The mechanics of an unbonded locked sand at low effective stresses

Abstract

Natural soil deposits are likely to have a structure resulting from particle bonding or interlocking or both. Recent research on natural sand having a predominantly locked structure with very little cement bonding has shown that, at low stresses, this material tended to mobilise its peak strength at or slightly before the onset of dilation. The intact material also displayed peak strengths very significantly greater than for comparable reconstituted samples. A fall and recovery in the stiffness of the material was observed prior to the onset of dilation. Dilation was associated with destructurisation, localisation and a loss of strength.
Understanding the behaviour before and after the start of dilation, and the role of destructurisation and localisation is essential to the development of reliable quantitative descriptions of the engineering behaviour of locked sands. This cannot be done in a conventional triaxial test with conventional instrumentation alone. To address this shortcoming, a digital image-based deformation measurement technique has been developed. Three digital cameras placed on radii at intervals of 120 degrees viewed on plan outside a transparent triaxial cell are used to capture images of the deforming samples at various instants. A suitable digital image correlation program has been written to analyse the captured images, using ray tracing to take account of image distortion due to refraction.
Linear Variable Differential Transformers (LVDTs) together with volume change measurement are used to characterise the behaviour of the material at small strains. The digital image-based technique has been used to determine the instant of onset of localisation, and the distribution of localisations within the sample as deformation progresses in a naturally locked sand. The implications of deformation localisation on the strength and mechanical behaviour of the material are studied. The effect of changing the confining pressure on the transition of deformation characteristics is also examined.

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ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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