## 3 Correction of the Electrical Resistivity of Calcareous Sand  
When the current passes through the contact point between the calcareous sand and the electrode, a contact resistance is generated at the contact point, mainly caused by the contraction resistance between the electrode and the medium and the oxide film on the surface of the metal electrode. Studies [16; 19; 20] have shown that, in the process of using the two-electrode method to test the resistivity of soil, the contact resistance between the sample and the electrode affects the results. To eliminate the influence of contact resistance, in this study, the contact resistance between the calcareous sand sample and the electrode was obtained through experiments and then corrected for the subsequent experimental results.  
To obtain contact resistance, sample boxes with lengths of 5, 10, 15, and 20 cm were made, as shown in Figure 3. This experiment used a digital bridge to test the resistance of calcareous sand samples under different test conditions to calculate the corresponding contact resistance.  
The following is an example to illustrate the correction method for the resistivity of calcareous sand using samples with a size fraction range of 1.5-2 mm. Figure 4 shows the relationship between the resistance and electrode spacing of samples with relative density \(D_{\mathrm{r}}=0.6\), saturation \(S_{\mathrm{r}}=80\%\) and \(100\%\), and size fraction of 1.5-2 mm. According to the linear fitting results shown in the figure, the contact resistance \(R_{0}\) with saturation values \(S_{\mathrm{r}}=80\%\) and \(100\%\) under this condition were \(0.0768\) k\(\Omega\) and \(0.0656\) k\(\Omega\) (intercept  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Influencing Factors** & **Size** & **Saturation/\%** & **Relative Density** & **Temperature/\({}^{\circ}\)C** & **Pore Water Type** & **Test Frequency/Hz** \\ \hline Pore water & 1–1.5 & & & & Pure water, tap water, NaCl & \\  & 1.5–2 & 100 & 0.6 & 20 & solutions with concentrations of & \\  & 2–2.5 & & & & 1\%, 2\%, 4\%, and 6\% & \\ \hline Temperature & 1–1.5 & & & & 5, 10, 15, 20, 25, & \\  & 1.5–2 & 100 & 0.6 & 30, 35, 40 & Tap water & 1000 \\ \hline Saturation & 1–1.5 & & & & & \\  & 1.5–2 & & 100 & 0.6 & 20 & Tap water & 1000 \\  & 2–2.5 & & & & & \\ \hline Relative density & 1–1.5 & & & & & \\  & 1.5–2 & & & 0.7, 0.8, 0.9 & 20 & Tap water & 1000 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test plans.  
of fitting curve), respectively. The calculation method for contact resistance under other conditions is the same as above.  
Based on the calculation results for the contact resistance between the calcareous sand sample and the electrode, the resistivity of the calcareous sand sample was corrected using the following formula:  
\[\rho=\frac{(R-R_{0})S}{L} \tag{4}\]  
Figure 4: Contact resistance test results of calcareous sand.  
Figure 3: Sample boxes with different electrode spacings. (**a**) electrode spacing of 5 cm; (**b**) electrode spacing of 10 cm; (**c**) electrode spacing of 15 cm; and (**d**) electrode spacing of 20 cm.  
In Formula (4), \(R\) is the test resistance, \(R_{0}\) is the contact resistance, and the meanings of the other quantities are the same as above.  
The resistivity in subsequent analysis was the result corrected using Formula (4).