## 3 Evaluation of Cyclic Behavior of OWT Foundation under Cyclic Loads  
### Testing System and Procedure  
The target load was selected as the design load of the OWT demonstration site. The ULS design load of 47,000 kN-m at the center of the foundation (i.e., the intersection point of the tower and ground surface) was adopted in design load scenarios. The load was assumed to be directed windward with a bucket support, such that it was extremely vulnerable to displacement, as shown in Figure 6. Based on the ULS load, the load amplitudes to be used for each experiment were calculated by the limit state definition introduced in [19]. The testing loads applied in each test are listed in Table 5.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Item**} & \multicolumn{2}{c}{**Tripod Bucket Foundation (70:1)**} \\ \cline{2-3}  & **Prototype (m)** & **Model (mm)** \\ \hline Tower diameter, \(\text{D}_{\text{T}}\) & 4.5 & 64 \\ \hline Skirt diameter, \(\text{D}_{\text{S}}\) & 6 & 86 \\ \hline Tower length, \(\text{L}_{\text{T}}\) & 78.4 & 1120 \\ \hline Foundation length, \(\text{L}_{\text{F}}\) & 20 & 286 \\ \hline Center-to-center, C & 20 & 286 \\ \hline Skirt thickness (t) & 0.07 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Dimensions of the OWT model and prototype.  
Figure 6: Direction of loading during testing.  
In these tests, the soil was set to be fully drained to exclude the effect of excess pore pressure on the foundation behavior during the loading tests. Although partially drained and undrained conditions may develop in reality, the main purpose of this study is to evaluate the interaction between the skirt and the soil in drained conditions. The OWT was inserted into the saturated soil by using the linear actuator at a constant rate of 0.1 mm/s, ensuring fully drained conditions during the installation process. The fully drained condition of the ground was confirmed based on the following criteria for the normalized penetration velocity (\(V\)):  
\[V\ =\ \frac{\
u\cdot d}{c_{v}}, \tag{3}\]  
where \(\
u\) is the loading rate (0.1 mm/s in the model scale), \(d\) is the bucket diameter (64 mm in the model scale), and \(c_{v}\) is the vertical coefficient of consolidation (3226 mm\({}^{2}\)/s in the model scale of [20]). In these experiments, \(V\) is 0.002, which is lower than the maximum V for the drained conditions (i.e., \(V<0.01\)) suggested by [21]. Hence, it can be concluded that the monotonic and cyclic behaviors of the bucket observed in these tests indicate a fully drained scenario.  
A 1-D horizontal actuator, operated with displacement control, was used to generate cyclic loadings at a centrifugal acceleration of 70 g. The loading actuator could apply the cyclic loading up to 50 cycles continuously. After 50 cycles, the loading sequence was paused for 2 s and then reapplied. To minimize the influence of the frequency, the loading frequencies of all the cyclic tests were maintained at 0.1 Hz. Laser sensors and linear variable differential transformers (LVDTs) were installed to measure the displacement of the tower and buckets of the tripod, respectively. In addition, a load cell was set up to quantify the load applied to each bucket. The data measured by the transducers were collected at a sampling rate of 10 Hz. Figure 7 shows a schematic of the test setup.  
The testing program consists of monotonic and cyclic tests. All testing conditions are listed in Table 5. Standard, O [19] proposed the following cyclic design loads: (1) the ULS based on the ultimate capacities, (2) the SLS that occurs approximately \(10^{2}\) times during the lifetime of the wind turbine, and (3) the FLS that occurs approximately \(10^{7}\) times during the lifetime of the wind turbine. The SLS and FLS are estimated to be 50% and 30% of the ULS, respectively [5].