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Experimental Study on Scour Around Monopile and Pile Group in Silty Bed Under Steady Flow Conditions
Shengtao Du, Yingxue Lv, Chaolin Wang, Guoxiang Wu, Fangfang Zhu and Yakun Guo

Local scour around pile foundations in silty seabeds remains difficult to predict because cohesive sediment behavior and pile interference can strongly modify erosion patterns. This study presents steady-current flume experiments on scour around a single pile and a 3 × 3 pile group installed in a silty bed simulated using quartz powder. The tests were designed based on incipient-motion similarity and conducted at a constant water depth of 0.40 m and approach velocity of 0.2125 m/s for 9 h. Bed morphology was surveyed hourly using a handheld laser scanner to obtain high-resolution three-dimensional point clouds, and flow conditions were characterized using an ADV. For the single pile, scour developed rapidly during the early stage and then approached a quasi-equilibrium depth while the scour-hole extent continued to expand. The temporal evolution was well fitted by an exponential function, yielding estimated equilibrium scour depths of 1.55D at the pile front and 1.40D at the pile rear. A distinctive silty-bed feature was that front and rear scour depths were comparable, indicating an enhanced role of wake dynamics relative to sandy-bed expectations. For the 3 × 3 pile group, inter-pile interference produced strong spatial heterogeneity and generally reduced scour compared with the single-pile case, with group-to-single scour depth ratios ranging from 0.14 to 0.89 depending on pile position. Notably, downstream piles exhibited non-monotonic evolution with scour depth peaking and subsequently decreasing. This result challenges monotonic equilibrium assumptions for predicting scour within pile groups.

Keywords: local scour, silty bed, pile group, flume experiment, Sscour mechanism, Scour depth