# Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand-Bentonite Mixture
## 4 Micro-Mechanical Behavior of Sand-Bentonite Mixture  
### DEM Model Set-Up  
The discrete element model was established to investigate the micro-mechanism of bentonite on the mechanical behavior of the sand-bentonite mixture. Firstly, the triaxial shear tests under different confining pressures were simulated. Figure 8 shows a three-dimensional DEM model to investigate the fine bentonite particle on the mechanical behavior of the sand-bentonite mixture specimen. The sand particles were modeled with the partial overlapping ellipsoidal particles, which can be observed in Figure 8a. The bentonite particle was modeled as the spherical particles, as shown in Figure 8b. As the calculation rate and the particle distribution of specimens in experimental tests are  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Maximum Deviator**} & \multicolumn{6}{c}{**Bentonite Content**} \\ \cline{2-7}  & \multicolumn{3}{c}{**Sodium Bentonite (Na\({}^{+}\))**} & \multicolumn{3}{c}{**Calcium Bentonite (Ca\({}^{2+}\))**} \\ \cline{3-7}  & & **5\%** & **10\%** & **15\%** & **5\%** & **10\%** & **15\%** \\ \hline \multirow{2}{*}{Confining pressure (kPa)} & 200 & 591.68 & 581.55 & 542.94 & 690.16 & 669.48 & 656.54 \\  & 300 & 936.20 & 925.97 & 865.15 & 1027.26 & 989.32 & 901.13 \\ \multirow{2}{*}{The first and second and second and third columns of Table 2} & 400 & 1217.75 & 1130.82 & 1107.21 & 1229.86 & 1196.67 & 1116.39 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test results of sodium bentonite–sand mixture and calcium bentonite–sand mixture.  
Figure 7: Working mechanism of sand–bentonite mixture.  
considered, the simulated size of sand particle was expanded to four times the size, and that of bentonite was 0.4 mm. The DEM model was established by fine particles of bentonite, which uniformly filled the skeleton of sand.  
The geometric dimension of the DEM models was the same as the specimens in experimental tests (39.1 mm in diameter and 78.2 in height). The sand-bentonite mixture model with a bentonite content of 10% shown in Figure 8c consisted of 38,912 pebbles and 28,510 balls. Considering that there is no bond strength between sand particles, a linear linkage model was adopted for the contact model of sand particles. However, given a certain bond strength between bentonite particles, a parallel connection model was adopted for the contact model of bentonite particles.  
The specific modeling processes are as follows.  
Step 1: Generated a cylinder with a diameter of 39.1 mm inside the specified domain. The upper and lower Boolean values of the cylinder were set to false so that the generated parallel walls could be used to apply axial pressure during the simulation of triaxial tests;  
Step 2: Imported clump template, generated sand particles of specified shape through clump distribute command and uniformly generate fine spherical bentonite particles;  
Step 3: Solved the model until the maximum unbalance force was less than \(5\times 10^{-5}\) N, which was regarded as a stable state. Then, the servo-controlled FISH function was imported;  
Step 4: Consolidated the model under the running environment of the FISH function. It was considered to be stable when the confining pressure change was less than 0.5%. Then the axial servo was released, and loading was started.