Chemically dependent mechanical properties of natural andesite rock fractures

Different engineering rock works are affected by chemical–mechanical coupling phenomena in rock mass. Friction, strength of fracture walls, shape of asperities, and roughness control hydraulic and mechanical behaviors of fractures, which are all affected by chemical solutions. In addition, the efficiency of some operations such as leaching depend on the fracture conductivity in a confined condition. To evaluate the effects of leaching agents on the mechanical and geometrical properties of andesite rock fractures, a set of laboratory-scale experiments were conducted on three types of andesite in a sulfate medium with different pH values. Although the cohesion and shear strength reduced by decreasing the pH value, the roughness and shear stiffness of fractures did not follow a unique trend. Scanning electron microscope images indicated that the reason is mainly due to the selective chemical dissolution of the more reactive minerals in acidic solution. It is shown that fracture conductivity is a strong function of confining stress, asperities resistance, and aperture in such a way that confining pressure decreases the conductivity appreciably and the reduction rate depends on the mineralogy of fracture surfaces.