Radial strain recordings from 24 triaxial tests on sandstone cores had been analysed in order to investigate if dilation could be related to localisation of shear bands. A weak azimuthal anisotropy was observed both in the hydrostatic and in the triaxial stress region. However, the results showed that the orientation of the principal strain directions shifted from the hydrostatic region to the triaxial region. Two different mechanisms relating to closing of microcracks and shear crack sliding might explain this observation. Another shift in the orientation of the principal strain directions was observed after peak stress and was followed by a significant increase in the strain anisotropy. No correlation was found between the orientation of the principal radial strain before peak stress and the orientation of the shear bands observed after peak stress. These results indicate that 1ocalisation of shear bands only affects the radial strain after peak stress in these materials.
The understanding of the failure mechanisms in sandstone is an important and significant part of many petroleum related geotechnical problems. Normally, failed materials appear as highly inhomogeneous, with more or less unaffected material surrounding regions of completely damaged material. The formation of 1ocalised failure zones alters the stress and strain geometry, and may thus have a strong impact on the behaviour of the rock. This is particularly relevant in nonhomogeneous stress situations, like in rocks surrounding boreholes or cavities.
Dilatancy is an important and significant aspect of rock deformation and it is closely related to the failure mechanisms. Large deformations and decrease of strength, which may cause borehole instability and sand production, accompany the dilation process. The dilatancy is here defined as the volume increase due to increase in the shear stress (see Larsen et al. 1998).
The transitions between dilating and compacting samples are related to the transition between brittle and ductile behaviour (Scott & Nielsen 1991). The micromechanical behaviour in the ductile regime is related to grain cracking and rotation and sliding of grains (Bernabe & Brace 1990). However, they found that the deformation in the brittle regime is related to frictional sliding of grain contacts localised in a narrow zone. Similar shear bands has also been observed in sandstone by Menedez et al. 1996.
Based on these observations the radial deformation in a stress triaxial test should be different for brittle and ductile sandstone. In this study we analyse the radial deformations recorded during triaxial tests on sandstone and find a measure of the radial anisotropy. Based on this analysis we can estimate the time of 1ocalisation of shear cracks in the cores. We also study the impact the radial anisotropy have on the volumetric strain and the Poisson's ratio.