How to mitigate the effects of scour on bridge piers through the use of combined countermeasures

Raleigh, Andrew (2015) How to mitigate the effects of scour on bridge piers through the use of combined countermeasures. [USQ Project]

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Abstract

Construction of a bridge pier in a flow of water will cause a disruption to the flow. Scour around bridge piers arises due to the separation of this water causing erosion of the sediment at the bridge pier and leading to the development of both horseshoe and wake vortices around the pier. The relationship between water and bridge piers in flowing streams creates a three dimensional field of flow. There is an additional pressure head upstream of the pier as the water hits the bridge pier which then curves downwards into the scour hole and a horseshoe vortex is formed. The accumulation of flowing water on the surface pushes back and creates a bow wave. The water also deviates around the pier as it continues its downstream flow and produces a shedding wake vortex. Local scouring occurs due to the action of the horseshoe and wake vortices. Local scour is the immediate change in the bed level surrounding an obstruction due to the restriction or change in the natural flow path. This reduction in the depth of bed level is called scour depth. Scour causes significant structural unpredictability. The bridge foundations are weakened and may eventually cause overall failure of the structure especially when there are floods as the volume and intensity of the moving flow increases so rapidly. Researchers have been trying for quite a long time now to find ways to reduce this scour occurring and thus increase the safety of bridge piers.

Experimentation to understand the scour process and the damage a horseshoe vortex causes is usually conducted in laboratories using straight flumes. Results of some studies noted in this literature review showed that the maximum depth of scour was highly dependent on the amount of time for which the experiment was conducted. It also identified that as the flow rate increased so did the level of erosion. Several engineering designs relating to this topic have been tested over the years and they are becoming more successful with time and research. Bridges are very necessary in our modern world so engineers must study the best ways to install bridge piers to create the least disturbance to natural or man-made waterways in turn reducing erosion around the pier.

This research was developed to understand and consequently aim to reduce scour through the economical design and best use of countermeasures around bridge piers. The project identified, through a literature review and design analysis, three combined countermeasures for bridge piers. Initially countermeasures were built for testing in the large flume. After preparing the large flume for testing and commencing the control test the large flume unfortunately failed. Due to this misfortune models were then built and tested in the small flume. The results compared the control pier and each separate countermeasure with the amount of erosion which occurred. A control pier for the purpose of this research was a circular pier without any countermeasure. Volumes and dimensions were calculated using a laser scanner (FARO) then further processed and modelled using mining software. These volumes were used identify the greatest reduction in erosion. These outcomes all showed a decrease in erosion when compared with the control pier but one combined countermeasure in particular, three collars and a plate, showed the greatest reduction but did not totally deny scour.

Results show that an effective reduction in local scour can be achieved through the use of three collars as a combined countermeasure but not totally eliminated.


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Item Type: USQ Project
Item Status: Live Archive
Additional Information: Bachelor of Engineering (Honours) Major Civil Engineering project
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Civil Engineering and Surveying
Supervisors: Foley, Joseph
Date Deposited: 07 Jun 2016 01:03
Last Modified: 07 Jun 2016 01:03
Uncontrolled Keywords: bridge piers, combined countermeasures, scour, water flow, erosion, bridge safety, flume
Fields of Research : 09 Engineering > 0905 Civil Engineering > 090599 Civil Engineering not elsewhere classified
URI: http://eprints.usq.edu.au/id/eprint/29269

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