A new methodology to delineate the compactive yield cap of two porous sandstones under undrained condition

We have developed an experimental methodology to investigate the compactive yield behavior of porous sandstones under undrained condition and to distinguish it from the conventional method we refer to it as “modified undrained”. In a conventional undrained test a sample is deformed while the confining pressure and pore volume are fixed. Our novel method is designed to simulate undrained experiments by maintaining a constant pore fluid volume during triaxial loading. However, unlike in typical undrained experiments where pore pressure is allowed to vary, the modified method maintains a constant pore pressure, and instead relies on the simultaneous manipulation of the confining pressure and axial stress to maintain a constant pore fluid pressure. We apply this method to two sandstones precompacted inelastically to a range of porosities. The stress paths from a modified undrained test map out isoporosity stress contours that would coincide with the yield caps, if the elastic strain is negligible. Conventional triaxial experiments under drained condition, in which strain hardening was observed, provide constraints on the yield caps for different values of plastic volumetric strains. While at high mean stresses the yield caps for plastic strains <3% correlate with the isoporosity contours, we observed that at high deviatoric stresses and plastic strains the yield caps diverged from the isoporosity contours. This point of divergence was concomitant with an upsurge of acoustic emissions, implying that the stress path for an undrained experiment has pushed into a stress regime beyond the current yield cap thus triggering inelastic damage.