SCOURING AROUND A GROUP OF PIERS

Abstract

Scour is caused by the erosive action of flow of water, which erodes and takes away sediments from the river bed, as well as from the area of bridge piers and abutments. This process of scour is well-known for its complicated behaviour, and this process becomes more complex for the group of piers. As per the literature available, scour is a major contributor for the collapse of the bridge structures. Hence it becomes essential for the researches to predict the scour depth accurately. To fulfill this necessity, the deep understanding of the flow-field around the piers and factors affecting the scour depth is important. Numerous studies are available in the literature on the flow-field and local scour, but majority of them are focused only on single pier. However, multiple piers are more common in the bridge designing due to economical and geotechnical considerations. Engineers across the world are predominately considering single pier characterization for the group of piers, although all bridge structures are laid on the group of piers. The design considerations developed for a single pier mostly ignores the most significant group effects for multiple piers such as: pier sheltering, pier spacing and mutual interference effect. It is demonstrated by the fact that inadequate research and development for the multiple piers have been observed. Therefore, it is utmost important to investigate the factors affecting the multiple piers and develop an equation by considering all the variables for accurate prediction of scour depth. The primary aim of this research is to investigate the effect of the pier spacing (Ps) between the piers and temporal scour development on scour depth and propose a new equation for scour depth estimation. In order to meet the objectives of this research, detailed experimental study has been conducted for, single pier, two piers, and three piers. In total, 232 experiments have been carried out for various pier spacing and pier arrangements on uniformly graded sediment in the Hydraulic Engineering Laboratory of Delhi Technological University, Delhi, India. Pier arrangements studied in this research are: single pier, two piers in tandem arrangement, two piers in side-by-side arrangement, two piers at an angle of 15 and 30 degrees to the direction of flow, three piers in tandem arrangement, three piers in side-by-side arrangement and three piers in staggered viii arrangement. The instantaneous 3-D velocity measurement is conducted by utilizing SonTek 16-MHz Micro-ADV (Acoustic Doppler Velocimeter) has been used. For profoundly understanding and continuous monitoring of scour evolution SeaTek Ultrasonic ranging system have been used. The Experimental observations for the temporal scour evolutions and scour hole development pattern for all the pier arrangements under various pier spacing has been presented in the graphical and pictorial form. The findings of this research shed light on the factors that causes scour: pier spacing, flow depth, Froude number, and scour development pattern. These are all important variables, which needs to consider in order to accurately predict the maximum scour depth. Results shows that, the flow-field around the group of piers is much more complex than single pier and pier spacing plays a major role in the scour depth estimation. The results also exemplify that non-dimensional scour depth increases with the increase in the Froude number and velocity of flow. The effect of pier spacing shows that the scour depth increases as the pier spacing increases at a certain Ps range, then it decreases. Furthermore, two semi empirical equations has been proposed for the tandem and side-by-side arrangements by using GEP, and the comparison of the proposed GEP equation with the well-known equation and experimental results shows that the GEP is better prediction tool for scour depth. Lastly, to evaluate the influence of various parameters on the scour depth sensitivity analysis has been conducted, and concluded that pier spacing plays an important role in scour estimation. The findings of this study can be further used for the evaluation of the research data.