This study investigated how soil properties affect levee erosion and foundation scouring by evaluating the behavior of loose and cohesive (mixed) soils beneath a rigid crest under overflow conditions and analyzing flow dynamics within the scoured hole to understand the scouring mechanism. Four cases were examined with varying overtopping depths (Od): LS-FS, LS-FM, and LM-FS, at Od = 2 cm, and LS-FM at Od = 3 cm, where 'L' stands for levee, 'F' for foundation, 'S' for sand (#8), and 'M' for mixed soil (20% silt + 80% sand #8). The results revealed distinct differences among the cases. Notably, erosion of the back slope in the LM-FS case was delayed fourfold compared to LS-FS. In the LS-FM case, breaching of the levee body was delayed by 1.6 times compared to the LS-FS case with a 2 cm overtopping depth. Moreover, different scour hole geometries with complex flow patterns occurred in different timespans. Particle image velocimetry (PIV) was utilized on two physical scoured hole models to analyze the flow behavior within these scoured holes. The PIV analysis revealed the formation of twin eddies, moving in opposite directions and shaped by the nappe flow jet, which was instrumental in the development of the scour holes. This study found that foundation cohesion is more essential than the levee body in delaying levee breaches under rigid crest. Additionally, it revealed the role of twin eddies, especially the levee-side eddy, in increasing the size of the scoured hole upstream and causing levee breaches.

The scouring effect is widely acknowledged as a primary contributor to the weakening in the bearing performance of offshore piles; it often results in asymmetric scour patterns around the pile. To meticulously examine the impact of three-dimensional asymmetric local scour on the lateral bearing performance of a single pile, the Boussinesq solution is employed to determine the effective stress within the soil encompassing the pile, considering the presence of a three-dimensional asymmetric local scour hole. Utilizing the strain wedge model, the calculation method for the lateral bearing performance of a single pile under the condition of three-dimensional asymmetric local scour is established. The validity of this approach is established, and parameter analysis unveils the effect of varying sizes of three-dimensional asymmetric scour holes on the mechanical properties and displacement performance of a single pile. The analysis reveals that, as scouring dimensions around the pile escalate, the impact of scouring on single-pile lateral displacement and internal forces intensifies, leading to a decrease in the lateral bearing performance of a single pile. At a constant scour depth, the bottom area of the upstream scour hole significantly influences the displacement performance of a single pile. When the bottom length Swb1 of the upstream scour hole grows by 1 time, 4 times, and 8 times, the lateral displacement of a single pile at a buried depth of 6 m is augmented by approximately 0.41%, 1.65%, and 2.06%, respectively. The simplified model obtained via the modified strain wedge model and Boussinesq solution can provide a theoretical basis for the preliminary design of a single pile under asymmetric scour hole conditions.