• Tina Højgaard Christiansen
  • Ida Folke Møller
4. term, Structural and Civil Engineering, Master (Master Programme)
This report is concerned with static and cyclic horizontal loading of monopiles used in offshore wind turbine foundation. Today the p-y method is used for determination of lateral bearing ca-pacity and deflections of piles. However, the reliability of the p-y method in connection with rigid piles, i.e. piles with low slenderness ratios, has been questioned.
In this project the efficiency of the p-y method is tested for a rigid pile and the design code rec-ommended p-y curves are investigated. In order to perform these investigations, a number of static pile experiments are executed as well as a number of p-y experiments.
All experiments performed in this project are 1g tests. Both experiments with dry and saturated sand are performed.
The p-y experiments indicate that the design code recommended p-y curves overestimate the initial stiffness and generally underestimate the ultimate bearing capacity. When applying the recommended p-y curves as nonlinear springs in a 2D Winkler beam model, the global response therefore underestimates the pile displacement to a given load and predicts an ultimate bearing capacity that is too low.
When applying the experimental p-y curves to a 2D Winkler model, the global response coin-cides with the results of the static experiments. This indicates the p-y method is a reasonable modelling approach when using correct input.
Alternative design methods are tested for static loading conditions. That is 3D modelling using Plaxis 3D foundation with non associated plasticity, and Abaqus with a user defined implemen-tation of Mohr Coulomb soil using associated plasticity. The best results are achieved using Abaqus.
9 cyclic experiments are executed with a displacement controlled test setup. The pile is loaded with 12,000-24,000 cycles. In the cyclic experiments, a steady state is observed. That is a condi-tion in which the soil properties have obtained their maximum potential and no longer changes. The results of the cyclic experiments indicate that the soil properties are improved due to cyclic loading, as both secant stiffness and ultimate bearing capacity are significantly increased.
Comparing the results of static tests performed in improved soil conditions after a cyclic test indicates the stiffness of the curves being the same as the one predicted by the design code rec-ommendation, however the design code significantly underestimates the bearing capacity.
Publication date17 Jun 2011
Number of pages93
ID: 53021001