## 1 Introduction  
Rock masses serve as the medium for the storage and flow of geological fluids, and often interact with water in engineering construction. These interactions occur in various projects, including dam foundations [1], reservoir slopes [2], and deep buried water diversion tunnels [3], where the rock masses are susceptible to different levels of softening due to groundwater or surface water [4]. Soft rocks with argillaceous cementation, as a complex rock with intense water-rock interaction, exhibit significant water-induced softening characteristics. The significant reduction in mechanical properties due to water softening is an important cause of geological disasters in many engineering soft rock masses. Therefore, studying the softening effect of water on soft rocks has significant practical importance. In recent years, numerous studies have investigated the effects of water on the mechanical responses of rocks. The water-induced weakening analysis mainly focuses on the influence of water content, immersion time, and wetting-drying cycles on rock strength. Many studies have shown that the mechanical parameters (compressive, tensile, shear strength, and Young's modulus) of rocks weaken with increasing water content, especially mudstone [5; 6; 7]. In addition, the water content can reduce the crack toughness [8]. Compared with water content, the relationship between the immersion time and the actual field situation is closer. The immersion time can significantly affect rock strength since the deep surrounding rock masses are immersed in water for months [9]. For wetting-drying cycles (WDC), it was found that the first several WDC significantly affect the strength of rocks, but there was no change in the following cycles [10; 11]. In essence, natural rocks are heterogeneous, and the macroscopic failure behavior and mechanical properties of rock materials under water-rock interactions are closely related to the types of minerals (e.g., minerals with different shapes, sizes, and mechanical properties) and microstructures (e.g., pores, mineral particle skeleton, etc.) [12; 13; 14]. The softening mechanism of water on rocks is complex, and many scholars have studied different issues using various techniques. [PERSON] et al. [15] investigated the evolution law of micro-fractures in mudstone during the water-rock interaction process based on CT scanning. [PERSON] et al. [16] studied the evolution characteristics of microstructures of red bed mudstone and sandstone under rainwater infiltration based on XRD and SEM, and found that the contact relationship of clay mineral particles changed from face-to-face to edge-to-edge and edge-to-face contact, leading to the loosening and porosity of the micro-structure. [PERSON] et al. [17; 18] proposed the concepts of diagenetic bonding (DG) and depositional bonding (DP) that support the macroscopic stability of rocks in order to study the impact of water on the macroscopic mechanical properties based on the polarizing microscope thin sections. However, obtaining and preparing rock samples for experiments and conducting experiments are redundant, and the microstructure of each rock sample is different, so the results of laboratory tests are almost irreproducible. In recent years, with the rapid development of computational technology, numerical methods have emerged as a promising experimental alternative tool to study the mechanical behavior of rocks at different scales. However, numerical methods based on continuous media (e.g., FEM, FDM, BEM) are difficult to capture the initiation and propagation of cracks during the fracture process of rocks, and cannot characterize the rock heterogeneity. Discontinuous simulation methods [19; 20] and continuum-discontinuum simulation methods [21; 22] overcome the drawbacks of continuous medium-based methods, and have obvious advantages in characterizing the special morphology and spatial distribution of material components and in studying the impact of micro-scale heterogeneity on the macroscopic deformation and failure process of rocks.