## 2 Materials and Methods  
### Physical Model Test  
The physical model tests were performed in the Geotechnical Laboratory of the Department of Civil Engineering of Iskenderun Technical University. In the physical model tests, the compression force was applied by the load motor fixed to the test box. While the loading device with speed control is employed, the loading arm can be moved up and down in different directions. Whilst performing the tests, the load was applied gradually, and the speed of the compression load was kept constant at each step. A rectangular steel-framed test box with internal dimensions of 1500 mm (length), 1200 mm (width), and 1000 mm (depth) was used. During the tests, an electronic load cell with a capacity of 20 kN was preferred to read the load values applied to model piles. The displacements of the model piles were measured by employing two different displacement transducers (LVDTs). The displacement values of the pile system were obtained for each load stage by calculating the arithmetic mean of the displacement considered at two different points that were equal distances to the center of the system. During the tests, the displacement and load values saved by the data logger were conveyed to the software, and they were converted into numerical values. In addition, the test apparatus with all related attachments is presented in Figure 2 to show the test setup consisting of the test box, loading system, sandy soil, model pile, load cell, and displacement transducers. The shaft lengths of model steel pipe piles employed in the tests were equal to 600 mm and the outer diameters of them were equal to 22 mm. Since the effect of upper helix diameter on the bearing capacity was one of the parameters investigated, the diameter of the lower helixes (D\({}_{1}\)) was kept constant at 60 mm while the diameters of the upper helixes (D\({}_{2}\)) were considered as 60 mm, 80 mm, 100 mm, and 120 mm. The pitch of the helix for the lower and upper plates was accepted as 0.3D\({}_{1}\) and 0.3D\({}_{2}\), respectively. To mention the investigated parameters and the test program, the number of helixes (N = 0, 1, 2), the spacing ratio between helixes (S/D\({}_{1}\) = 1-8), and the diameter of the upper helix (D\({}_{2}\) = 60 mm, 80 mm, 100 mm, and 120 mm) were considered as the variables. The test program is given in Table 1, and the effects of these parameters on the bearing capacity of the helical piles are presented in detail thereafter. Figure 2: _Cont._  
Various methods are available to prepare a sand body in the laboratory like vibration, compaction, and the raining technique. In physical model tests, having uniform strata is significant to allow for the repeatability of tests [33]. Considering the foregoing methods, the raining technique was selected to acquire the required relative density of the sand. In the physical model tests, sand samples brought from Iskenderun sand quarry were used. In accordance with ASTM D2487-11 [34], they were washed and sieved through sieve number 10 (2 mm dia.) and sieve number 80 (0.18 mm dia.). Following this, they were dried in an oven at the end of the sieving process. Afterward, sand samples were sieved through predetermined sieves, taking into account the standard mentioned above, and the grain size distribution was determined (see Figure 3). The soil class was identified as SP based on the grain size distribution curve. The sieve analysis results are presented in Table 2.