Laboratory study on scour development around a monopile in layered soils

Scour in layered soils remains an understudied topic. Existing predictive frameworks are based on limited experimental campaigns and often assume the characteristics of the lower layer, leading to over or underestimations. This study aims at addressing existing gaps through physical modelling of scour around a monopile under a steady current in clear-water. Layered, non-cohesive soils were tested, with variations in uniformity coefficient, and upper layer thickness. In double-layered soils, scour depth lies between that of the upper and lower sands in single layer configuration. For fine sand over coarse sand, a thin upper layer produced scour depths similar to that of a single layer of the coarser sand, while a thick upper layer limited scour to its own thickness. For coarse sand over fine sand, the uniformity coefficient of the upper layer is an important factor. In this case, scour depth is closer to that of the single layer coarse sand as the uniformity coefficient increases. Additionally, in general, the scour extent of the upper layer seems to be a good predictor of scour extent in double-layered soils. These findings may improve the design of scour protection measures, with particular implications to dynamic scour protections.