’—are seen in the resulting images which have been traditionally associated with the directions of major principal stresses in the equivalent continuum. In this paper the passage of polarised light through a single spherical particle under stress is studied experimentally and analytically. The effect of placing the particle within a layer of particles, a layer of thickness 2–3 particles, and within a mass of particles is investigated experimentally. The appearance of light stripes is a visual reinforcement of effects seen at the particle level provided the level of stresses in individual particles is low. The implications for quantitative photoelastic interpretation of granular media are discussed.
Glass beads, when immersed in a liquid having the same refractive index, provide an idealised gra... more Glass beads, when immersed in a liquid having the same refractive index, provide an idealised granular material which can be used both for full-field determination of displacements and hence strains and also, by viewing under polarised light, information concerning the internal stresses within the granular assembly. A simple test box has been used to subject this granular material to rotation of principal strain axes and to encourage the development of a shear mechanism. Results from image analysis are presented to show detail of shear strains and volumetric strains. The images obtained using polarised light have also been subjected to digital manipulation. The nature of the patterns that are revealed is subtle and appears to occur at a scale quite different from the strain patterns.
Combining integrated photoelasticity and high resolution particle image velocimetry (digital imag... more Combining integrated photoelasticity and high resolution particle image velocimetry (digital image correlation) provides a tool which can be used to study granular materials at different scales. We have used an experimental arrangement which subjects an assembly of small 3D glass grains to plane strain conditions at the macro-scale. While the quantitative interpretation of individual and incremental photoelastic images in terms of continuum stress quantities presents challenges, some comparisons of strain increment and qualitative stress increment data are presented with concentration on the regions of apparent strain localisation.
Extended Abstracts Volume (332 pages) + full paper CD-ROM (1772 pages), 2008
ABSTRACT: In order to undertake a flood risk analysis, the performance of different flood defence... more ABSTRACT: In order to undertake a flood risk analysis, the performance of different flood defence structures under varying load conditions needs to be represented. This paper reports on work undertaken through the FLOODsite Project (Task 4) in bringing together available information on failure modes for a number of representative flood defence structure types, to support the development and implementation of system wide models for flood risk assessment. The work comprised a review of structures and failure modes which ...
ABSTRACT The evolution of shear zones in cohesionless sand for earth pressure problem of a reta... more ABSTRACT The evolution of shear zones in cohesionless sand for earth pressure problem of a retaining wall is experimentally investigated using a non-invasive method called particle image velocimetry. It is an optical technique for measuring surface displacements from successive digital images. Small scale laboratory tests are performed for active and passive cases of a rigid retaining wall subjected to horizontal translation, rotation about its toe and rotation about its top. Attention is focused on the effect of initial sand density on distribution of volumetric and deviatoric strain. The results for initially dense sand are qualitatively compared with corresponding ones obtained with X-rays at Cambridge University.
A method of plastic limit analysis of cohesive slopes reinforced with geotextiles is presented. F... more A method of plastic limit analysis of cohesive slopes reinforced with geotextiles is presented. First, the theoretical model of reinforced cohesive soil is presented, then the formulation of boundary value problems by a method of characteristics described.The bearing capacity of reinforced slopes and the failure surfaces are obtained as the solution to the system of governing equations.The method proposed allows for studying the influence of design parameters, like the unit weight of the soil, the angle of internal friction, cohesion, and the amount of reinforcement, on the stability of reinforced soil structures.
The intention of the paper is to check the capability of a discrete element method (DEM) to simul... more The intention of the paper is to check the capability of a discrete element method (DEM) to simulate a pattern of quasi-static shear zones in initially dense sand. Discrete calculations were carried out with a rigid and very rough retaining wall, undergoing passive and active horizontal translation, rotation about the top and rotation about the toe. To simulate the behavior of sand, the three-dimensional spherical discrete model was used allowing for grain rolling resistance. The geometry of calculated shear zones was qualitatively compared with experimental results of laboratory model tests using X-rays and Digital Image Correlation technique (DIC), and quantitatively with finite element results obtained with a micro-polar hypoplastic constitutive model. The results show that a discrete model is able to realistically predict the experimental pattern of shear zones in the sand interior. A satisfactory agreement with experiments and finite element calculations was achieved.
The use of glass ballotini as a granular material provides the opportunity to simultaneously stud... more The use of glass ballotini as a granular material provides the opportunity to simultaneously study internal stress fields and internal fields of deformation as a sample is submitted to boundary perturbations. Digital image correlation makes use of the visible fabric of the material to deduce a field of displacements from one digital photographic image to the next. If the glass granules are immersed in a fluid having the same refractive index, then observation with polarized light exploits the photoelastic properties of the glass to reveal information about the stresses. Again, comparison of digital photographs enables changes in stress conditions from one image to the next to be discovered. Tests performed in a simple loading device which forces rotation of principal axes in parts of the granular mass are presented to demonstrate the unique potential of this dual experimental configuration.
