## 3 Development of Fluid-Solid Coupling Similar Materials
### Water Resistance Experiment and Analysis of the Results
#### 3.4.1 Water Immersion Test  
In the water immersion test, when the amount of paraffin wax in the sample was 0, we placed it in water and it started to dissolve immediately, so it was not necessary to study. Under a certain mass ratio of river sand, calcium carbonate powder, and gypsum, the added amounts of paraffin wax were 3.33%, 3.70%, 5.00%, 5.56%, 6.67%, and 7.41%. The samples were weighed in water for 24 h to obtain 24 h water absorption values, and  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & **Ratio of** & **Ratio of** & **Ratio of** & **Water** & **Permeability** \\  & **Solid–Liquid** & **Paraffin** & **River** & **Softening** & **Absorption** & **Coefficient** \\  & **Components** & **Wax–Petroleum** & **Sand–Calcium** & **Coefficient** & **Coefficient** & **\%** & **10\({}^{-7}\) cm/s** \\  & & **Jelly** & **Carbane-Gypsum** & & **\%** & **10\({}^{-7}\) cm/s** \\ \hline B-2 & 8:01 & 1:01 & 30:03:07 & 0.783 & 7.78 & 1.16 \\ B-3 & 8:01 & 1:01 & 40:03:07  
then the softening coefficient of similar materials was obtained by uniaxial compressive strength and degree test. 1. Water Absorption  
The water absorption characteristics of similar materials are mainly characterized by water absorption parameters; the lower the water absorption, the stronger the non-hydrophilicity [10]. The formula for calculating water absorption is as follows:  
\[a=(m_{w}/m_{d})\times 100\% \tag{7}\]  
where \(a\) is the water absorption; \(m_{w}\) is the water absorption of the specimen after 24-h immersion; and the increment in the specimen's mass before and after immersion was calculated. \(m_{d}\) is the mass of the sample before soaking. According to the experimental data, the influence curves of the amount of paraffin on the 24 hour water absorption (24H water absorption) and softening the coefficient of the sample were drawn and shown in Figures 5 and 6, respectively. According to the comprehensive analysis shown in Figure 5a,b, river sand is the main aggregate under different ratios of solid materials, mainly constituting the pore volume of the sample. At the same time, the main pore of the sample through which water absorption occurs is filled by calcium carbonate and gypsum, that is, the value of 24H water absorption does not change significantly. When the proportion of solid materials is fixed, the paraffin wax content of the cementing agent increases, and the 24H water absorption is mainly related to the pore volume of the standard sample. From Figure 5b, it can be seen that the effective pore volume of water absorption decreases with the increase in paraffin wax content. The mixture of molten paraffin wax and petroleum jelly was used in binding the similar aggregate materials of the water-resisting layer, and paraffin was the main sealing component of the water-absorbing pore of the sample, so the value of 24H water absorption rate decreased gradually. Therefore, in Figure 5b, with a river sand-calcium carbonate-gypsum ratio of 40:3.7, as the paraffin wax content increases from 3.33% to 7.41%, the value of 24H water absorption of the similar materials sample generally decreases from 10.19 to 2.87. Among  
Figure 5: The influence curve of the content of paraffin wax and the percentage of the auxiliary cementing agent on the water absorption of the similar materials. (**a**) The influence curve of the content of the auxiliary cementing agent on the water absorption. (**b**) The influence curve of the content of paraffin wax on the water absorption. them, when the paraffin wax content accounts for 5%, the 24H water absorption value suddenly increases to 8.65. (2) Softening coefficient  
Among the hydraulic properties of similar materials, the softening coefficient is an important comprehensive indicator of changes in water ratio and strength of the materials. It mainly characterizes the decrease in strength and increase in flexibility (decrease in elastic modulus) of similar materials after being saturated with water.