Modified Shields number for sediment incipient motion around a pile with impact of three-dimensional seepage in a porous seabed: Fid-Bau Portal

Modified Shields number for sediment incipient motion around a pile with impact of three-dimensional seepage in a porous seabed

Zhai, Hualing / Jeng, Dong-Sheng / Guo, Zhen / Sui, Titi

Abstract

Abstract Sediment incipient motion is one of main forms to undermine the stability of piled-type foundations in the coastal environment. Unlike previous studies ignoring the impact of seabed seepage, the three-dimensional seepage is included in the process of sediment incipient motion in this study. A new modified coefficient ( $R$ ) representing the ratio of modified Shields number and conventional Shields number is introduced to investigate the impact of three-dimensional seepage on sediment incipient motion. It is found that a discontinuity of 3D modified coefficient ( $R$ ) occurs at the phase with zero flow velocity. The horizontal seepage extremely affects the hydrodynamic characteristics and the sediment incipient motion beside the pile. However, the horizontal seepage can be ignored at the positions before and behind the pile. The parametric study indicates that larger wave height, current velocity and soil permeability and smaller water depth, shear modulus and saturation degree remarkably increase the three-dimensional modified coefficient ( $R$ ) around the pile and further promote the sediment incipient motion and scour.

Highlights • A modified Shields number ( $θ_{x y z}$ ) with 3–D seepage was proposed based on the physical process. • The influence of 3–D seepage on sediment incipient motion around the mono-pile was investigated. • The horizontal seepage greatly impacts the sediment motion beside the pile, which can’t be ignored. • Larger wave height and current velocity increase the influence of the 3D modified coefficient. • Larger soil permeability and smaller shear modulus increase the effects of the 3D Shields number.