## 1 Introduction  
Helical piles have been used as the foundations of offshore structures over 100 years [1] to resist tension and compression. The typical helical pile consists of a central shaft and a serious of steel helixes welded on it, as shown in Figure 1. Helical piles have been employed extensively to support both tensile and compressive forces in telecommunication towers, energy transmission, distribution lines, solar panels, and onshore wind turbine foundations. Helical piles can be installed with torque and minor compression, quickly and conveniently. Due to the advantages of helical piles, such as: the fast installation process, high uplift capacity and the convenience for recycling, it is being treated as a kind of novel foundation for offshore wind turbines by some scholars [2; 3]. If helical piles were used as the foundation of an offshore wind turbine, there are three important parts for geotechnical design: (i) the installation process of helical piles; (ii) the loading procedure of helical piles after the installation process; (iii) the recycling process when the loading procedure of helical pile is finished. For the installation process of helical piles in sand, some scholars have done some research through model tests [4; 5; 6; 7; 8] and centrifuge tests [9; 10]. Based on the test results, there are several findings and contributions: (1) the installation process of a helical pile would cause some disturbance to the surrounding soil and the soil traversed by the helix is displaced laterally, which causes the lateral soil pressure to increase during the installation process of a helical pile; (2) based on the tests result and inspired by the lead screw driver theory, some semi-empirical solutions were proposed to predict the installation torque of helical piles in sand; (3) there is a linear relationship between the uplifting capacity of a helical pile, \(Q_{\mathrm{u}}\), and the corresponding torque, \(T\), required for installing helical piles to the desired depth; the ratio between \(Q_{\mathrm{u}}\) and \(T\) is the so-called torque correlation factor (\(K_{\mathrm{T}}\)). The value of \(K_{\mathrm{T}}\) is influenced by the sand's relative density and the shape of the helical piles. For the loading procedure of helical piles after installation process, the main research objective is the uplift capacity of helical piles in sand. Many scholars treated single-plate helical piles as circular plate anchors due to the similar behavior under a tension load when the pitch of helix was small compared with the size of its outer diameter, and a number of single gravity experiments were conducted [11; 12; 13; 14; 15; 16; 17; 18; 19; 20] to invest the uplift capacity of circular plate anchors and helical piles in sand. Figure 1: The definitions of typical helical pile. Besides, [PERSON] et al. [21] conducted a series of uplift tests in dense silica sand through geo-centrifuge to analyze the influence of the relative embedment depth of the helix, number of helices and the spacing of adjacent helixes on the uplifting capacity of helical piles in dense sand. [PERSON] et al. [22; 23] analyzed the uplift-capacity of circular plate anchor through FEM. After finishing their service, helical piles could be recycled from sand easily with reverse torque, just like the removal process of a screw nail because of their particular shape. The recycling process of helical pile is the key point for the reuse of a helical pile and the cost reduction; however, studies on the recycling process of helical pile is still rare and there is no basis to confirm the equipment for the recycling process. Therefore, some relevant study about the topic is very necessary.