Acoustic emission and compaction creep of quartz sand at subcritical stress

Abstract

Time-dependent fluid-assisted mechanisms such as stress-induced dissolution and subcritical crack growth play an important role in porosity reduction and compaction of granular material in nature. Previous compaction creep experiments on loosely packed, well-rounded quartz sand at subcritical effective pressure show that the rates of compaction increase with the presence of pore water, effective pressure (confining pressure minus pore pressure), and temperature due to operation of fluid-assisted mechanisms. We have investigated the role of cracking during creep compaction of quartz sand by monitoring acoustic emissions (AE). Experiments on water saturated St. Peter quartz sand packs (255 ± 60 []m grain size, initial porosity ~32%) and quartz powder made of disaggregated Arkansas novaculite grains (35 ± 12 []m grain size, initial porosity ~40%) were conducted at a pore fluid pressure of 12.5 MPa, at effective pressures, P[e], of 15 to 105 MPa, and at temperatures, T, of 24 to 225 °C. Volumetric strain rates were allowed to decrease to approximately 10⁻⁸s⁻¹ before experiments were ended. Our experiments show decelerating creep after application of effective pressure, that volumetric strain, β, increases approximately linearly with log time, and that β rates are dependent on P[e] and T. The ratio AE to β is constant with time at low T and P[e], but at high T and P[e] the ratio decreases with time. We interpret this to indicate that, for higher P[e] and T, there is a gradual transition in the dominant strain mechanism with time from critical crack growth to an acoustically quiet mechanism, such as subcritical crack growth or pressure solution.