Stepped Foundations

When building the approaches for a stone arch bridge, sloped ground is often encountered; after all, stream banks are usually sloped. When building the approach foundations, then, the builder is left with two options: Either the foundations should be at the same level as the arch foundation the whole length of the approach, or the foundations should be stepped into the bank.

Why Stepped Foundations

First and foremost: Never build a wall on a slope. The slope should be turned into steps like a staircase. The reason? Masonry is held together by gravity, and if the wall is laid up a slope, there is nothing to keep the wall from sliding slowly downhill as the years go by!

Deep Foundations

Deep foundations are certainly less vulnerable to scour, but there are other factors at play. For one thing, approaches are unlikely to be scoured unless there is a stream running up against them; obviously the bridge should be designed so that water runs under it, not washes against it! So it follows that approaches do not need to have as rugged of a foundation as the arch does.

Another consideration is that deep foundations means deep wall. This extra depth means more work and more material, and then requires extra wall thickness for stability. After all, we cannot rely on the relatively soft soil to resist swaying of the wall long term; once the ground is broken, the wall must be built according to its full height. In other words, an extra deep foundation means that the wall must be built more robustly to account for its increased height. Thus, an extra deep foundation may not actually gain anything structurally at all! Furthermore, in some cases if the foundation of the approaches is built to the same depth as that for the arch, you will end up with more wall underground than aboveground.

The Solution: Stepped Foundations

To avoid these problems of increased expense and decreased stability, the foundations can be stepped into the banks of the stream. They still must have a reasonably solid base (no loose topsoil for a foundation!) but as long as this criterion is met, the wall can be built stepped back into the bank. Thus, while the foundation of the arch is the lowest point of the bridge, up in the banks where the ground is higher the approaches can be built with a shallower yet stable foundation.

The foundations for these approaches do not start at the same depth as the arch. Rather, they quickly step up into the bank. We also like to add loose riprap underground against the approaches as an extra layer of unseen protection.

Of course, the foundations for the approaches still must be placed in solid subsoil rather than soft topsoil.