Preliminary experiments of local scour around pile groups embedded in sandy soil are carried out under steady clear-water conditions. Four types of pile groups including the single pile, tandem arrangements, side-by-side arrangements and 3×3 arrangements of piles are considered. According to flow rate, water depth, and sediment condition, the pile spacing of tandem arrangements and side-by-side arrangements are designed as one, two, four and eight times the pile diameter, while that of 3×3 pile groups is designed as four times the pile diameter. It is observed that the interference of local scour between two piles decreases as the spacing increases for the case of side-by-side and tandem arrangements. When the pile spacing is large enough, the two piles behave as two isolated piles. For the case of 3 × 3 pile groups, the maximum local scour depth occurs at the front piles and the shape of the scour-hole varies with different pile spacing. It is also observed in this study that the scoured material from the front piles will be trapped inside the scour hole of the back ones, which mitigates the scour around the back piles. The complex interaction among piles are analyzed, and the pile group effect of scour is proposed to explain the observations in the present study.
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