# Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand-Bentonite Mixture
## 4 Micro-Mechanical Behavior of Sand-Bentonite Mixture
### Simulation Results and Discussion  
The mesoscopic parameters of the DEM model were preliminarily set according to the previous research results. The mesoscopic parameters were calibrated by comparing the numerical results with the experiment results. Table 3 shows the calibrated parameters for further analysis. The mesoscopic parameters that were calibrated were the basic parameters for the further simulated experimental test. The simulation results to be compared with the experimental test results were the results of the digital test that were conducted in the DEM simulation. The basic parameters that were calibrated and the compared results were different but related. Figure 8: DEM models for the Triaxial shear specimens: (**a**) sand; (**b**) bentonite; (**c**) sand–bentonite mixture. Figure 9 shows the results of the simulated triaxial tests under various confining pressures. Figure 9a,c,e shows the simulated maximum deviational stress, and the errors of maximum deviational stress between test and simulation are shown in Table 4. It is noted that the results from numerical simulations are in good agreement with the experimental results. However, the strain corresponding to the maximum deviating stress was different from the experimental results due to the difference in particle size between test and simulation. Figure 9b,d,f shows the volumetric strain in simulation. In the shearing process, dilatancy was followed by shrinkage, and the dilation became more significant with the increase in bentonite content. Figure 10 shows the nephograms of contact force distribution for the sand-bentonite mixture specimens with various bentonite contents, which were of an X-shaped distribution along the shear surface. The red particles represented the contact force greater than 20% of the maximum contact force, and the blue particles were for that lower than 20% of the maximum contact force. The green particles show the distribution of the fine bentonite particles in the specimen. It can be seen from the nephograms that the contact force distribution trended to be more uniform with the increase in bentonite content, especially under the condition of low confining pressure. The particles changed from red particles to blue particles with the increase in bentonite content; in other words, the probability of smaller contact force increased with the addition of bentonite. The larger contact force (red particles) turned to distribute more uniformly with the addition of bentonite, along with the gradual blurring of the boundary of the X-shaped distribution area. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Bentonite Content (\%)**} & \multirow{2}{*}{**Confining Pressure (kPa)**} & \multicolumn{3}{c}{**Maximum Deviational Stress (kPa)**} & \multirow{2}{*}{**Error**} \\ \cline{3-4}  & & & **Experimental Results** & & \\ \hline \multirow{4}{*}{0} & 200 & 719.15 & 751.87 & 0.044 \\  & 300 & 687.92 & 674.62 & 0.0198 \\  & 400 & 704.58 & 701.25 & 0.0047 \\ \hline \multirow{4}{*}{5} & 200 & 1102.6 & 1084.2 & 0.0169 \\  & 300 & 1009.3 & 937.5 & 0.0766 \\  & 400 & 1020.2 & 1043.7 & 0.0220 \\ \hline \multirow{4}{*}{10} & 200 & 1403.3 & 1339.0 & 0.0478 \\  & 300 & 1316.7 & 1072.3 & 0.226 \\ \cline{1-1}  & 400 & 1329.2 & 1315.9 & 0.0106 \\ \hline \end{tabular}
\end{table} 
Table 4: Errors of maximum deviational stress between test and simulation.