Moisture dynamics controls rock elasticity from weakening to stiffening

Water is ubiquitous in crustal rocks and has been independently shown to increase and decrease rock elasticity via adsorption-induced weakening and saturation-related stiffening. Yet, the interplay of how weakening and stiffening effects concurrently control the rock elasticity remains unclear. Here, we examine the acousto-mechanical behavior of a free-standing sandstone subjected to gradual water infiltration with a downward-moving wetting front over 7 days. Using time-lapse ultrasonic and digital imaging techniques, we observe elastic weakening ahead of the wetting front, which is explained by an analytical model linking P-wave velocity decrease, adsorption-induced expansion and surface energy decrease established at the grain scale. As the wetting front moves through the probed region, the weakening effect diminishes, and P-wave velocity begins to increase, consistent with findings in granite. This shift is attributed to saturation-related stiffening, supported by an analytical model of partial water saturation. A numerical model simulating the profile of water saturation and vapor further validates these observations. Our research sheds light on a key question in rock deformation: how weakening and stiffening effects jointly control rock deformation during progressive wetting.