Study On Scour And Protection Of Well-shaped Group Piles Under Unsteady Wave Current

Many bridge collapses are caused by the gradual decrease of the buried depth caused by water scour around the pier.Large-span bridges are mostly based on pile groups.The water resistance of the pile groups causes local scour nearby,the bed surface around the pile foundation is reduced,the pile foundation buried depth is reduced,and even endangers the safety of bridge.Therefore,the scour development process,scour mechanism and safety protection around the bridge pile foundation are engineering issues worth studying.In this paper,the scour development law of pile group and riprap protection of pile group under the condition of periodic unsteady wave flow are studied,which is of practical significance to bridge engineering.Through the analysis of the results of the 30 groups well-shaped group pile scour experiment and the36 groups pile scour protection experiment,as well as the theoretical analysis,the papers studied various influencing factors under the protection with and without riprap.The main contents and conclusions are as follows:?1?Under wave and current conditions and other conditions being the same,the local scour depth of pile group increases with the increase of wave height and water depth ratio;it increases with the decrease of pile spacing;it is gradually reduced with the increase of sediment particle size d₅₀,but the change is nonlinear,and its cause is analyzed in conjunction with the sediment starting formula.Under constant flow condition,the local scour depth of pile group decreases first and then increases,with the change of pile spacing G/D from 1.0?1.5?2.0.?2?The difference of the maximum scour depth between the central section of the pile group and the central section of the pile becomes smaller and smaller with the pile spacing decreases,which indicates the pile spacing has an effect on the local scour depth of the group pile.When the pile spacing is larger,the effect is smaller,it is consistent with the conclusions under the condition of constant flow.?3?The influencing factors of the scour depth of well-shaped group piles,such as pile spacing,wave height to water depth ratio,time,average flow velocity,etc.,are analyzed by multiple regression analysis,and the empirical expression for calculating the local scour depth of pile group was obtained by fitting.?4?Under wave and current conditions and other conditions being the same,the local scour depth of the pile group decreases with the increase of the relative thickness of the riprap layer;it decreases with the increase of the non-uniform coefficient of the riprap.The smaller the ratio of wave height to water depth,the protection effect is closer to that under the condition of constant flow with the same average water depth.To some extent,increasing the non-uniform coefficient of riprap or the thickness of riprap layer can effectively reduce the local scour depth of pile group under the condition of constant flow.?5?The influencing factors of the scour depth of well-shaped group piles under the protection of riprap,such as the thickness of the riprap layer,the non-uniform coefficient of the riprap,the ratio of wave height to water depth,and time,etc.,are analyzed by multiple regression analysis,and the empirical expression for calculating the local scour depth of pile group under riprap protection is obtained by fitting.