Flow-altering countermeasures against local scour at bridge piers

Abstract

The ability to protect bridge piers and abutments against scour is critical to bridge safety. Excessive pier scour can cause high maintenance costs or even bridge collapse resulting in the interruption of traffic and possibly death. Up to now, various types of countermeasures have been recommended to reduce scouring around pier foundations. The countermeasures against pier scour can be broadly classified into two categories: (1) flow-altering and (2) bed-armouring countermeasures. This thesis focuses on the first category of countermeasures. Based on a comprehensive review of the up-to-date studies on various types of flow-altering countermeasures, these can be classified into four main groups based on their shapes and performances: (1) openings through piers, (2) pier attachments, (3) bed attachments and (4) other devices. After evaluation and considering all the countermeasures, a few countermeasures were recommended for further investigations. Among them, five countermeasures were selected for analysis in the present thesis. They are transverse sacrificial piles, collar, threading, pier slot and bed-sill. The first part of the experiments, were performed in clear-water scour condition, near threshold condition of sediment movement. In this part, countermeasures were tested individually. Afterwards, countermeasures that showed appropriate efficiency in scour depth reduction were combined one by one. Results of this part showed that a collar, pier slot and sacrificial piles may show efficiency of about 30% in scour depth reduction; however, threading cannot be assumed as a reliable pier scour countermeasure. Results show that the blockage ratio of sacrificial piles is an important parameter, since by increasing the number of piles from three to five, the efficiency reduces. In this part also optimum size and location of a pier slot were criticised and best configuration based on previous and present studies was proposed. In fact, the best configuration can be suggested as a pier slot half as long as the flow depth plus maximum scour depth. In this case, the pier slot near the water surface may not significantly increase the efficiency. A combination of sacrificial piles with a collar and sacrificial piles with a slot showed that they cannot reduce the scour depth significantly with respect to each individual countermeasure. In particular, a combination of a collar and sacrificial piles increases the rate of scouring with respect to a single collar. At the end of this part, a combination of a bed-sill and cut-collar was selected as the best configuration. Two forms of this countermeasure were proposed for further studies. In the first proposed countermeasure, the use of a sill as wide as the channel width was recommended. This form, although reliable to prevent souring at the upstream part of the bed-sill, may not be applicable for some practical purposes. Therefore, in another attempt, a shorter bed-sill was also proposed, protected with two side-walls, in order to reduce the risk of scour intrusion in front of the bed-sill. These two proposed countermeasures showed efficiencies of 68.5% and 64.5% in scour depth reduction, however owing to the use of a border around the collar rim, the scour did not reach the pier body. In next part, the later form of proposed countermeasure that consists of a shorter bed-sill and seems to be more practical, was evaluated in unsteady flow tests with the peak flow intensity slightly greater than the threshold for the inception of sediment movement. In this section of the thesis, circular and round nose and tail rectangular piers were tested in both configurations of single and two in-line piers. The results of unsteady flow tests also show that the proposed combined countermeasure can protect pier foundation during a flood event. Specifically, it seems that the proposed countermeasure is more effective when it is applied to round nose and tail rectangular piers than circular ones