The mechanics of an unbonded locked sand at low effective stresses

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Description/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 signicantly greater than for comparable
reconstituted samples. A fall and recovery in the stiness 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 quantita-
tive 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 devel-
oped. 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 Dierential Transformers (LVDTs) together with volume change mea-
surement 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 locali-
sation, 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 eect of changing the con-
ning pressure on the transition of deformation characteristics is also examined.