| Local scour of pier foundation is an important factor causing bridge flood damage.Existing research results mainly focus on local scour of pier foundation under single pier and double pier conditions,and there are few studies considering the factors of multiple piers and pier spacing.Based on the engineering construction practice,it is significant to carry out experimental study on the influence of multi-row cylindrical pier spacing on local scour in sandy riverbed.In this paper,three pre-tests on local scour characteristics of single pier with three flow velocities(0.15m/s,0.18m/s,0.20m/s)were carried out,and three test pier spacing(10cm,20 cm,40cm)were proposed.In consideration of the engineering practice,the arrangement forms of equal or unequal spacing of serial double-row piers,triple-row piers and four-row piers were determined.Based on this,an experimental study on the influence of pier spacing on local scour was carried out considering pier spacing and velocity,and the influence and evolution law of pier spacing and pier number changes on flow structure around pier,average velocity along the passage and scour pit characteristics were investigated.The main conclusions are as follows:(1)Under the condition of the same velocity,spacing,tandem more upstream side of the 1# pier scour pit,the largest number of tandem piers on the upstream side of the 1#pier weeks have little impact on flow structure and scouring characteristics,and due to the blocking effect 1# pier scour other pier weeks after reduced,other pier weeks flow have the similar structure change rule.(2)Under the condition of the same flow rate and the number of piers,the change of tandem spacing has a certain influence on the flow structure and scour pit around pier 1#.For 1# pier,the maximum velocity around pier and scour pit were slightly larger when the spacing was 10 cm,and the scour pit characteristics were similar to those of single pier when the spacing was 20 cm and 40 cm.The larger the pier spacing is,the less the influence of front pier on water flow and sediment replenishment,and the shielding effect gradually disappears.The scour pit depth of rear pier also increases with the increase of pier spacing.(3)The experimental results show that for the same arrangement,the scour pit around each pier increases with the increase of flow velocity under the condition of clean water scouring.Moreover,with the increase of velocity,the interaction between adjacent piers increases.When the velocity was 0.15m/s,the adjacent piers had little influence when the spacing was 20 cm or more.However,when the velocity is 0.20m/s and the spacing between adjacent piers is 40 cm,the front pier has some influence on the back pier.(4)On the basis of the existing formula for calculating local scour,a formula for calculating local scour depth of multi-row piles is proposed,which is suitable for the change of distance,considering the arrangement of multi-row piles,pile spacing and flow velocity,etc.Combining with the test results,the calculation value of maximum scour depth is analyzed with the test value,and the rationality of the formula is verified. |
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