Dry, damp, or drenched? The effect of water saturation on the frictional properties of clay fault gouges

Clay minerals often constitute a significant proportion of fault cores, yet the physics of clay friction, in particular the effect of free and interstitial bound (interlayer) water, is poorly understood. Understanding the behaviour of this typically frictionally weak component is fundamental to constraining the larger scale behaviour of fault zones. In this study, laboratory experiments were conducted on synthetic clay fault gouges under carefully controlled saturation states. Samples were sheared at room temperature under triaxial pressure at the following conditions: water saturated; room humidity; thermally dried then sheared at room humidity; in a vacuum at room humidity; thermally dried then sheared in a vacuum; in a vacuum and thermally dried within the pressure vessel. Friction coefficient (mu) is shown to increase with 'dryness' by a factor of 3 in 2:1 smectite and a factor of 2 in 1:1 sheet silicate, with largest increases in thermally dried samples. Analysis of constitutive frictional parameters (a-b) show that gouges become less stable with 'dryness'. The amount of displacement or time required to establish a new steady state mu upon a change in sliding velocity (d(c)) decreases markedly in drier samples. Results suggest that the presence of water is key in promoting time and slip dependent frictional changes, constraining operative grain-scale deformation mechanisms to those that are fluid assisted. They also highlight the optimum laboratory procedures to investigate the frictional properties of clay-bearing materials.