## 3 Development of Fluid-Solid Coupling Similar Materials
### Water Resistance Experiment and Analysis of the Results
#### 3.4.1 Water Immersion Test
It is called the softening property and is represented by the softening coefficient. \(K_{\rm R}\) is defined as the saturated compressive strength of rock specimens (\(\sigma_{\rm cw}\)) and dry compressive strength (\(\sigma_{\rm c}\)). The calculation formula is as follows:  
\[K_{\rm R}=\sigma_{\rm cw}/\sigma_{\rm c} \tag{8}\]  
where \(K_{\rm R}\) is the softening coefficient of the similar materials, \(\sigma_{\rm cw}\) is the saturated compressive strength, and \(\sigma_{\rm c}\) is the dry compressive strength. The test results of the softening coefficient are shown in Table 2 and Figure 6.  
The softening coefficient parameter is directly related to the uniaxial compressive strength of the specimen after being saturated with water. When the ratio of solid materials is fixed, and with the increase in paraffin wax content in the binder, the 24H water absorption decreased gradually. That is, the water content of the sample decreased after water absorption. Therefore, the weakening effect of water on the strength of the sample also shows a decreasing trend. From Figure 6b, it can be observed that, when the proportion of paraffin content is 3.33-6.67%, the softening coefficient increases slightly with the increase in paraffin content in similar materials. When the river sand-calcium carbonate-gypsum ratio is 30:3:7, the softening coefficient of the sample increases slowly from 0.677 to 0.758 with the increase in paraffin wax content from 3.33% to 6.67%. In addition, from Figure 6a,b, we can observe that the softening coefficients of the samples, for ratios of river sand-calcium carbonate-gypsum of 30:3:7, 40:3:7, and 50:3:7, were very close for paraffin contents of 3.33% and 6.67%, respectively. When the proportion of paraffin is between 3.33% and 6.67%, the softening coefficient increases with the increase in the proportion of the auxiliary cementing agent; at the same time, the water absorption strength of the sample is stronger. This is because the samples with the same proportion  
Figure 6: The influence curve of the content of paraffin wax and the percentage of auxiliary cementing agent on the softening coefficient of the material. (a) The influence curve of the percentage of calcium carbonate and gypsum on the softening coefficient. (b) The influence curve of the content of paraffin wax on the softening coefficient. of paraffin wax had the same amount of water absorption, but calcium carbonate and gypsum play a role in the auxiliary cementation process, so that the specimen is further enhanced after the strength added by water absorption. However, when the proportion of paraffin wax was 7.41%, the softening coefficient of the samples with the three ratios of river sand-calcium carbonate-gypsum was reversed. The river sand-calcium carbonate-gypsum ratio was 50:3:7, and the softening coefficient of the samples with the proportion of calcium carbonate-gypsum was 14.84%, which suddenly dropped to 0.448. When the amount of river sand is too large, the content of calcium carbonate-gypsum that helps in the cementation process is not enough to complete the hydration-cementation process of a large number of aggregates. The increase in the paraffin wax content reduces the water absorption of the sample, and the inclusion of the auxiliary cementing agent cannot be completed, so this situation occurs. Through comprehensive analysis, it can be observed that, from the softening coefficient change of the similar material after immersion, the strength of the model sample after immersion is slightly reduced compared to that without immersion, with a reduction range of 0.35-0.2. The influence of water absorption on the strength of similar materials of water-resisting layers is a rather complex process.