Laboratory model tests to assess the effects of scale and stress on the stiffness of laterally loaded foundations in sand

Offshore wind farms have the ability to meet the energy demands of the USA four times over, and are located offshore where less turbulent, high-speed winds can generate electricity efficiently. The most popular option for offshore foundations are monopiles, which are characterized as large diameter steel pipe pile suited for relatively shallow depth (20 – 30m) so they have a large aspect ratio (length divided by diameter or L/D) of between 3 and 8. The most common design methods for laterally loaded monopiles rely on p-y curves, which were developed for relatively small-diameter and slender piles and might not be suited to predict the behavior of larger diameter and smaller L/D ratios. In order to evaluate the suitability of p-y curves to predict the load – displacement behavior of laterally loaded monopiles a consistent laboratory testing methodology to perform 1-g lateral load tests in sand was developed. The aim of this research is to compare the results from the 1- g lateral load tests to compared to the predictions of a newly developed 3D Finite Element Method (FEM) analysis as well predictions from the American Petroleum Institute (API), which utilizes the p-y method. A repeatable and consistent methodology to conduct a large number of 1-g lateral load tests in sand using spheres to model the state of stresses around a monopile for varying sphere diameters and embedment depths was developed. The sand was characterized by its dynamic properties, which were used as input parameters for the 3D FEM analysis. A total of 50 1-g model tests were conducted and analyzed. The results indicate that the 3D FEM is able the non-linearity in initial stiffness if accurate dynamic properties are used. Additionally, p-y were not able to capture the initial non-linearity in the load – displacement behavior and underestimated the initial stiffness for all tests.