Experimental Study On Local Scour Mechanism Of Bridge Pile Groups

The safety of the bridge is affected by the local scour around the bridge foundation.Pile group foundation is widely used in bridge engineering.Compared with the single pier,the flow field structure and the mechanism of water-sediment interaction around pile group are more complex.This thesis focus on the the engineering phenomenon of local scour around bridge pile group foundation,the local scour mechanism was explored through the in-depth study of(1)the scour process around the pile groups;(2)the evolution of flow field and hydrodynamic characteristics around the pile group;and(3)the relationship between water and sediment.It is of great theoretical and engineering significance to improve the accuracy of local scour prediction and ensure the safety of bridge structure.In this thesis,the local scour process of pile groups under different pile group arrangement and Froude(Fr)number was studied based on flume experiment.The flow pattern around the pile group and the characteristics of horseshoe vortex in front of the pile group under different pile spacing and flow conditions were revealed based on particle image velocity(PIV)technology.The evolution of flow field in the scour hole with scour development and the spatial distribution of the three-dimensional flow field were obtained by using the acoustic Doppler velocimeter(ADV).The distribution of suspended load concentration between piles under oscillatory flows was obtained by image processing method,and the relationship between water and sediment was further established through theoretical analysis.The main innovative findings and contributions are drawn as following:(1)Based on flume test,the scour evolution law of different pile groups was observed,and the function of scour depth around pile groups with time was obtained.By comparing the results of equilibrium scour depth between pile groups and single pile,the influence of pile spacing and Fr on the equilibrium scour depth of pile groups was analyzed,and the predictive formula was modified.By analyzing the shape of equilibrium scour hole under different pile groups,the empirical relationships between the scale characteristics of scour hole and pile spacing and Fr were obtained.(2)The instantaneous flow fields at different longitudinal sections around the pile groups on the flat bed were measured by PIV.The influences of flow conditions,pile spacing and pile arrangement on the time-averaged,transient and dynamic characteristics of the horseshoe vortex were discussed.The flow field of pile group and single pile was compared,and the influence of the change of pile spacing on the complex flow structure and vortex shedding in the area between piles was discussed.(3)The three-dimensional flow field around the 2 × 2 pile group within the scour hole was measured by ADV.The time-averaged flow field and turbulent characteristics in different scouring stages were compared,and the contributions of time-averaged and turbulent quantities to scour were explored.The spatial distribution of flow field around the pile within the scour hole was analyzed.Through quadrant analysis and energy spectrum analysis,the scale characteristics of flow burst and turbulence within the scour hole were analyzed.(4)The experiment of scour around tandem piles was carried out in an oscillatory flume.The flow field and the movement of sand particles were simultaneously measured by using PIV.Based on image processing technology,the distribution characteristics of suspended load concentration between pile groups were obtained,as well as the contribution of suspended load transport to scour was analyzed.Combined with the dynamic characteristics of the flow field,the mechanism of sediment suspension was revealed,and the relationship between the flow characteristics and the suspended load movement was clarified.Finally,based on the diffusion theory,the relationship between the flow characteristics and the suspended sediment transport was established.