Abstract: Aging deterioration of the soft interlayer of carbonaceous shale in acid-rich environment is a key factor inducing geotechnical engineering instability. To investigate the deterioration of shear behavior of the weak interlayer under acid corrosion and the underlying acid corrosion mechanism, the Permian weak interlayer from the Huangshan limestone mine in Sichuan province was selected as the study object. Direct shear tests on weak interlayer specimens were carried out after soaking in oxalic acid solution, along with pH monitoring, scanning electron microscopy, and wave velocity measurements. The results show that some mineral components in the weak interlayer react more readily with oxalic acid solution. The pH value of the immersion solution increases with time and eventually shows slightly alkaline. The H⁺ generated by oxalic acid dissociation significantly increases microscopic porosity by dissolving cemented minerals, leading to the deterioration of the longitudinal wave velocity of rock acoustic waves. This deterioration is specifically manifested as an increase in longitudinal wave velocity and a gradual decrease in the wave velocity change rate. Microscopic damage further propagates to the macroscale, specifically manifested as a significant evolution of the shear failure morphology with increasing acidity. Moreover, the stress drop phenomenon during shear failure stage of the weak interlayer after acid erosion is significantly weakened, and the plastic characteristics are notably enhanced. As immersion time increases and solution pH decreases, the cohesion and internal friction angle of the weak interlayer decrease, and the shear strength shows a tendency of gradual deterioration. The reduction in strength is concentrated in the early stage of immersion (0−7 d). Finally, a statistical damage model for rock under chemical-loading was established based on statistical damage theory. The model results were compared with indoor direct shear test results to verify the accuracy and validity of the model.

Key words: acid corrosion, weak interlayer, shear characteristics, acid corrosion mechanism, constitutive model