# Effect of Initial Granular Structure on the Evolution of Contact Force Chains
## 3 Results and Discussion
### Effect of Initial Granular Assembly Conditions on the Structure of the Contact Force Chain
It is worth noting that one can find the great similarity between the pattern of the distribution of the contact force chains shown in Figure 7 and the experimental data of [PERSON] [23] showing the transmission of contact forces along the lines inclined at 60 degrees relative to each other in the assembly consisting of the equal cylinders. In the case of a granular assembly where initial local structural defects existed, as shown in Figure 8, when a uniaxial compressive load was applied (UI), the support behavior was significantly different from that of a granular assembly where there were no structural defects (UU, Figure 7). It can be seen that the contact force chain structure was aligned with the main stress direction (i.e., the vertical direction), and not diagonally. This result is consistent with previous findings by other researchers [7; 8; 9; 10; 36]. However, it is worth noting that in this study, the contact chain structure was clearly arranged in the direction of major principal stress _only_ when there was an initial structural defect. Afterward, as the load increased, the number of vertical contact force chains increased, and the photoelastic reaction simultaneously become brighter. At load levels of \(\geq\)2000 N, it was observed that contact force chains began to form near the foundation plate in the diagonal direction, as well as the vertical direction. However, the number of diagonally developed contact force chains was much lower than that shown in Figure 7 (i.e., under UU conditions). Figure 9 shows the results of quantitative detection of the particle force in the photoelastic images of UU and UI using digital RGB photoelastic analysis. Figure 9a,b show the average particle force and the sum of the force that the particles receive for the detected overall granular assembly, as the load increased. As is shown, the UU and UI results were similar, regardless of the presence of local defects in the granular assembly. Figure 9c shows the average force of the particles that were classified as the main force chain, and the results indicate that the UI received a larger average particle force than the UU condition for the same load. In the case of UU, this can be interpreted to mean that all of the particles shared the external load evenly, while in the case of the UI, only specific structures (i.e., vertically arranged structures) share the load. Figure 9d shows the frequency of particles that were classified as the main force chain. At low load levels (\(<\) 2000 N), the particle frequency in the UU that was classified as the main force chain was greater than that in the UI, while at high load levels (\(>\) 2000 N), the particle frequency in the UI that was classified as the main force chain was higher than that in the UU. Figure 9e shows the sum of the forces of the particles that were classified as the main force chains. Initially, the main force chain of the UU structure seemed to support the external load better than the main force chain of the UI structure. However, as the load increased, the main force chain of the UI structure showed better support of the external load than the main force chain of the UU structure.