## 2 Materials and Methods
### Influence of Various Factors on the Water Absorption Process of Soft Rock  
In rock engineering, water absorption by soft rock is the result of the combined action of internal and external factors. In different geological environments, the influence of different factors on the water absorption process needs to be considered. This section analyzes the experimental results for seven factors: clay mineral type, clay mineral content, vertical stress (porosity), cation type, anion type, concentration and pH value. Under the influence of various factors, the water content and water absorption velocity of rock are shown in Figures 6 and 7, respectively. The experimental design in Table 1 in this study was designed according to the seven factors and three levels in the orthogonal experiment,  
Figure 8: Influence of different levels on the water absorption process of rocks. The numbers above the columns are the sum of the normalized calculation results of the four response indicators. Figure 7: Water absorption velocity \(v\) with time \(t\) as affected by: (**a**) clay mineral type (CMT) and clay mineral content (CMT), (**b**) vertical stress (VSs), (**c**) cation type (CTs) and anion type (ATs) and (**d**) concentration (CCs) and pH value (pH). By analyzing Table 2 and Figure 8, it can be seen that seven factors have different degrees of influence on rock water absorption. Among the fitting parameters, \(A_{1}\) and \(A_{2}\) are less affected by seven factors, while other fitting parameters are greatly affected by seven factors. The specific surface area and swellability of montmorillonite are much higher than those of illite and kaolinite, which leads to slower water absorption velocity in rocks containing montmorillonite (Sections I and II last longer, \(t_{1}\) and \(t_{2}\) are larger, \(a\) is smaller) and the change rate of water absorption velocity (\(b\)) is smaller. The effect of clay mineral content on rock water absorption cannot be ignored. With the increase in clay mineral content, the duration of Section I (\(t_{1}\)) shortened, the duration of Section II (\(t_{2}\)) increased and the change rate of water absorption velocity decreased (\(b\)). The clay mineral type and clay mineral content have little effect on the saturated water content (\(w_{0}\)) of the rock. Generally speaking, the smaller the vertical stress is when the rock is synthesized, the higher the rock porosity, the better the rock permeability and the greater the water content and water absorption velocity [44,45]. However, when the vertical stress is too large, it will cause secondary fracture of the rock, and although its porosity is small, its permeability is very good [42]. Therefore, in this study, when the vertical stress is small and large (5 MPa and 15 MPa), the rock water absorption velocity is fast and the saturated water content is high. Regarding the influence of water solution on the water absorption process of rocks, the research mainly includes the interaction of ion type, concentration and pH value with clay minerals in rocks [22,26]. In this study, the saturated water content and the change rate of the water absorption velocity of the rock in the monovalent cation-containing solution are larger, and the solution containing Ca\({}^{2+}\) makes the rock water absorption velocity slower. Among the anions, the saturated water content of the rock in the SO\({}_{4}^{2-}\)-containing solution is the smallest, and the change rate of the water absorption velocity is the largest, while the water absorption velocity of the rock in the CO\({}_{3}^{2-}\)-containing solution is the slowest.