## 5 Conclusions  
The lateral load response of monopile foundations in clay was studied using 3D numerical modeling and the 1D standard p-y method from common codes. The model was validated against a large-diameter monopile test by the PISA project team at the Cowden site. The methods were used to evaluate the lateral response of monopiles with different geometries under small (service) and large (ultimate) displacements. The study also identified the contribution of various resisting components to the lateral capacity of monopiles with different slenderness ratios and explored design aspects like minimum embedment length criteria. The main conclusions are summarized as follows:  
* The results of the FE and 1D DNV p-y models show that, as the L/D ratio increases for a specific diameter (embedment length increase), the lateral capacity also increases, but the rate diminishes with greater embedment length. This trend is more pronounced at small displacements. For large displacements, increasing embedment length leads to a continuous capacity increase, with the rate declining at an L/D ratio of 8 for both methods. For small displacements, this decline occurs at an L/D ratio of 6, beyond which additional capacity gains are negligible. This suggests that for small displacements or service criteria, a shorter embedment length can be chosen compared to what is needed for ultimate lateral capacity. * The results showed that among the three minimum embedment length criteria analyzed, the zero-toe-kick criterion led to excessively large embedment lengths. In contrast, the DNV recommended and vertical-tangent criteria provided shorter lengths, with the DNV recommended criterion being the shortest. * The FE model predicts a higher capacity than the 1D DNV p-y method, particularly at large displacements. This discrepancy arises because of the difference between the distributed lateral reaction on the monopile in the FE model and the p-y curves considered in the 1D DNV p-y method. Additionally, the 1D DNV method does not consider supplementary resisting components such as vertical shaft shear forces, base shear, and the base moment. * The results showed that the maximum and minimum differences between the 3D FE model and the 1D DNV p-y method were 41% and 11% at large displacements, and 32.5% and 13.3% at small displacements. As the slenderness ratio increases, the difference decreases, averaging 13% at an L/D ratio of 10 across all monopile diameters for both displacement domains. This trend is expected, as a more slender monopile better matches the conditions for which the 1D DNV p-y method is designed. Thus, for L/D ratios of 10 or higher, the 1D method can predict capacity with reasonable accuracy. * Comparing the 1D DNV p-y method with the 1D FE-extracted p-y curves showed that as the slenderness ratio increased from 4 to 10, the results of the two models converged, reducing the impact of supplementary resisting components. However, the results never fully aligned, highlighting differences in the p-y curves used. * The primary resisting moment across all slenderness ratios comes from the distributed lateral load along the monopile shaft (\(M\)CR\({}_{\rm p-y}\)). This contribution increases with slenderness, rising from about 87% at an L/D ratio of 4 to 96% at an L/D ratio of 10. The results showed that the base moment \(MCR_{BM}\) minimally contributed to the moment resistance of the monopile, with about 3 percent at an L/D ratio of 4 and nearly zero at an L/D ratio of 10. Except for an L/D of 4, where vertical shear stresses along the pile shaft \(MCR_{M-\theta}\) contributed more, \(MCR_{M-\theta}\) and base shear \(MCR_{BS}\) had similar contributions at other L/D ratios. Conceptualization, A.K. and D.L. ; methodology, A.K. and D.L. ; software, A.K. ; validation, A.K. ; formal analysis, A.K. ; investigation, A.K. ; resources, D.L. ; data curation A.K. and H.P. ; writing--original draft preparation, A.K. ; writing--review and editing, A.K. and D.L. ; visualization, A.K. and H.P. ; supervision, D.L.