Experimental Study And Calculation Method Of Local Scour Around Bridge Caisson Foundations

Local scour has become one of the most important factors affecting the safety of the construction and the operation of bridge foundations.With more and more caisson foundations are used as the foundations of large bridges,it is very important to study the local scour of caisson foundations in the stage of settled and suspended situation.This thesis analyzed the scour mechanism of bridge foundation in different influencing factors,the flow field around foundation and the sediment movement and summarized the research situation of the calculation formula of local scour around bridge foundation in the equilibrium scour depth and the scour depth with time development.This thesis expounded the research on the flume experiments of the influencing factors and characteristics of local scour.The research status of numerical simulation was also summarized in this thesis.The research status and the shortcomings of local scour around caisson foundation are also discussed.Therefore,in this thesis,the theoretical analysis,the flume experiments and CFD numerical simulations were used to study the local scour around the caisson foundation of bridge.(1)This thesis introduced,compared and analyzed the existing calculation formula of local scour depth of bridge foundation and summarized the characteristics of each formula.The advantages and disadvantages of the calculation formulae in Chinese and American codes are summarized.The sensitivity of different influencing factors(velocity,water depth,foundation width and sediment particle size)in the existing formula was analyzed.The applicability of each formula was studied through the selected experimental data and field measured data.A new formula of calculating the local scour depth is established by using the method of dimensionless analysis and the multiple regression analysis.The correction coefficient K₃of field-measured data was proposed and the reliability and accuracy of the new formula are verified.(2)A series of flume experiments of local scour around caissons were designed and carried out.Three types of caisson models(circular,square and diamond)and two types of inflow currents(unidirectional and tidal)were used to study the local scour characteristics under the action of the unidirectional and tidal currents and the local scour characteristic of caissons suspended in different positions in water.The sensitivity calculation formula of caissons with different cross-sections was proposed,which can reflect the sensitivity of caissons with different cross-sections under the action of tidal current with the change of inflow direction.The influence of the clearance c on the minimum scouring velocity V_i,minand the maximum scour depth,and the difference of local scour characteristics of suspended caisson under the action of unidirectional and tidal currents were also studied.(3)The CFD software Flow-3D was employed to study the local scour around the settled caisson.Based on the fully developed stable flow field section and live-bed scour section,the time of the numerical simulation can be significantly shortened and the calculation efficiency can be improved.The reliable of the numerical models was verified by the experiments of local scour around caissons in this thesis and the other tests of local scour around the large-scale caisson of a certain bridge.Based on the experimental conditions in this thesis,the cross-section of caisson,the ratio of length and width and the angle of inflow current were studied by numerical simulations.The unknown correction coefficients in the new formula proposed in Chapter 2 were determined:the shape correction coefficient K₁ and the angle coefficient K₂.Based on the test conditions of a certain large bridge,the radius of guide angle of caisson was studied and the influence coefficient₁^?was proposed in this thesis,which filled the blank of domestic and international codes.(4)Based on the experiments of suspended caisson in Chapter 3 and the CFD numerical simulation method in Chapter 4,the local scour around suspended caisson was studied by Flow-3D.It was found that the cause of local scour is the acceleration of velocity at the clearance between the caisson bottom and the sediment in the process of local scour.The horseshoe vortex can be ignored in the mechanism of local scour around suspended caisson.Based on the results of numerical simulations,the calculation method of local scour depth around suspended caisson was put forward and verified.The calculation method of the development of scour depth with time around the settled caisson and the suspended caisson was established and verified using multiple groups of experimental data.Finally,considering the influence of different sinking time when the suspended sinks to the same position,a calculation formula of different sinking time and real-time scour depth was proposed in this thesis,which can provide important reference for the judgment of local scour depth in the sinking process of caisson.