Study On Local Scour Of Bridge Pier With Vibration Loadings

Local scour of bridge pier is an important factor affecting the safety of the bridge,and it is also a crucial issue that should be paid attention to in the design of bridge pier.As the economic development and the requirement of human activity,the bridge career in China is developing rapidly while the number and scale of large cross-river and cross-sea bridges are increasing.However,once the bridges are destroyed,not only transportation will be affected seriously,but also people's lives and property will be threatened.At present,although there are many research achievements about local scour at bridge pier on non-cohesive sediment bed,but there are few studies considering fine sediment bed due to the complexity of viscous sediment.In addition,the phenomenon of pier vibration is common caused by vehicles,wind,waves and so on,nevertheless,the influence of vibration loadings on local scour is never considered in the existing researches.In particular,the fine sediment around the pier will fluidize under the long-term vibration loadings and become higher potential for incipient motion and scour,which results in the local scour depth measured in field is larger than the predicted value by the existing formulae.Therefore,studying on the local scour of bridge pier under vibration loadings,further research on local scour mechanism and scientifically predicting local scour depth are of practical significance for improving safety of wading bridges and preventing bridge collapse accidents.Firstly,taking a cylindrical pier as an example in this thesis,the threedimensional flow around pier is analyzed in detail by means of numerical simulation,and the effect of flow around pier on local scour is explored.Combined with the numerical results and relative researches,the basic features of the local scour is summarized.The results indicate that the flow around pier is a complex three-dimensional flow movement.And the diving flow,the horseshoe-shaped vortex and the trailing vortex around pier are significant flow structures forming the local scour of pier.Secondly,the experiments on the clear-water scour of pier under different vibration loadings are conducted in a flume.The measured velocity around the pier and the numerical simulation results are compared.The law of pier local scour with vibration frequency and amplitude is analyzed,whose influence on the local scour is reflected by vibration strength.And the differences of local scour on the sediment beds with different particle sizes are compared.The results indicate that the numerical simulation results are in good agreement with the experimental results of measuring point velocity.The vibration loading of pier plays an important role in the local scour,and the effects are different between non-cohesive sediment bed and fine sediment bed.For fine sediment bed,the local scour of pier is intensified under vibration loadings caused by sediment rheology.The maximum scour depth of pier are positively correlated with the vibration strength,however,the maximum scour depth tends to be stable when the vibration strength reaches a certain extent.And the smaller particle size of the bed sediment,the greater effect of pier vibration is on local scour,the higher fluidization degree is under vibration loadings resulting in larger local scour depth.For non-cohesive sediment bed,the influence of vibration loadings is weaker,which makes the sediment become more compact,resulting the slight reduction of the local scour depth.Finally,the local scour depth formula of pier considering the vibration load is discussed.Based on the analysis of the experimental data,the dimensionless vibration coefficient and sediment particle size coefficient are derived to modify Gao's local scour depth formula under clear scour condition,so that the new formula can be applied to calculate the maximum scour depth on the sediment bed with different particle sizes considering vibration loadings,which provides a calculation basis for bridge design and reasonable prediction for embedded depth of bridge pier.