Some observations of volumetric instabilities in soils

The direct shear apparatus was developed for soil testing because it reproduced the shear failure surfaces that formed a part of the failure mechanism of many geotechnical systems. Typical shear tests on dense sands show a softening
load:displacement response which is associated with significant volumetric expansion. While shear localisations can be detected from discontinuities in marker layers, the change in density can be detected using radiography. The progressive formation of shear localisations is readily observed in situations which impose a discontinuity of boundary displacement and these can naturally be interpreted as precursors to a failure mechanism. However, more subtle patterns of volumetric strain or density localisation can be observed in situations where no such obvious boundary displacement discontinuity exists but the sand body is subjected to a more general shearing. Such patterns have a structure which is clearly related to the size of the sand particles. Several examples of such patterns are presented and implications for soil testing and for model tests on soils are discussed.