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The paper reports new observations of strain localisation following dynamic impact. Two dimensional physical model tests have been performed to investigate the response of dry granular soils to the impact of a free falling steel plate. The tests have been performed to study the mechanics of the densification of soils during dynamic compaction, which is a widely used process to improve the performance of deep soil deposits by repeated dropping of large weights onto the ground surface. High speed photography and digital image correlation techniques have enabled the deformation patterns, soil strains and strain localisations to be observed. Tests have been performed on sand with a range of densities and a sand-silt mixture. The results have shown that the soil deformations are comprised of a conventional bearing capacity mechanism at the surface and a series of compaction bands that propagate downwards beneath the impacting plate. Similar patterns are observed in all tests, however as the compressibility of the soil increases the contribution from the bearing capacity mechanism decreases. [ABSTRACT FROM AUTHOR]

This paper describes the results of triaxial compression tests carried out at effective cell pressures ranging from 12.5 to 100 kPa to investigate the influence of fabric structure on the yield and failure of intact Reigate silver sand. In some of the tests, a digital image-based technique was used to determine the instant of onset of strain localisation, and the distribution of strain localisations within the specimen as overall deformation progressed. Comparative tests on intact and reconstituted specimens showed that fabric structure in the intact material allows the mobilisation of stress ratios close to peak before the onset of dilation, and increases the shear modulus at a given effective cell pressure and strain. Localisation was found to start at or after the onset of dilation, with a tendency to delay at increasing effective cell pressure. More localised deformation was observed at low effective cell pressures. Consistency between the critical state strengths of intact and reconstituted specimens is demonstrated, provided that the effect of shear band geometry is taken into account in stress analysis. [ABSTRACT FROM AUTHOR]

In-situ x-ray tomography has been used to follow deformation processes in 3D during two triaxial compression tests, one on a specimen of bio-cemented Ottawa 50-70 sand and the other on a specimen of the non-cemented sand. The global stress-strain responses show that the bio-cementation process increases the shear strength (peak deviator stress is approximately doubled), and causes the material to exhibit a linear behaviour up until peak, as well as increasing the dilatancy angle. The residual strength of the two samples is very close at large strain. Quantitative 3D digital image analysis (porosity, cement-density and strain field measurements), reveals that a dilatant shear band gradually develops pre-peak in the reference material. The cemented sample however undergoes an abrupt change of deformation mechanism at peak stress: from homogeneous deformation to localised dilatant shearing, which is associated with a local loss of cementation. [ABSTRACT FROM AUTHOR]

Tests have been performed on an analogue two-dimensional granular material in a special laboratory apparatus that allows the application of general stress or strain conditions. Digital image correlation of pairs of consecutive photographs taken during the tests has enabled fields of displacement and hence strain to be determined. Thus direct observation of internal displacements and strains has been possible for a series of general strain increments with different orientations of principal strain and different imposed angles of dilation. This analysis has successfully provided clear evidence of evolving internal structures of deformation. The observed evolving structures consist of bands of localised deformation and ‘cells’ of low deformation between the bands. The orientations of the identified localised features and cells are seen to depend on the applied strain path. The characteristic features and dimensions of the bands have been approximately identified. [ABSTRACT FROM AUTHOR]