When circularly polarised light is passed through a granular material under boundary stresses pat... more When circularly polarised light is passed through a granular material under boundary stresses patterns—‘light stripes’—are seen in the resulting images which have been traditionally associated with the directions of major principal stresses in the equivalent continuum. In this paper the passage of polarised light through a single spherical particle under stress is studied experimentally and analytically. The effect of placing the particle within a layer of particles, a layer of thickness 2–3 particles, and within a mass of particles is investigated experimentally. The appearance of light stripes is a visual reinforcement of effects seen at the particle level provided the level of stresses in individual particles is low. The implications for quantitative photoelastic interpretation of granular media are discussed.
Glass beads, when immersed in a liquid having the same refractive index, provide an idealised gra... more Glass beads, when immersed in a liquid having the same refractive index, provide an idealised granular material which can be used both for full-field determination of displacements and hence strains and also, by viewing under polarised light, information concerning the internal stresses within the granular assembly. A simple test box has been used to subject this granular material to rotation of principal strain axes and to encourage the development of a shear mechanism. Results from image analysis are presented to show detail of shear strains and volumetric strains. The images obtained using polarised light have also been subjected to digital manipulation. The nature of the patterns that are revealed is subtle and appears to occur at a scale quite different from the strain patterns.
Combining integrated photoelasticity and high resolution particle image velocimetry (digital imag... more Combining integrated photoelasticity and high resolution particle image velocimetry (digital image correlation) provides a tool which can be used to study granular materials at different scales. We have used an experimental arrangement which subjects an assembly of small 3D glass grains to plane strain conditions at the macro-scale. While the quantitative interpretation of individual and incremental photoelastic images in terms of continuum stress quantities presents challenges, some comparisons of strain increment and qualitative stress increment data are presented with concentration on the regions of apparent strain localisation.
Extended Abstracts Volume (332 pages) + full paper CD-ROM (1772 pages), 2008
ABSTRACT: In order to undertake a flood risk analysis, the performance of different flood defence... more ABSTRACT: In order to undertake a flood risk analysis, the performance of different flood defence structures under varying load conditions needs to be represented. This paper reports on work undertaken through the FLOODsite Project (Task 4) in bringing together available information on failure modes for a number of representative flood defence structure types, to support the development and implementation of system wide models for flood risk assessment. The work comprised a review of structures and failure modes which ...
ABSTRACT The evolution of shear zones in cohesionless sand for earth pressure problem of a reta... more ABSTRACT The evolution of shear zones in cohesionless sand for earth pressure problem of a retaining wall is experimentally investigated using a non-invasive method called particle image velocimetry. It is an optical technique for measuring surface displacements from successive digital images. Small scale laboratory tests are performed for active and passive cases of a rigid retaining wall subjected to horizontal translation, rotation about its toe and rotation about its top. Attention is focused on the effect of initial sand density on distribution of volumetric and deviatoric strain. The results for initially dense sand are qualitatively compared with corresponding ones obtained with X-rays at Cambridge University.
A method of plastic limit analysis of cohesive slopes reinforced with geotextiles is presented. F... more A method of plastic limit analysis of cohesive slopes reinforced with geotextiles is presented. First, the theoretical model of reinforced cohesive soil is presented, then the formulation of boundary value problems by a method of characteristics described.The bearing capacity of reinforced slopes and the failure surfaces are obtained as the solution to the system of governing equations.The method proposed allows for studying the influence of design parameters, like the unit weight of the soil, the angle of internal friction, cohesion, and the amount of reinforcement, on the stability of reinforced soil structures.
The intention of the paper is to check the capability of a discrete element method (DEM) to simul... more The intention of the paper is to check the capability of a discrete element method (DEM) to simulate a pattern of quasi-static shear zones in initially dense sand. Discrete calculations were carried out with a rigid and very rough retaining wall, undergoing passive and active horizontal translation, rotation about the top and rotation about the toe. To simulate the behavior of sand, the three-dimensional spherical discrete model was used allowing for grain rolling resistance. The geometry of calculated shear zones was qualitatively compared with experimental results of laboratory model tests using X-rays and Digital Image Correlation technique (DIC), and quantitatively with finite element results obtained with a micro-polar hypoplastic constitutive model. The results show that a discrete model is able to realistically predict the experimental pattern of shear zones in the sand interior. A satisfactory agreement with experiments and finite element calculations was achieved.
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Papers by D. Lesniewska