## 2 Experimental design
### Test Device  
In previous similar experimental studies, researchers usually simplified the marine environmental load. The main focus is on the characteristics of the low-frequency cycle of wind, wave and current loads, and the completely one-way or two-way load form determined by experience is adopted, and only the regular action of equal-amplitude loads is considered. Therefore, the amplitude and frequency of the load cannot be changed arbitrarily in a certain test, but this is inconsistent with the actual sea state. In order to better solve this problem, a complex dynamic load loading system was independently developed in this experiment in the early stage of design. This loading system can apply the load time  
Figure 5: The simplified diagram of load.  
Figure 6: The result of combined wave and current load.  
history results obtained by theoretical calculation to the model structure in a custom way, which can realize asymmetric and irregular ocean load loading.  
As shown in Figure 7, the whole loading system is divided into four parts: model box, reaction frame, actuating device and data acquisition device. Because the iterative algorithm is embedded in the servo control system of the actuator, it can realize dynamic loading in a large frequency domain and meet the requirements of precisely controlling the load amplitude and frequency in this model test. The loading system can realize control methods such as force control and displacement control, so that the dynamic load time history obtained by theoretical calculation can be accurately applied to the model structure.  
For dynamic loading model tests, boundary effects cannot be ignored. In the dynamic test, [PERSON] et al. (2010) [21] proposed that when the distance between the model structure and the rigid boundary of the model box is greater than three times the pile diameter, the boundary effect caused by dynamic loading can be ignored. For the test conditions of this study, the distance between the bottom of the model pile and the bottom of the rigid box is 0.7 m, which is greater than ten times the pile diameter. The vertical distances between the model pile wall and the rigid box side wall are 0.47 m and 0.72 m, respectively, which are also greater than 7 times the pile diameter. Therefore, the dynamic loading boundary effect of the model test in this study can be ignored. The specific relative position relationship is shown in Figure 7.  
The bending moment of the pile body during the test was monitored by 24 strain gauges arranged on both sides of the pile body, and the spacing between each two groups of strain gauges was 0.06 m. Among them, 1 pair is at the soil surface, 2 pairs are above the soil surface, and 9 pairs are below the soil surface. By arranging the strain gauges symmetrically on both sides of the pile body, the bending moment of the pile body of the pile section can be obtained by using the bending moment calculation method.  
For the measurement of pore water pressure at different positions around the pile during the test, 8 pore water pressure gauges were used, and the arrangement of the pore pressure gauges adopted the arrangement suggested by [PERSON] (2003) [22]. In the shallow soil, four pore water pressure gauges are arranged before and after the axis of the pile body, which are divided into two categories: far field and near field. Two pore water pressure gauges are arranged in the middle soil body and the deep soil body at the position of one pile diameter before and after the axis of the pile body, which are only arranged in the near field.  
The above two types of test data are collected by the JM3841 dynamic strain acquisition instrument, and the sampling frequency of this model is 50 Hz; for the measurement of  
Figure 7: Sketch map of test device.  
pile displacement and load, the dial indicator and the data acquisition system that comes with the actuator are used to collect. The distribution positions of the sensors in the whole test system are shown in Figure 8.