Study On The Influence Of Pile Group Arrangement And Non-uniform Sediment On Local Scour Around Piles

With the need of marine resources development and the transportation industry,the construction of some large pile-based foundations such as cross-sea bridges and other hydraulic structures has been increasing.The local scour around the piles has potential threats to bridge's safety,and it has become the focus of attention of scholars all around the word.Because there are many factors affecting the scour around the pile groups,the understanding of it is not enough.In addition,there are few research results about the effect of the non-uniform sand scour around the pile.In this paper,the scouring of group piles in several simple layouts is experimentally studied,the law and mechanism of scouring for different group piles in constant flow and wave current conditions are analyzed,and the effects of non-uniformity coefficient on scouring are summarized.The main conclusions are as follows:(1)The pile group effect has an important influence on local scour of group piles,such as reinforcement effect and shield effect between piles,the vortex shedding from front row piles and the compression of horseshoe vortex.For the conditions of this test,under the same flow velocity,the scouring depth and range of the large-diameter single pile is the largest,and the scouring depth of the 2 × 1 double-pillar arrangement is the smallest;The scouring of 2 × 2 double-row layout is affected by the rear piles,which makes the scour depth greater than 2 × 1 double-pile arrangement.The large water blocking width and the influence of vortex shedding from front piles makes the maximum depth of quincunx arrangement pile groups greater than the scour depth of 2 × 2 double-row arrangement.The caculation of the equivalent pile diameter of group piles is carried out,and the equivalent pile diameter can be used both under constant flow and wave current conditions.(2)Under the wave and current conditions,the relative scour depth around single pile increases linearly with the increase of the ratio of wave heigh to water depth.For the condition of the group pile arrangement,the larger ratio has more obvious scour enhancement effect,which makes the relative scour depth appear a inflection point with the increase of the ratio of wave heigh to water depth.For the same wave conditions,the group pile arrangement also has a certain effect on the scour depth and scour rate,and the law of influence is the same as the constant flow.(3)The non-uniformty of sediment has a greate impact on erosion.Under different flow conditions,the scour depth decreases as the non-uniform coefficient of sediment increases,.When the non-uniform coefficient is large,the impact on scour is more obvious.,and when the non-uniform coefficient is small,the depth is close to that of uniform sediment.(4)The formula of initial velocity of non-uniform sediment around the pile with considering the non-uniform coefficient of sediment is given,but its applicability needs further study...