An experimental study of fracture processes during dilatant hardening

The processes of dilatant hardening have been proposed as a potential cause of slow slip, a poorly understood mode of slip along plate boundary faults. Previous experiments and numerical models show that fault rupture and slip under high pore fluid pressure increases the duration of the stress drop during fault rupture and slip as a result of dilatant hardening. While previous research has focused on the mechanical changes to fault rupture and slip, few consider how damage itself can vary during dilatant hardening and affect subsequent slip and deformation. We conducted a suite of triaxial compression experiments on intact rock cores of Crab Orchard sandstone to characterize the mechanical effects of dilatant hardening and relate them to the macro- and microstructures that developed using micro-CT imaging and petrographic analysis. Experiments were conducted at an effective confining pressure of 10 MPa, with confining pressures between 10 and 130 MPa and pore pressures between 1 and 120 MPa. At all deformation conditions, the Crab Orchard sandstone exhibited multiple stress drops punctuated by episodes of friction sliding. With increasing pore fluid pressure, we document an increase in the cumulative stress drop attributed to slow periods of faulting, and a corresponding decrease in the cumulative stress drop occurring during rapid periods of faulting. Preliminary structural observations suggest a correlation between more discrete localized fracturing at low pore fluid pressures and more distributed fracturing at higher pore fluid pressures. Our results indicate that with increasing pore fluid pressure, dilatant hardening plays an increasingly more prevalent role in how strain is accommodated during fault rupture and slip, resulting in differences in both the mechanical behavior during failure and the structures that may exist in the rock record.