Hydro-chemical erosion mechanism on shear properties and morphology of limestone joints

Understanding the evolution of micromorphology and shear behavior of joints is essential for evaluating the stability of rock engineering in an acidic environment. Limestone joint samples from the Three Gorges Reservoir area, China, were subjected to acidic erosion using sulfuric acid solutions with varying pH values and immersion times. The influence of acid erosion on joint morphological characteristics and shear mechanical response was experimentally investigated using a large-scale rock direct shear testing system. Results indicated that key joint roughness parameters showed a slightly increasing trend from 1.75% to 19.23% with increasing acid exposure. Conversely, the peak shear strength of the joints decreased nonlinearly with erosion time and acid concentration. In the initial 10–20 days of immersion, shear strength dropped from 8.19 to 6.91 MPa, with the rate of decrease subsequently slowing. Under strongly acidic conditions (pH ≤ 3), strength reduction was significantly greater than under weakly acidic conditions (pH ≥ 5), reaching a 20.6% decrease from the initial value after 80 days at pH = 1. The surface damage ratio of joints increased from 13.59% to 19.71% as pH decreased from 7 to 1. Microstructure observations revealed that localized dissolution areas and craters with a diameter of 9.0 μm appeared on joint surfaces initially, contributing to increased roughness. However, with prolonged exposure, acid infiltrated the rock matrix, triggering secondary crack expansion and pore structure degradation, thereby reducing matrix strength. Despite a minor increase in joint roughness from acidic erosion, the significant attenuation of rock matrix strength was the dominant factor controlling the degradation of joint shear strength.