The weakening effect of water on the brittle failure strength of sandstone

Abstract

Pore fluids affect the brittle failure strength of rocks by physico-chemical interactions between solids and fluids and by mechanisms associated with changes in effective stress controlled by drainage conditions. The two classes of mechanisms are often hard to separate in the field and in the laboratory. We performed triaxial deformation experiments at different strain rates that allowed for a systematic variation of the drainage state with the aim to identify the contribution of physico-chemical effects on the failure characteristics of three sandstone varieties (Ruhr sst, Wilkeson sst and Fontainebleau sst) covering a range of porosities, hydraulic properties and mineralogical compositions. The applied strain rates ranged from 10−7 to 10−3 s−1 comprising the critical strain rates for internal drainage estimated from experimentally determined hydraulic properties. Tests were performed at room temperature and an effective pressure of 20 MPa. Under these conditions all samples exhibited brittle failure and hence dilatancy hardening occurred when deformation proceeded too fast to maintain effective drainage. The strain rates observed for the onset of dilatancy hardening agreed within an order of magnitude with calculated critical strain rates. At strain rates low enough to ensure internal drainage the physico-chemical effects on strength were deduced from the comparison of tests on dry and saturated samples. The relative weakness of saturated samples was similar for the three sandstones and rather moderate. We did not find a distinct relationship between the weakening effect of water and the presence of clays. The peak in work performed on a sample systematically preceded that of the applied axial stress potentially pointing to an irreversible micromechanical instability before actual macroscopic failure. Considering work in addition to axial stress may provide valuable complementary constraints for a micromechanical interpretation of processes related to different states of internal drainage.