The effect of scour on monopile lateral behaviour

A series of centrifuge experiments examining the effect of scour hole formation on the lateral ultimate limit state (ULS) behaviour of monopiles is presented. Model foundation tests were performed at 100 gravities (100g) of a prototype pile 5 m in diameter with a pre-scour hole embedment of L/D = 5. The testing schedule saw four separate scour cases - 0.5D, 1.0D and 1.5D local scour (a conical formation around the monopile) and 1.0D global scour (a complete loss of embedment). Observations reveal a significant reduction in lateral stiffness and ultimate capacity with an increase in scour depth. In addition design codes are proven to be conservative in their recommendations for dealing with scour. Local p – y curves along the depth of the monopile are presented and exemplify the effects of overburden stresses from soil outside of the scour formation; it would appear that at depth, the effects of scour on soil strength become negligible.