# Horizontal Loading Performance of Offshore Wind Turbine Pile Foundation Based on DPP-BOTDA
## 4 Conclusions  
In this paper, the distribution strain values along the length of the pile under each load were obtained. These distributed strain values can intuitively reflect the overall deformation of the test pile. According to the \(F\)-\(\varepsilon_{\text{max}}\) curve, the horizontal bearing capacity of the pile foundation can be judged. In this test, the maximum strain at the load of 900 kN was 1436 \(\mu\varepsilon\), and the position was at \(-19.63\) m. Through the linear relationship between the load and the maximum strain, we can get a strain coefficient under a load of 1.631 \(\mu\)\(\varepsilon\)/kN. The maximum stress was 295.8 MPa, which did not exceed the yield stress of Q345 steel. The stress ratio of 0.86 indicates that the pile was still in the elastic stage, and this pile-bearing capacity can reach 900 kN, which satisfies the standard of horizontal load capacity of offshore wind turbines. Secondly, the distributed displacement values of the test pile are calculated by the strain values. The maximum displacement generated under the 900 kN load was 0.79 m. Finally, in order to verify the true reliability, the calculated displacement data were compared with the measured displacement data by dial indicators. The MAPE value was only 0.03548.  
Figure 11: Displacement data at +10.5 m elevation collected by the dial indicators during the test.  
Figure 12: Displacement data at +10 m elevation collected by the dial indicators compared with displacement data obtained by the distributed strain calculation under horizontal loads.  
Compared with traditional electrical strain sensing systems, the transmission signal of distributed optical fiber sensing technology is a light wavelength signal, which is resistant to corrosion and lightning and subsequently is not subject to meteorological conditions or geographical locations. For the similar wind turbines and different types of piles, such as steel pipe piles, reinforced concrete piles, and steel-concrete piles, due to the similar force characteristics in the soil, this method is also applicable to the test of their bearing capacity. However, the DPP-BOTDA technology needs an optical circuit, and the optical fiber may be damaged during the piling process. So, the protection and survival rate of optical fiber deserves attention. We believe that the proposed DPP-BOTDA can provide accurate distributed measurements for many offshore wind turbine pile foundations.  
Z.Z. (Data curation, Investigation, Writing--original draft preparation); PG. (Data curation, Investigation); J.X. (Validation, Formal analysis); B.W. (Formal analysis); H.L. (Supervision, Writing--Review, Editing, and Validation); Y.D. (Supervision, Funding acquisition, Writing--Review, Editing, and Validation). All authors have read and agreed to the published version of the manuscript.  
This work was funded by the National Key Scientific Instrument and Equipment Development Project of China 2017 YFF0108700; National Natural Science Foundation of China 61575052. Photographs in this paper were taken by authors.  
The authors declare no conflict of interest.