## 3 Findings and Discussion
### The Effect of the Distance between Helixes
While the physical model test and 3D numerical analysis results showed similar behavior, no significant capacity increase was found in the 2D numerical analysis results (see Figure 5i). In other words, the optimum spacing ratio between helixes was obvious  
Figure 6: The effect of the distance between helixes on the failure mechanism. in the results of the physical model tests and 3D analyses, whereas this finding was not observed in the results obtained from the 2D analyses. Based on the physical model test results, a failure load of 359 N was obtained at S/D\({}_{1}\) = 1, 520 N was obtained at S/D\({}_{1}\) = 3, and 364 N was obtained at S/D\({}_{1}\) = 8, and therefore, the maximum load was found at S/D\({}_{1}\) = 3. In the 3D numerical analysis results, the maximum pile capacity was determined to be S/D\({}_{1}\) = 3-4. In other words, in the case where the buried depth of the upper helix was more than 5D\({}_{2}\), the pile capacity was determined to be the maximum in both the physical model test and the 3D numerical analysis. It was understood from the results that the effect of the soil zone over the upper helix on the pile capacity is more important than the distance between the helixs. It can be said that in helical piles at a buried depth of less than 5D\({}_{2}\), a shallow foundation behavior rather than a deep foundation behavior is effective for the upper helix, and therefore, the contribution of the upper helix plate to the capacity is less. Plus, there was not a significant capacity increment in the 2D analysis results as the spacing ratio between helixs increased. According to these results, if the multiple helical pile is to be used, it is recommended that the distance between the helixs and the buried depth of the upper helix be adjusted to be more efficient. Therefore, the recommended buried depth of the upper helix for this study is a value greater than 5D\({}_{2}\). The effect of the distance between helixes on the failure mechanisms obtained from the 3D finite element model is presented in Figure 6. It is known that multiple helical piles have cylindrical or individual failure mechanisms depending on the distance between helixes [20; 30; 48]. Therefore, this situation was clearly confirmed when the failure mechanisms were examined. While the failure mechanism in closely spaced multiple helical piles exhibited cylindrical behavior, as the distance between the helixes increased, an individual failure mechanism was obtained (see Figure 6). The intensity in the cylindrical failure mechanism was more pronounced at the spacing ratio between helixes of S/D\({}_{1}\) = 1. As the distance between helixes increased (e.g., S/D\({}_{1}\) = from 1 to 2), although the failure mechanisms appeared to be roughly cylindrical, the intensity began to cluster around the helixes, and this intensity decreased along the shaft between two helixes. It was observed that at S/D\({}_{1}\) = 3, the most intense zone was still around the helix, and this intensity around the helixes was preserved at all increasing spacing ratios. It was noticed that the interaction between the helixes decreased, and the failure mechanism reached the soil surface along with the spacing ratio of S/D\({}_{1}\) = 4. In other words, the failure mechanism showed a transition from a deep foundation to a shallow foundation at this spacing ratio. The decreases in helical pile capacity occurring after these spacing ratios (e.g., S/D\({}_{1}\) = 4, 5, 6, 7, and 8) can be explained by this behavior change observed in the failure mechanisms.