One of the worst enemies of bridges is scour. Scour involves water digging holes in the streambed around and, eventually, under piers and substructure elements. Modern bridges are built with scour in mind. Piers and abutments are placed deep into the streambed. However, many stone arch bridges were built with shallow abutments that often rest on the streambed itself. Therefore, these bridges can be vulnerable to scour. Here are 3 ways to prevent scour:
Preventing Scour: Underpinning
The most ideal solution when it comes to preventing a stone arch bridge from being scoured out and collapsing is underpinning. Underpinning consists of extending the foundations downward by carefully digging below the bridge, and then supporting the bridge from below with a firm, scour-resistant material.
There are several ways to go about this, but the basic idea is to add much deeper foundations to the bridge, so that scour is no longer a concern. This solution is expensive and must be done carefully. As one is digging below an existing structure, care must be taken to prevent the structure from collapsing into the excavations.
Preventing Scour: Streambed Paving
Another solution is to try to prevent scour under the bridge altogether. This can be done by paving under the bridge with concrete or some other scour-resistant material. This new solid streambed under the bridge forms a barrier between the water and the bridge’s foundations, effectively protecting the bridge from scour.
One thing to note is that paving the streambed does not merely consist of throwing material under the bridge. Loose rubble under the bridge can actually encourage scour by increasing the water velocity and turbulence. A good streambed paving is tight and smooth, allowing the water to flow freely under the bridge, while blocking it from digging at the substructure below.
Keep in mind, scour can still take out the streambed paving starting at its edges, eventually reaching the bridge. This will take some time, however. As the failure of the streambed pavement will occur before the failure of the bridge foundations, ample time should be available for corrective measures before the bridge itself is seriously threatened.
Preventing Scour: Aprons
An even simpler and relatively inexpensive solution to the scour problem is the use of concrete aprons around the piers and abutments of the bridge. In Kansas, this is a popular solution, and appears to be remarkably effective. The water simply cannot dig under the piers; it has to undermine the apron first, which takes time. The concrete apron is something of a localized streambed paving and a highly economical solution.
More information on scour aprons can be found here.