Study On The Mechanism Of Local Scour Reduction Around Cylindrical Pier Using Permeable Sacrificial Pile

In recent years,the bridge construction business in China has made world-renowned achievements.However,local scour of bridge foundation has become one of the main causes of bridge collapse accidents.Therefore,it is of great significance to carry out research on local scour reduction measures and mechanism of bridge foundation to reduce water damage accidents caused by local scour and ensure the safe operation of bridges.Solid sacrificial pile protection is a proven scour reduction measure,but its own scour is very serious,which is a great threat to the safety of the sacrificial pile itself.Therefore,a new form of filled gravel type permeable sacrificial pile was proposed in this paper.Firstly,through literature research and theoretical analysis,the influence of each factor on the scour reduction effect of permeable sacrificial piles was analyzed in terms of water flow characteristics,channel characteristics,riverbed characteristics,and protection structure characteristics.The main influencing factors of permeable sacrificial piles studied in this paper were determined as follows: gravel filling particle size,arrangement distance(distance between sacrificial piles and bridge piers),diameter ratio(ratio between sacrificial piles and bridge piers),and inundation rate(Ratio of the distance between the top of the sacrificial pile and the water surface to the water depth).Secondly,the influence law of each factor on the reduction of local scour of permeable sacrificial piles under clear water conditions was studied in terms of local scour of sacrificial piles themselves and the effect of scour reduction on cylindrical bridge piers through water tank model tests.The test results show that the local scour reduction effect of permeable sacrificial piles on bridge piers is close to that of solid sacrificial piles,and it can also reduce its own local scour depth up to 28.17%.The scour reduction rate of the bridge pier and the overall scour reduction rate of the permeable sacrificial pile and the bridge pier are the highest when the gravel filling size is 0.2-0.25 D,the arrangement distance is 3.0D,the diameter ratio is 1.0,and the inundation rate is 0.2,reaching 70.42% and 32.08% respectively,which is the best combination of permeable sacrificial pile protection effect.Finally,ANSYS Fluent numerical simulation software was used to simulate some of the test conditions and reveal the impact reduction mechanism of permeable sacrificial piles in terms of flow field,pressure field,vortex system and bed shear stress.The simulation results show that the permeable sacrificial pile allows part of the water flow to pass through the pile,which can effectively reduce the amount of water falling in front of the pile,weaken the bottoming effect of the falling water on the riverbed,and play a role in reducing its own local scouring depth.At the same time,the pressure difference between inside and outside the permeable sacrificial pile makes the water flow in the pile flow outward in the direction perpendicular to the isobaric line,which weakens the flow velocity around the pile,decomposes the vortex system and weakens the vortex strength,making the low-flow velocity area behind the pile increase significantly,thus significantly reducing the local scouring of the bridge pier.In the process of water bypassing,high shear stress area will be generated on both sides of the sacrificial pile and the bridge pier.Compared with the solid sacrificial pile protection,the maximum shear stress is reduced by about 27.7% when using permeable sacrificial pile protection.On the other hand,the range of low stress area after permeable sacrificial pile is larger and the protection range is bigger.The variation law of riverbed shear stress near permeable sacrificial pile and bridge pier is consistent with the variation law of local scour characteristics of permeable sacrificial pile and bridge pier in the model test.