The bedrock used for underground construction has obvious traces of hydrodynamic scouring damage, and the mechanical properties of bedrock especially are severely damaged under a groundwater environment. On this basis, considering the excavated bedrock under various saturations, the uniaxial compression test of diorite is carried out. Meanwhile, scanning electron microscopy (SEM), electron energy spectroscopy (EDS) and X-ray diffraction (XRD) are used in the experiment. The variation law of the elastic p-wave velocity and microstructure and the response characteristics of the strength, deformation and mechanical parameters of rock under different flow rates and pH values are analyzed in detail. The results indicate that: (1) Saturations with a faster flow rate and lower pH value cause greater relative changes in the elastic longitudinal wave velocity of the samples. (2) The uniaxial compressive strength of the samples under various treatment conditions showed a decreasing trend. Compared with the dried samples, the uniaxial compressive strength of the samples under saturation with field flow rate v = 300 mm·s−1 and pH = 1 decreased by 46.08%, and the strength decreased by 35.67% under saturation with a field pH value = 6.56 and flow rate v = 900 mm-s−1. (3) The saturation with a stronger acidity, greater flow rate and longer action time causes the apparent dense structure of the diorite sample to be loose and accompanied by microcracks, which weakens its macromechanical properties. (4) Acid and hydrodynamic saturation produce water–rock chemical and physical effects on diorite, which weaken the connection force between mineral particles and the friction between fracture surfaces, reduce the elastic modulus, increase Poisson’s ratio and accelerate the failure of diorite.

In order to study the deterioration characteristics of the tunnel surrounding rock under the scouring of flowing groundwater, we ratioed similar acidic solutions based on groundwater composition. The microstructure of granite samples cored on site, deformation features, and evolution characteristics of mechanical parameters under saturations with different flow rates and various pH values were analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). The results indicate the following: 1) compared with the static water condition, the higher flow rate produces greater relative changes in granite’s microstructure, porosity, mass, and pH of the immersed solution. Moreover, the rate of change is relatively fast in the early stage and gradually slows down until it reaches a stable state. 2) Compared with the natural dry condition, the solution with a lower pH value causes the structural framework of the specimen to loosen, the mass loss degree to increase, and the porosity dispersion to intensify. 3) The dynamic water is more sensitive than the static water to the elastic vertical wave velocity of granite. Compared with the dry sample, the elastic longitudinal wave velocity of the sample in acidic solution with pH = 2 and flow rates of 0, 150, and 300 mm·s−1 for 49 days decreased by 8.7, 10.9, and 13.5%, respectively, which accelerates the instability and failure of the granite surroundings.