# Undrained Behavior of Microbially Induced Calcite Precipitated Sand with Polyvinyl Alcohol Fiber
## 4 Results
### Effect of Fiber Inclusion  
The effect of fiber inclusion on shear strength and permeability was described in this section. The results for sand treated with 4% CC and with four different fiber contents (FC = the weight of fiber/the weight of dry sand) (0.0%, 0.2%, 0.4%, and 0.6%) are summarized in Table 4.  
Figure 4: Engineering properties of Ottawa sand treated by MICP: (**a**) Mohr’s stress circles; (**b**) cohesion and friction angle.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multicolumn{3}{c}{**Calcite Content (CC) (\%)**} \\ \cline{2-4}  & **0** & **2** & **4** \\ \hline Cohesion (kPa) & 0 & 34.2 & 93 \\ Friction angle (\({}^{\circ}\)) & 35.3 & 37.8 & 39.6 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Strength properties of samples with different CCs and without fiber.  
Figure 5 shows a typical result for the undrained triaxial test with different fiber contents (FCs) and a confining pressure of 300 kPa. The deviatoric stress increased continuously as the axial strain increased, regardless of the fiber contents. The peak deviatoric stress increased as more fiber was included. [PERSON] et al. [24] found similar results for reinforced cemented sand with 0.5% fiber. An excess pore pressure showed less dilative behavior as the fiber content increased. The relationship between the stress ratio and the axial strain showed a similar pattern to treated sand without fiber. The peak principal stress ratio increased as the cementation ratio and fiber content increased, and it decreased as the confining pressure increased [25,26].  
Figure 6 shows the relationship between the peak stress ratio and peak principal stress ratios versus the confining pressure. The peak stress ratios of the treated sand with fiber were much higher than that of the sand without fiber. The peak stress ratios were less affected by the fiber content.  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Test ID**} & **Average CC** & **Fiber** & **Confining** & **Hydraulic** & **Peak** & **Peak Fore** & **Peak Stress** & **Peak** & **Peak** \\  & **(\%)** & **Content (\%)** & **Pressure** & **Conductivity** & **Deviatoric** & **Pressure** & **Ratio** & **Princapal** & **Stress Ratio** & **CC (\%)** \\ \hline FOC30(4) & & & & 50 & \(4.07\times 10^{-5}\) & 874 & \(-107.4\) & 2.61 & 20.9 & 4.0 \\ \hline FOC100(4  
Figure 7a shows the Mohr's stress circles and failure envelope for the treated sand with and without fiber under three confining pressures. From these stress circles, the cohesion and friction angle were calculated and are compared in Figure 7b and Table 5. Both the cohesion and friction angle increased as the fiber content increased. The strength gradually increased up to a fiber content of 0.2% and then it remained constant. In this study, the optimum fiber content was approximately 0.4%. [PERSON] et al. [18] discovered that the UCS rapidly increased as the PVA fiber content increased up to 0.8% when the CC was high. [PERSON] et al. [17] showed that the shear strength of MICP treated sand with polypropylene fiber increased up to a ratio of 0.3%. Therefore, the optimum fiber content for sand treated by MICP could be influenced by the CC and testing methods.  
Figure 6: Peak stress ratio and peak principal stress ratio for samples with different fiber contents against confining pressure: (**a**) peak stress ratio; (**b**) peak principal stress ratio.  
Figure 7: Engineering properties of Ottawa sand treated by MICP and fiber: (**a**) Mohr’s stress circles; (**b**) cohesion and friction angle.  
As the fiber content increased, the average hydraulic conductivity decreased slightly due to the reduction in the void volume caused by the fiber and calcite within the sand particles. However, the effect of fiber inclusion on permeability appears to be minor compared to the precipitation of calcite [18].