On the loading conditions for pore fluid stabilization of failure in crustal rock

During shear failure in rock, fracture damage created within the failure zone causes localized dilation, which, under partially drained conditions, results in a localized pore fluid pressure drop. The effective normal stress within the failure zone therefore increases, and with it the fracture and frictional strengths. This effect is known as dilatancy hardening. Dilatancy hardening may suppress rupture propagation and slip rates sufficiently to stabilize the rupture and postpone or prevent dynamic failure. Here, we study the loading conditions at which the rate of dilatancy hardening is sufficiently high to stabilize failure. We do so by measuring the local pore fluid pressure during failure and the rate of dilatancy with slip at a range of confining and pore fluid pressures.

We performed shear failure experiments on thermally treated intact Westerly granite under triaxial loading conditions. The samples were saturated with water and contained notches to force the location of the shear failure zone. For each experiment, we imposed a different combination of confining pressure and pore fluid pressure, so that the overall effective pressure was either 40 MPa or 80 MPa prior to axial deformation at 10^-6 s^-1 strain rate. Dynamic shear failure was recognized by a sudden audible stress drop, whereas the stress drop during stabilized shear failure took longer and was inaudible. Local pore fluid pressure was measured with in-house developed pressure transducers placed on the trajectory of the prospective failure.

At effective pressures of 40 MPa and 80 MPa, we observe stabilized failure for a ratio λ (imposed pore fluid pressure over confining pressure) > 0.5. For λ < 0.5, we observe dynamic failure. Of two experiments performed at λ = 0.5 and 80 MPa effective pressure, one showed stabilized failure and one failed dynamically. For λ > 0.5, we observe pore fluid pressure drops in the failure zone of 30-45 MPa for 40 MPa effective pressure, and 60 MPa for 80 MPa confining pressure. The local pore fluid pressure during dynamic failure (λ < 0.5) is 0 MPa, strongly suggesting local fluid vaporization. Of the two experiments at λ = 0.5, the dilation rate with slip is higher for the dynamic failure relative to the stabilized failure.

We show that with increasing effective pressure, dilatancy hardening increases as the local pore fluid pressure drop during failure becomes larger. For λ < 0.5, dilatancy hardening is insufficient to stabilize failure because the local pore fluid pressure drop is larger than the absolute imposed pore fluid pressure. Near λ = 0.5, small variations in dilatancy control rupture stability. For λ > 0.5, dilatancy hardening is sufficient to suppress dynamic failure.