# Experimental Study on Engineering Properties of Cemented Soil with High Water Content
## 4 Analysis and Discussion
### Shear Index  
The confining pressure of the cement mixing pile in engineering is generally smaller than 400 kPa. Therefore, the total shear index obtained by using the test data within the confining pressure of 400 kPa was more of guiding significance to the actual project in the unconsolidated-undrained triaxial test of the cemented soil. The failure principal stress line method was adopted to calculate the shear indices in this study [28]. The vertex of Mohr's circle (\(\sigma_{1}+\sigma_{3}/2\), \(\sigma_{1}-\sigma_{3}/2\)) under different confining pressure was considered as the failure point. The failure point of the Mohr's circle was fitted with the least square method to obtain the failure stress line. The expression of the failure stress line is shown in Equation (2). The internal friction angle and cohesion of the cemented soil can be obtained by converting the intercept and slope of the principal stress line and the specific calculation method of the index is shown in Equations (3) and (4). \[K_{f}:(\sigma_{1}+\sigma_{3})/2=a+(\sigma_{1}-\sigma_{3})/2\cdot\tan\alpha \tag{2}\]  
\[\varphi=\sin^{-1}(\tan\alpha) \tag{3}\]  
\[c=a/\cos\varphi \tag{4}\]  
where \(K_{f}\) is the failure stress line obtained by fitting failure points; \(a\) is the intercept of the failure stress line; tan\(\alpha\) is the slope of the failure stress line; \(\varphi\) is the angle of the internal friction; and \(c\) is the cohesion. Relevant parameters obtained by the method of fitting the failure principal stress line are listed in Table 4, where \(\varphi\) and \(c\) are the shear index of the cemented soil. As shown in Figure 12, the value of the internal friction angle was almost not affected by the curing time and increased slightly with the cement content. The correlation between the cohesion of the cemented soil and cement content is shown in Figure 13. This figure shows that the cohesion increased stably when the cement content increased. Moreover, the cohesion also increased with the curing time. Figure 13 also shows that when the specimens were cured for 28 days, the cohesions were 375.1 kPa, 675.3 kPa, and 894 kPa when the cement contents were 6%, 12%, and 18%, respectively. The cohesion increased to 894 kPa as the curing time reached 28 days when the cemented content was 18%. The shear strength of the cemented soil was an important factor in the process of load transfer for the DCM pile, jet grouting pile, and PGP pile. The correlation between the shear strength and the compressive indices of the cemented soil also needs to be investigated. In the unconsolidated-undrained triaxial test, the shear resistance of the cemented soil was mainly provided by the bonding force. The internal friction angle of cemented soil was 1.45-2.34 times that of the intact soil in this study. In contrast, the cohesion of the cemented soil increased significantly with the curing time and cement content. The cohesion of \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Cement Content/\%** & \multicolumn{4}{c}{**6**} & \multicolumn{4}{c}{**12**} & \multicolumn{4}{c}{**18**} \\ \hline
**Curing Time/d** & **3** & **7** & **14** & **28** & **3** & **7** & **14** & **28** & **3** & **7** & **14** & **28** \\ \hline \(\varphi/^{\circ}\) & 9.7 & 8.4 & 8.5 & 9.5 & 13.2 & 12.1 & 12.6 & 13.6 & 14.0 & 14.1 & 14.6 & 16.2 \\ \(c/\mathrm{kPa}\) & 188.2 & 286.8 & 353.9 & 375.1 & 355.3 & 535.7 & 607.7 & 675.3 & 461.4 & 654.9 & 781.4 & 894 \\ \(R^{2}\) & 0.913 & 0.955 & 0.923 & 0.977 & 0.988 & 0.903 & 0.999 & 0.997 & 0.997 & 0.999 & 0.987 & 0.994 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Shear index of the cemented soil. cenented soil was 13.4-68.8 times that of the intact soil in this study. When the UCS of the cemented soil increased from 447 kPa to 2353 kPa, the corresponding cohesion increased from 188.2 kPa to 894 kPa. The correlation between the UCS and cohesion of cemented soil is arranged as shown in Figure 14. As shown from the picture, the cohesion grew almost linearly with the UCS of the cemented soil.