Creep of a porous rock and associated acoustic emission under different hydrous conditions

Abstract

Static fatigue of a polycrystalline porous rock (iron ore) was studied by performing multistep uniaxial creep tests under partially saturated conditions, and the impact of water saturation on creep was analyzed. Recorded strains and acoustic emissions (AE) show that water saturation induces a strong increase in AE activity and dilatant inelastic volumetric strain and a decrease in Young's modulus and in the b value of the Gutenberg-Richter law (i.e., the relative number of large-amplitude events increases) as the rock approaches failure, indicating that microfracturing plays an important role in the creep process. Water saturation accelerates static fatigue through hydromechanical coupling and subcritical stress corrosion cracking. The chemical reactions involved in the corrosion of iron ore and leading to a decrease in its intrinsic mechanical properties are described. These reactions play a major role in the static fatigue of iron ore, which on a large scale is probably the main mechanism explaining certain collapses in underground iron mines. It is also shown that creep straining of iron ore is reversible after stress removal, indicating that it results also from a time-dependent viscoplastic mechanism (i.e., dislocation creep). Copyright 2009 by the American Geophysical Union.