FRACTURING IN SATURATED ROCKS UNDERGOING TRIAXIAL DEFORMATION USING COMPLEX ELECTRICAL CONDUCTIVITY MEASUREMENTS: EXPERIMENTAL STUDY

Abstract

Frequency dependent complex electrical conductivity measurements have been made on sandstones saturated with distilled water during triaxial deformation in both drained and undrained regimes. The resulting electrical and mechanical data show how the rock undergoes compaction, followed by dilatancy due to new crack formation, crack growth, interlinkage and failure as axial strain is increased. Electrical data are particularly good at indicating how early the formation of new cracks begins, showing that the quasi-linear portion of the stress-strain curve for triaxial deformation of saturated rocks does not represent truly elastic behaviour, but the combined effects of (i) crack closure perpendicular to the strain axis and (ii) the formation of tensile cracks parallel to the strain axis. A difference in the stress-strain behaviour between the drained and undrained samples was also observed, with the undrained samples developing a pronounced strain-softening phase before failure. The experimental data have also been used to derive the volumetric porosity, electrical porosity, cementation exponent and electrical tortuosity of the pore/crack network during deformation. The relative importance of crack closure and dilatation (a) during the progress of deformation and (b) between crack populations, controls these parameters and the electrical data over a wide range of frequencies. However, the frequency dependence of the micro-structural parameters and the electrical data was found to be not affected significantly by the hydrostatic pressurisation or the triaxial deformation. The development of large scale crack connectivity is observed to be confined to just prior to failure, and is controlled by the loss of cracks perpendicular to the axis of current flow and deformation.