# Undrained Behavior of Microbially Induced Calcite Precipitated Sand with Polyvinyl Alcohol Fiber
## 4 Results
### Effect of Calcite Treatment  
Table 2 summarizes the testing conditions and compares the peak values and hydraulic conductivity. Figure 2 shows a typical result of the undrained triaxial test on sand with and without calcite treatment and a confining stress of 50 kPa. Deviatoric stress and mean effective stress are defined as \(\mathrm{q}=\sigma^{\prime}{}_{1}-\sigma^{\prime}{}_{3}\) and \(\mathrm{p}^{\prime}=(\sigma^{\prime}{}_{1}+2\sigma^{\prime}{}_{3})/3\), respectively. The stress-strain curve for untreated sand showed a continuous hardening behavior, but for treated sand showed stiffer behavior at the initial loading stage. A positive excess pore pressure was generated initially, but this became negative due to dilation regardless of the CC. As the CC increased, the dilation decreased due to the cemented particles. [PERSON] and [PERSON] (2013) obtained similar results for biocement with MICP. The relationship between the stress ratio (\(\mathrm{q}/\mathrm{p}^{\prime}\)) and axial strain is shown in Figure 2. The ratio for the untreated sand reached a peak value and then remained constant. On the other hand, the ratio for the treated sands reached a peak and then decreased slightly. The trend for the principal stress ratio (\(\sigma^{\prime}{}_{1}/\sigma^{\prime}{}_{3}\)) was similar to that of the stress ratio, as shown in Figure 2. This kind of stress-strain behavior has also been observed ([PERSON], 2013). \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Test ID**} & \multicolumn{2}{c}{**Average**} & \multirow{2}{*}{ \begin{tabular}{c} **Calcite** \\ **Content (CC)** \\ **(\%)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Fiber** \\ **Pressure** \\ **(kPa)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Confining** \\ **Consecutive** \\ **(m/s)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Hydraulic** \\ **Desiatoric** \\ **Desiatoric** \\ **Stress (kPa)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Peak Pore** \\ **Pressure** \\ **(kPa)** \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} **Peak Stress** \\ **Ratio** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Peak** \\ **Principal** \\ **Stress Ratio** \\ \end{tabular}  } & \multirow{2}{*}{**CC (\%)**} \\ \cline{6-6}  & & & 50 & \(9.03\times 10^{-3}\) & 725 & \(-219.1\) & 1.45 & 3.8 & 0.0 \\ \hline CSC100 & & & 100 & \(9.09\times 10^{-3}\) & 876 & \(-207.6\) & 1.47 & 3.9 & 0.0 \\ \hline CSC300 & & & 300 & \(9.01\times 10^{-3}\) & 1363 & \(-193.2\) & 1.46 & 3.8 & 0.0 \\ \hline F050(2)Figure 3 shows the relationship between the peak and peak principal stress ratios, and the confining pressure. Untreated sand had almost constant values of peak stress ratio (q/p\({}^{\prime}\) = 1.4) and peak principal stress ratio (\(\sigma^{\prime}_{1}\)/\(\sigma^{\prime}_{3}\) = 4), regardless of the confining pressure. These peak values increased as the CC increased. On the other hand, both peak stress ratios of the treated sands decreased as the confining pressure increased. This is because the cementation, caused by the calcite precipitated between the sand particles, is destroyed as the confining pressure increases. If the confining pressure becomes much higher, the treated sand may show similar behavior to the untreated sand. Figure 3: Peak stress ratio and peak principal stress ratio for samples with different CCs against confining pressures: (**a**) peak stress ratio; (**b**) peak principal stress ratio. Figure 2: Typical results of undrained triaxial tests with different CCs. Figure 4a shows the [PERSON]'s stress circles at different confining pressures. The friction angle and cohesion were obtained from the [PERSON]'s stress circles and compared for different CCs. Table 3 summarizes these strength parameters. As the CC increased, both the cohesion and friction of the treated sand increased due to the cementation of sand particles [23; 24].