I discussed inverted-arch foundations a while back and wanted to briefly return to the topic with this picture. That’s a beautiful example of an inverted arch, carrying the load from two piers in a church apse. The most important thing about it is that it has performed its job properly for over one hundred years. But…
Unless the stones are shaped exactly the right way, invert aches don’t work the way arches do. It doesn’t really matter how the tops of ordinary (right-side-up) arch voussoirs are shaped because gravity will ensure that the vertical load gets though to the arch proper. But in an upside-down arch, the load below, on the soil, may not be evenly distributed. Most inverted arches probably distribute their loads unevenly, with the load following a more or less 45-degree spread from the pier out and down to the bottom. That distribution is quite similar to what you’d get if you built masonry stepped footings instead of inverted arches. And, of course, stepped footings are much easier to build.
The foundations of a lot of 1800s churches that I’ve seen were built the same way: the entire footprint of the building was excavated to a uniform depth, along with some extra area around the perimeter. The foundation walls were built, then the cellar and perimeter filled in to the appropriate elevations, then the superstructure built. So if the inverted arch above looks sort of shallow, it’s not. Most of it is buried below the cellar floor.
The workmanship of the arch voussoirs is good, as is all of the rest of the masonry. This is truly obsolete structure – not only would we not build this today, but we think they were wrong to build it then – but it’s well done.