With the increasing capacity of offshore wind turbines, the diameter of wind power monopiles has been continuously growing, leading to a significant decrease in the length-to-diameter ratio (L/D). Existing methods primarily focus on correcting the p-y curve due to increased pile diameter but fail to adequately consider the impact of changes in resistance components distribution resulting from a decreased L/D. This study aims to analyse the pile-soil interaction of large-diameter horizontally loaded monopiles by examining the distributed resistance components. Through finite element analysis and verification via engineering pile testing, the paper explores the resistance composition, deformation characteristics, and changes in resistance components of large-diameter monopiles. The findings reveal that the pile-soil horizontal resistance primarily governs the lateral bearing capacity of large-diameter monopiles for small range of length-to-diameter ratios (4 similar to 10). It is found that the deformation mode of monopiles is controlled by the pile-soil relative stiffness. A p-y resistance component curve for large-diameter horizontally loaded monopiles under various interlayer states of sand and clay was proposed as a function of pile-soil relative stiffness. For engineering practice, a simple but useful method for evaluating and correcting the lateral bearing capacity of monopiles was demonstrated based on the proposed resistance component curve.

Under long-term horizontal cyclic loading, the evolution characteristics of the loading and unloading stiffness of piles are an important representation of pile-soil interaction. However, research in this area is limited, particularly regarding the impact of factors like pile-soil relative stiffness. In this study, laboratory tests with a long-term horizontal cyclic loading strategy were conducted to study various factors, including different cyclic amplitude ratios (zeta b), cyclic load ratios (zeta c), and pile-soil relative stiffness (T/L) in sandy soil, on dynamic pile head stiffness. The results show that the normalized cumulative displacement increases with the number of cycles and the ratio of T/L but tends to decrease as zeta c increases. As zeta b increases, the normalized cyclic loading stiffness also rises, while it has little effect on the normalized cyclic unloading stiffness. On the other hand, as zeta c or T/L increases, the cyclic loading stiffness increases while the unloading stiffness decreases. Based on these observations, prediction formulas for normalized cumulative displacement and cyclic loading and unloading stiffness were established and confirmed with test results. The findings of this study provide methodological references for establishing models of pile-soil interaction under cyclic loading and for predicting loading and unloading stiffness under different influencing factors.