A series of experiments were conducted in a large flow-structure-soil interaction flume to analyze the scour development and pore-water pressure response around a monopile foundation under the action of combined waves and currents. In the experiments, the scour depth and pore pressure response around the pile were measured simultaneously. The experimental results indicate that the maximum equilibrium scour depth due to waves plus currents is greater than a linear sum of those caused by waves and currents respectively. This nonlinearity effect is particularly obvious when the sand-bed condition under currents or waves alone is in clear-water regime. The maximum equilibrium scour depth normalized with pile diameter is closely dependent on the Froude number with increasing the wave-induced water particle velocity meanwhile the current velocity keeping constant. The wave-induced pore pressure gradient around the monopile under the wave trough weakens the buoyant unit weight of the surrounding sand and induces the sand-bed more susceptible to scouring.
In recent decade years, quite a few monopile foundations have been utilized in shallow-water subsea locations for economically constructing fixed-bottom offshore structure systems, e.g. oil platforms, offshore wind farms and long-span cross-bay bridges. Local scour and soil liquefaction usually occurs around the pile due to various hydrodynamic loads, e.g. waves, unidirectional currents and tidal. As such, the bearing capacity of the monopile is reduced and the stability of costal structures could be threatened. In most parts of coastal and shelf seas, scour process becomes more complex owing to the coexistence of waves and currents than those in the cases of waves or currents alone. The local scour around a monopile or pile groups under steady flow or current alone was originally studied for the scour protection design of the bridge piers in the river.
