The existence of mudstone weak interlayers has a significant impact on the stability of open-pit coal mine slopes. Under the combined influence of rainfall and groundwater, the mechanical properties of the mudstone of weak interlayers deteriorate, leading to a local loss of bearing capacity of the slope and further accelerating the overall instability of the slope. In order to investigate the changes of macroscopic and mesoscopic structures, mechanical failure behavior, and the damage evolution mechanism of water-immersed mudstone, non-destructive water immersion experiments and uniaxial compression experiments were conducted. The results indicate that the main causes of macroscopic structure failure of water-immersed mudstone are the initiation, propagation, and mutual penetration of micro cracks. The mesoscopic structure characteristics of water-immersed mudstone are primarily manifested by increased surface smoothness, increased occurrence of small-scale pores, the presence of a dense network of fissures on the surface, and fusion of mineral unit boundaries. With the increasing immersion time, the quality, relative water content, and peak strain increase, while the uniaxial mechanical parameters and energy parameters decrease. In addition, a statistically damaged constitutive model for mudstone considering the coupling damage of water immersion and low-stress loading was established, and the model is consistent with experimental results. Finally, the water-softening characteristics of mudstone are caused by the propensity of clay minerals to expand and disintegrate upon water contact, changes in pore structure, variations in mineral types and distributions, and the presence of pore water pressure. This study provides valuable insights into the water–rock deterioration mechanism of mudstone and the stability of slopes containing weak interlayers.

There is an obvious impact effect of on-site blasting on the slope coal mass of open-pit mines, so it is of great significance to study the dynamic mechanical response characteristics of coal rock for slope stability control. In this paper, first, the mineral composition and microstructure of lignite from open-pit mine are analyzed, and it is found that the content of non-organic minerals in lignite such as clay accounts for more than 24.40%; meanwhile, the rock sample has obvious horizontal bedding characteristics and mainly micro pores and transition pores inside; further, there are obvious banded areas with high water content in the rock, which has the same extending direction as the beddings. Based on the SHPB test system, the dynamic compression tests of lignite with different impact velocities are carried out. The results show that there is a significant hardening effect caused by the increase of strain rate on the dynamic mechanical parameters of rock samples, and the stress–strain curve has obvious “double peak” characteristics; meanwhile, the macroscopic crack of the rock appears at the first stress peak and disappears after further compression until the interlayer fracture occurs; further, the fracture fractal dimension of lignite increases linearly with the impact velocity, revealing that the fragmentation of rock samples increases gradually. In addition, with the increase of impact velocity, the input energy and dissipated energy of rock samples increase linearly, while the elastic property increases slowly and at a low level. The bedding characteristics of lignite and the wave impedance difference between the layers cause the high-reflection phenomenon in the process of stress-wave propagation, and then produce the obvious tensile stress wave in the rock sample, which finally results in the interlayer fracture failure of the rock.