## 3 Analysis of the Penetration Test Results
### The Influence Analysis of Hydraulic Gradient \"i\" on the Permeability Coefficient, \(k_{v}\)  
When \(\sigma_{r}\) values of 2 and 4 MPa were applied to the clayy sand samples, respectively, permeability tests with different hydraulic gradients were performed while keeping their \(\sigma_{r}\) constant, and the relationship curve between \(k_{v}\) and \(i\) under the same \(\sigma_{r}\) could be obtained, as shown in Figure 13. The curve shows that the variation law of low- and high-\(\sigma_{r}\) permeability coefficients with an increasing \"\(i\)\" was completely different. \begin{table}
\begin{tabular}{c c c c} \hline \multicolumn{2}{c}{**Low-Confined Water Infiltration**} & \multicolumn{2}{c}{**High-Confined Water Infiltration**} \\ \hline
**Confining** & **Recovery Ratio** & **Confining Pressure** & **Recovery Ratio** \\
**Pressure (\(\sigma_{r}\))/MPa** & \(k_{\text{unload}}/k_{\text{load}}\) & **(\(\sigma_{r}\))/MPa** & \(k_{\text{unload}}/k_{\text{load}}\) \\ \hline
1 & 0.12 & 1 & 0.12 \\
2 & 0.20 & 2 & 0.42 \\
4 & 0.49 & 4 & 0.50 \\
6 & 0.56 & 6 & 0.65 \\
8 & 1 & 8 & 1 \\ \hline \end{tabular}
\end{table} 
Table 4: Recovery ratio (\(k_{\text{unload}}/k_{\text{load}}\) ) of the loading–unloading permeability coefficient of the clayy sand samples. Figure 13: _Cont._  
The permeability of clayey sand is different from that of sandy and clayey soil, and its \(k_{v}\) is simultaneously affected by the seepage characteristics of sandy and clayey soil. The seepage of sandy soil obeys [PERSON]'s law. Because of the viscous resistance of adsorbed water, the pore water can seep only if the \(i\) is greater than the initial hydraulic gradient. The infiltration process of clayey sand in the bottom aquifer of a thick alluvium is influenced by the initial hydraulic gradient of clayey soil, the non-Darcy infiltration of clayey soil, the Darcy infiltration of sandy soil and the drainage effect of clayey sand samples; each factor has a different influence on the permeability characteristics under different infiltration conditions. The concrete manifestations were as follows: When the \(\sigma_{r}\) was low, clayey sand was dominated by Darcy infiltration of sandy soil, and when the head pressure difference was 50 kPa, there was a smooth seepage channel inside the sample. With the increase in \(i\), the effective back pressure (\(\sigma_{rp_{1}}\), its value calculation meets \(\sigma_{rp_{1}}=\sigma_{r}-p_{1}\)) remained unchanged, while the effective base pressure (\(\sigma_{rp_{2}}\), its value calculation meets \(\sigma_{rp_{2}}=\sigma_{r}-p_{2}\)) kept increasing, resulting in a decrease in the size of the whole seepage channel and a gradual decrease in the \(k_{v}\), as shown in Figure 13a,b. When the \(\sigma_{r}\) was high, the seepage process was influenced by the non-Darcy seepage of clayey soil and the Darcy seepage of sandy soil. Under a high \(\sigma_{r}\), the pores were compressed, the soil skeleton was closed, and the viscous particles blocked the seepage channel. When the head pressure difference was 50 kPa, there was almost no smooth seepage channel in the sample. At this time, the changes in the \(\Delta Q_{1}\) and \(\Delta Q_{2}\) were mainly due to the drainage effect caused by a decrease in the \(p_{2}\). As \(i\) increased to the initial hydraulic gradient, during the transition from non-Darcy seepage of clayey soil to Darcy seepage of sandy soil, the \(k_{v}\) gradually increased. Then, with a continuous increase in the \(\sigma_{rp_{2}}\), the size of the whole seepage channel of the sample decreased, while the \(k_{v}\) also gradually decreased; that is, the \(k_{v}\) gradually increased and then decreased with \(i\), as shown in Figure 13c,d. This law is also reflected by changes in the \(\Delta Q_{1}\) and \(\Delta Q_{2}\). When the \(\sigma_{r}\) was low, the changes in the \(\Delta Q_{1}\) and \(\Delta Q_{2}\) were highly consistent and synchronized, basically meeting [PERSON]'s law.