PHYSICAL MODELLING OF PILE TIP DAMAGE ARISING FROM IMPACT DRIVING

There have been a number of incidents worldwide where pile tip damage has occurred during pile driving, generally requiring costly remedial actions. The number of such occurrences has increased recently, consistent with the trend of increasing diameter tubular piles, which are increasingly thin-walled relative to the diameter, used to support offshore wind turbines either as monopiles or as part of a jacket structure. As the wind industry moves into new regions of the world, challenging soil conditions are becoming more common, in particular with increasing risks from embedded boulders or partially weathered soft rocks including chalk and limestone. Pile tip damage may occur as a rather abrupt tip 'crumpling', or as a more gradual progressive 'extrusion buckle'. Both are likely to be triggered by localised heterogeneous hard zones within the sediments, with the rate of growth determined by the stiffness of the surrounding soil matrix. The paper reports some results from a pilot series of centrifuge model tests where thin-walled piles were driven in flight into a sand bed containing thin layers of either 'gravel' (representing boulders of up to 30% of the pile diameter), or weakly cemented material. Tip damage varying from barely perceptible to rather extreme was triggered, resulting in increasing driving resistance and premature refusal.