## 1 Introduction  
Helical piles, which are one of the special deep foundation types, are widely used in many fields of geotechnical engineering due to their ease of application and functionality. These types of piles that can resist various forces, such as compression and uplift forces, are manufactured effortlessly and brought to the relevant region via preparation outside the project site, and they are placed into the soil body by pushing or turning in an inclined or perpendicular way through a hydraulic torque machine. Helical piles were recommended by [PERSON] in the first half of the 1800s with the idea of finding a solution for safely anchoring ships in harbors and supplying strong foundations for lithothouses in weak soil in shallow water [1]. Since that time, helical piles have been applied in the foundations of marine structures (bridges, oceanfront piers, lighthouses, etc. ), electricity transmission and telecommunication towers, structures on slopes, buried pipelines, renewable energy source structures such as solar panels and wind turbines, shoring structures, damaged or long-term damaged structures, historical structures, and the construction of many other structures [2; 3; 4; 5]. Piles that support the offshore jacked/tripod structures are exposed to combined axial (uplift and compression) and lateral forces (Figure 1). However, helical piles are superior to conventional pile foundations [6; 7; 8] because they provide more resistance due to the helix [9; 10]. Despite the abundance of usage areas of helical piles, the increase in application, and the advantages they provide, it can be seen that research and design methods for helical piles are relatively limited compared to conventional pile foundations, and previous studies mostly focus on the uplift behavior [11; 12; 13; 14; 15; 16; 17; 18; 19]. In addition, it can be expressed that numerical efforts regarding the failure mechanism of helical piles are quite limited [11; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Therefore, since there is a necessity in the literature, albeit at a limited level, the compression behavior of helical piles has been investigated using physical model tests and two-dimensional (2D) and three-dimensional (3D) finite element models. This study may be assessed as a reasonable effort where the behavior of helical piles has been investigated via a physical model test first in the laboratory, and following that, 2D and 3D numerical analyses have been performed in order to observe the performance of the finite element method. A simplified linear geometry for helixes was established in a 2D finite element model whereas a real geometry for helixes, which was a more realistic approach, was created in a 3D finite element model. It has been observed that the valuable previous efforts on the subject so far are generally concentrated in the theoretical and experimental framework. However, in addition to the limited number of 3D numerical studies on the subject, the number of parameters investigated in such studies was quite limited compared to experimental studies. Moreover, theoretical studies related to helical piles focused on determining the net value of the bearing capacity by considering several assumptions while experimental studies generally focused on the parameters that affect the bearing capacity (number of helixes, spacing ratio between helixes, helix diameter, soil density, etc.) [29; 30; 31; 32]. In the experimental studies, where the effect of helix number on the bearing capacity was investigated, the helix diameter was generally kept constant, whereas the number of studies in which the effect of the location of the upper helix plate was investigated was quite limited. Therefore, it is thought that it is quite essential to study the effect of a large number of spacing ratios between helixes on the bearing capacity experimentally, and this effort provides notable novelty to the present study.