Fracturing in saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements: experimental study

Frequency dependent complex electrical conductivity measurements have been made on sandstones saturated with distilled water during triaxial deformati on in both drained and undrained regimes. The resulting electrical and mech anical data show how the rock undergoes compaction, followed by dilatancy d ue to new crack formation, crack growth, interlinkage and failure as axial strain is increased. Electrical data are particularly good at indicating ho w early the formation of new cracks begins, showing that the quasi-linear p ortion of the stress-strain curve for triaxial deformation of saturated roc ks does not represent truly elastic behaviour, but the combined effects of (i) crack closure perpendicular to the strain axis and (ii) the formation o f tensile cracks parallel to the strain axis. A difference in the stress-st rain 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 vol umetric porosity, electrical porosity, cementation exponent and electrical tortuosity of the pore/crack network during deformation. The relative impor tance of crack closure and dilatation (a) during the progress of deformatio n and (b) between crack populations, controls these parameters and the elec trical data over a wide range of frequencies. However, the frequency depend ence of the micro-structural parameters and the electrical data was found t o be not affected significantly by the hydrostatic pressurisation or the tr iaxial deformation. The development of large scale crack connectivity is ob served to be confined to just prior to failure, and is controlled by the lo ss of cracks perpendicular to the axis of current flow and deformation. (C) 2000 Elsevier Science B.V. All rights reserved.