# Effect of Initial Granular Structure on the Evolution of Contact Force Chains
## 4 Conclusions  
In this study, the effect of initial granular structural defects on the support behavior of granular assemblies was confirmed through photoelastic model testing when assemblies, which were stacked with model particles, were subjected to uniaxial compressive or shallow foundation loading. The macroscopic analysis of the load-displacement relationship and the microscopic observation of the force transmission path inside the granular assemblies revealed that when the granular assembly was subjected to uniaxial compressive loading, there was no significant difference in the macroscopic load-displacement curve, regardless of the initial structural defects of the assembly. However, the distribution of the internal granular contact force chain structure in the photoelastic image was found to differ completely. In the case of homogeneous granular assemblies, contact force chains formed diagonally under loading, and the structure gradually expanded. However, in granular assemblies with initial structural defects, contact chains formed in the direction of the major principal stress, and the main force transmission structure was maintained and expanded until the end of the loading process. As a result of quantitative analysis of the photoelastic images, in the case of the granular assembly without initial defects, the particles were distributed evenly to support the external load, while in the case of the assembly with defects, only specific structures (i.e., vertically arranged structures) were found to share the load. When the granular assembly was subjected to a shallow foundation loading, a large difference was found in the macroscopic load-displacement curve depending upon the presence of initial structural defects. The maximum peak load of the granular assembly with initial defects was approximately 68% of the maximum peak load of the assembly without defects. The structures of the internal contact force chains in the photoelastic images were also found to differ substantially between the two assemblies. In the case of the granular assembly without initial defects, contact chains that spread diagonally at both ends of the foundation were observed, while in the case of the assembly with initial defects, it was confirmed that the contact force chain structure had a foundational width and was formed in the vertical direction (i.e., the major principal stress direction), as in the uniaxial compressive load. The expansion of local imperfections in the lower part of granular assemblies was observed when shallow foundation loading was applied by using PIV. From the initial state before loading, irregular voids in the lower part of the assembly expanded toward the central part of the assembly along with the load. Expanded zigzag-shaped voids broke the contact force chains of the particles. Likewise, according to the presence of an initial local instability with the granular assembly, a completely different type of contact force chain structure developed from the initial loading stage to the final failure stage. In particular, it was confirmed that the presence of imperfections greatly affected the bearing capacity during shallow foundation loading and buckling of the contact force chain structure due to particle rearrangement and rotation was found to occur more frequently. As a result, it appears that the behavior of the granular assembly (under uniaxial compressive loading) is significantly affected by the initial structural conditions of the assembly. However, since this experiment was performed under very limited conditions, it is necessary to verify the various cases and generalize the conclusions with respect the effect of initial defects on granular support behavior. Conceptualization, [PERSON]-H.J. and [PERSON]-H.P. ; methodology, K.-H.P. ; software, [PERSON]-H.P. and T.-Y.K. ; validation, [PERSON]-H.J., [PERSON]-H.P. and T.-Y.K. ; formal analysis, [PERSON]-H.J. and [PERSON]-H.P. ; investigation, [PERSON]-H.P. ; resources, [PERSON]-H.J. ; writing--original draft preparation, [PERSON]-H.P.