Effects of sediment densification and strengthening on scour around monopiles using mangrove-inspired skirt piles

The intricate prop root system of mangrove forests forms a natural barrier that traps the sediments and reduces coastline erosion, which provides a design inspiration to mitigate local scour around a monopile foundation. In this study, a ring of mini skirt piles with various spacings was proposed to mimic the mangrove prop root system. It is hypothesized that, in addition to hydraulic benefits, installation of the skirt piles also densifies and strengthens the sediments around the monopile and thus enhances the shear strength against erosion. The hypothesis was first tested with laboratory flume experiments considering four different installation sequences. The discrete element method was then used to model the pile installation process and investigate the evolution of sediment density and stresses. The flume tests validated that installation of the skirt piles reduces scour potential. The simulation results revealed that the installation of skirt piles causes densification of the sediments and strengthens the contact forces. Such effects were more pronounced when skirt pile spacing was smaller. Both numerical and experimental results indicate that the installation of skirt piles provides geotechnical benefits as part of the scour countermeasure.