Large-diameter (4 to 6 m) rigid monopiles are often used as foundations for offshore wind turbines. The monopiles are subjected to large horizontal forces and overturning moments and they are traditionally designed based on the p-y curve method. The p-y curves recommended in offshore design regulations are developed for long flexible piles with diameters up to approximately 2.0 m and are based on a very limited number of tests. Hence, the method has not been validated for rigid piles with diameters of 4 to 6 m. The primary issues concerning the validity of the standard p-y curves for large diameter rigid piles are: The initial stiffness of the curves and description of the static behaviour, including ultimate bearing capacity. The behaviour with respect to cyclic loading both stiffness degradation and ultimate bearing capacity. The aim of the present research is to investigate the static and cyclic behaviour of large diameter rigid piles in dry sand by use of physical modelling. The physical modelling has been carried out at Department of Civil Engineering at the Danish Technical University (DTU.BYG), in the period from 2005 to 2009. The main centrifuge facilities, and especially the equipment for lateral load tests were at the start of the research in 2005 outdated and a major part of the work with the geotechnical centrifuge included renovation and upgrading of the facilities. The research with respect to testing of large diameter piles included: Construction of equipment for preparation of sand samples. Testing of preparation method. Modification of available loading equipment for lateral load tests. Calibration of equipment. Completion of a series of lateral load tests. The lateral load tests carried out represent prototype piles with a diameter of 1, 2 and 3 m with embedment lengths of 6, 8 and 10 times the diameter. The tests have been carried out with a load eccentricity of 2.5 m to 6.5 m above the sand surface. The present report includes a description of the centrifuge facilities, applied test procedure and equipment along with presentation of the obtained results.
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Technical University of Denmark, Department of Civil Engineering, 2011