## 3 Findings and Discussion
### The Effect of the Upper Helix Diameter  
In this section, the effect of the upper helix diameter was investigated using the physical model tests and 2D/3D finite element models. In accordance with this purpose, the multiple helical pile (N = 2) was considered in this series of tests. While examining the upper helix diameter, the distance between helixes was kept constant at S/D\({}_{1}\) = 3, which is the optimum spacing ratio, to eliminate the effect of other parameters on the results. The physical model tests were carried out for four different upper helix diameters such as D\({}_{2}\) = 60 mm, 80 mm, 100 mm, and 120 mm. The results of the load-displacement relationship obtained from the physical model tests and 2D/3D finite element models are presented in Figure 9; also, the failure mechanism results acquired from the 3D finite element model, which is in better agreement with the physical model tests, are given in Figure 10.  
Figure 9: The effect of the upper helix diameter on the load–displacement relationship.  
With the increase in the upper helix diameter, the pile capacity increased significantly. When the lower and upper helix diameters were the same, the helical pile capacity was approximately 520 N. As the upper helix diameter increased, the capacity was determined to be 580 N, 690 N, and 875 N, respectively. When the upper helix diameter was twice the lower helix diameter, the capacity increased by approximately 70% (Figure 9). This increase in capacity can be explained by the growth of the soil zone which the upper helix plate resists while compressive load is being applied. Thus, the upper helix plate in the multiple helical piles became a determining parameter for the capacity. Based on this result, in cases where multiple helical piles will be used in loose sandy soils, it may be recommended to make the upper helix plate larger compared to the lower helix plate to obtain maximum efficiency from the helical pile and to generate the required torque/capacity. When the results obtained from the physical model tests and finite element models were examined, it was determined that the physical model test and 3D finite element model results were in good agreement, and therefore, it can be expressed that the 3D model quite successfully reflected the physical model test behavior. On the other hand, the results of the 2D model were slightly below the test results.  
The effect of the upper helix diameter on the failure mechanisms obtained from the 3D finite element model is presented in Figure 10. When the failure mechanisms were examined, it was seen that in the helical pile with helixes of equal diameter, the failure mechanism showed an individual intensity around the helixes but created a cylindrical behavior with low intensity along the shaft between the helixes. As the upper helix diameter increased, the increase in capacity was reflected in the failure mechanism as a decrease  
Figure 10: The effect of the upper helix diameter on the failure mechanism.  
in displacement intensity. With the upper helix diameter of D\({}_{2}\) = 80 mm in the helical pile, there was displacement intensity around the helix plates even though it was not as intense as with the diameter of D\({}_{2}\) = 60 mm. This intensity reached its minimum level at D\({}_{2}\) = 120 mm. It can be stated that the increase in plate diameter caused a change in the failure mechanism behavior. As a result of this, the cylindrical formation changed to an hourglass appearance that widens in the area where the upper helix is located and narrows in the area where the lower helix is located.