Concrete Slabs Against Bridge Scour. Is This A Good Typology of Protection?

Flood events occur year after year all around the world. They are a threat to the environment and society, and highlight the vulnerability of our infrastructures. Particularly, bridges over rivers, a key element in the connection of territories, are often affected and damaged by these events. Bridge-river interaction during floods are one of the main causes of bridge failure. For example, roughly 50% of bridges collapses between 1989 and 2000 in the United States occurred at river crossings during floods (Wardhana & Hadipriono, 2003). Pier and abutment scour is the major cause, but not the only one.

This research studies the behavior of a typology of bridge protection against river erosion. Many bridges are protected by placing a concrete slab over the erodible bed material, spanning all the width of the river cross section. By doing so, both piles and abutments are supposedly protected from the action of the flow. However, the collapse of several structures that had this kind of protection indicates that this is not always the case.

The study deals with the problem from a theoretical and an experimental point of view. It also takes into account valuable information gathered from field surveying campaigns. It is meant to achieve several aims: to increase the knowledge of the erosive process, to better use that knowledge in order to adapt and improve this typology of protection, and to improve as well the general methodology to assess vulnerability of bridges at river crossings, proposed by Vallés-Morán (2011). This methodology is based on field surveying campaigns.

The performance of the protection is explained using the results of experimental tests conducted in the Hidraulics Laboratory of the Universitat Politècnica de València (Spain), by means of physical modelling without scale effects. Real bridges with severe scour problems that serve as individual case studies are also presented.

Finally, it is concluded that this is not a good typology of protection by its own, according to the way it is usually implemented on the bridge. Scour holes simply move downstream the protected area, still threatening both the protection and the bridge foundation. Scour depths are significant enough to lead to bridge failure, as has been the case in many documented cases.