Experimental modelling of lateral loads on large diameter mono-pile foundations in sand

The last years several offshore wind farms have been completed. Such farms typically consist of a few dozen wind turbines. The majority of these wind turbines is founded on mono-piles with a diameter of 4–5 m and is designed according to standards that use the p-y method for lateral loading conditions. However, this p-y method is not validated for such diameter piles. Full-scale field tests and model pile tests with properly scaled stress conditions subjected to lateral loads are scarce. This Thesis investigates the effect of a diameter increase on the lateral bearing capacity of 2.2 m and 4.4 m diameter mono-piles in dry sand. Static displacement controlled and one-way cyclic force controlled model pile load tests were performed using the Delft University of Technology geotechnical centrifuge. Results show that the smaller 2.2 m diameter pile has a lower secant and tangent soil- pile stiffness when compared to the 4.4 m diameter pile. In all cases the tangent stiffness is about 50% higher than the secant stiffness. Also, a force acting higher above soil surface yields a lower initial system stiffness. Results furthermore show that an increase in pile diameter with a constant slenderness or L/D-ratio, relative density Id and load excentricity e, leads to a significant increase in static lateral capacity. The secant and tangent stiffness also significantly increase with increasing diameter. The accumulation of lateral pile displacements as function of the number of applied one-way cyclic lateral loads has been investigated using 500 force controlled cycles. For an increasing number of load cycles the pile head displacements increase whilst the rate of accumulation decreases. However, additional loading cycles, up to e.g. 100000, are recommended. For static loading conditions experimental results have been compared to the default API formulation for laterally loaded piles. This default formulation shows a significant overestimation of the initial stiffness. However, after incorporating a stress dependent secant stiffness E50, which has been derived from triaxial compression tests on model sand, good agreement is found between the experimental results and the modified API formulation for pile displacements