# Experimental Study of Scour Hole Depth around Bridge Pile Using Efficient Cross-Section
## 1 Introduction  
Local scour around a bridge pier is one of the most critical challenges that can jeopardize the stability of the bridges [1; 2; 3]. The scour results in altering the riverbed level, thereby causing a major structural problem for bridge piers. Local scour is amplified during extreme weather events such as floods and hurricanes, causing currents and vortices past bridge piers. Scouring is even more pronounced in the typically grouped pile foundation because of the boundary separation on the upstream side and vortex shedding on the downstream side of the grouped foundation [4; 5; 6]. It comes as no surprise that the study of local scour depth at bridge piers has received considerable attention and the literature devoted to this subject is vast. Various forms of equations have been proposed that describe mathematically the relationship between the scour hole and various influencing parameters. Unlike individual pier, scouring in the pile groups appears not to have been examined extensively, mainly because the group effect seems similar to that of the individual pier. Nevertheless, this expectation may not be realized because of [PERSON]'s study [7] on scouring in binary piles. The first systematic study of two piles have been made by [PERSON] [7] in which the effect of pile spacing was discussed keeping pile diameter constant. [PERSON] [7] introduced a dimensionless parameter S/D, which is the ratio between the face to face spacing of piles in a group and individual pile diameter D. This study shows that when the piles are spaced at face-to-face distance equal to pile diameter, i.e., S/D = 1, the depth of scour at the front pile equals the scour due to an equivalent individual pile with the width twice that of a pile in the group. With increasing pile spacing, the effect of the reinforcing factor can be seen in the first pile. The reinforcing factor effect reaches its maximum in S/D = 2.5 and exists upto S/D = 11. A similar investigation has been reported by [PERSON] and [PERSON] [8] in which both the pile diameters and spacing were considered variables. [PERSON] and [PERSON] [8] examined the effects of pile spacing on the maximum depth of scour at bridge piles. The results showed that by increasing the spacing between the piles, the amount of scouring reduces. This study was extended to find the effect of different S/D ratios on the scour depth [9]. They showed that if the ratio S/D > 11, then the scouring depth at the front pile is equal to the value of an individual pile. In addition, investigating the ratios S/D = 0.5, 1, 2, 4 and 6, they concluded that the scour depth reduces when the pile spacing increases. To be specific, for S/D = 0.25, the maximum depth of scour was found to be 50% more than a single pile. In a continuation of this study, [PERSON] [10] reported the effect of pile spacing on the scour depth. It was found that for the ratio S/D = 1, scour depth is 1.95 times greater than that of an individual pile. Contrary to [PERSON] et al. [8] investigation, [PERSON] et al. [11] reached different conclusions from a similar study. [PERSON] et al. [11], from a study of piles at ratios S/D = 0, 0.5 and 1, observed that the adjacent pile scour holes interfere with each other and form a big hole at the location of upstream piles. However, at these S/D ratios, when the minimum spacing increases either to 2D or 4D, scour holes and scour ridges around the piles do not interfere. This study also showed that by increasing the spacing between the piles, scour depth decreases, such as for the ratio S/D = 6, the scour depth approaches that of a single pile. The review of the literature highlighted above has focussed primarily on circular piles. Attempts have also been made to find the most efficient pile cross-sectional dimensions that can effectively reduce the scour depth. For example, the scour parameters of the cylindrical pile group have been considered by [PERSON] et.al. [12], investigating the effect of cylindrical pile diameter on the scour hole depth and volume.