Spatiotemporal water distribution governs anisotropic mechanical weakening and localized tensile failure in sandstone

The mechanical behavior of sandstone is particularly sensitive to changes in water distribution, as water can easily permeate the pore structure. This study investigates the weakening effects and failure modes in sandstone using both experiments and numerical simulations under uniform and imbibition conditions. T 2 spectra were measured to examine the spatiotemporal alterations in pore water types. The uniaxial compressive strength and elastic modulus (Young’s modulus) were tested, while stress fields were derived from numerical simulations to analyze the damage mechanism and tensile failure of sandstone. Key findings: (1) the film thickness of bound water progressively increased to transform into capillary and bulk water under uniform conditions, whereas all types of pore water increased concurrently during imbibition. (2) The increase in saturation degree induced a three-stage variation in physical and mechanical properties. At the same saturation degree, the bearing capacity of sandstone under imbibition conditions was higher than that under uniform conditions. (3) The differences in pore water types, elastic modulus, stress thresholds, and acoustic emission (AE) counts were remarkable at the medium saturation stage (20%–60%). (4) AE localization and numerical simulations corroborated that the sudden drop of tensile stress at the wet-dry interface facilitated localized damage accumulation under imbibition conditions. Through analysis of microscopic damage and the spring model inferred, it was inferred that, under uniform conditions, the weakening effects impacted all mineral particles. However, under imbibition conditions, the anisotropic weakening effect (directional damage due to saturation gradient) was only restricted to the wet zone, where short-term damage at the wet-dry interface led to localized tensile failure.