What is a foundation arch?
A foundation arch is a masonry arch built in the ground, spanning between foundation piers so that less brickwork is needed than for a continuous foundation wall. Instead of a solid wall along its full length, builders used to construct piers only at the load-bearing points and connect them with arches. On piled foundations, the piles sit under the piers, at the feet of the arches. Such arches are usually below ground or visible in a cellar. In Dutch this is called a spaarboog (literally “saving arch”). It differs from an inverted arch, which curves downwards between piers to spread their load into the ground.
How it works
- Load goes to the piers. The arch carries the wall above and directs its weight into the piers. The ground between the piers is relieved, but the piers and any piles below them carry more load.
- Saving material. Less brickwork and less excavation than a continuous strip foundation.
- Not a relieving arch. A relieving arch sits above an opening in a wall and keeps load off a lintel or frame. A foundation arch replaces part of a solid foundation wall.
Application
Foundation arches are found mainly in old buildings, castles, town walls and historic cellars. During alterations or foundation repair:
- Recognise the arch. Look in the cellar or crawl space for arches between heavier piers; the wall above is then not supported along its full length.
- Have the piers assessed. Because all load passes through the piers, their condition is decisive. Include them in any foundation survey.
- Do not simply break through. An opening in or just below the arch disrupts the arch action. Consult a structural engineer first.
- Excavate with care. Never undermine the feet of the piers without temporary support; an arch that loses its support will crack or drop.
- Watch for cracks. Cracks in the arch or sunken piers indicate differential settlement of the supports.
Common mistake: treating the space under a foundation arch as free room for services or a cellar extension, without realising that the piers carry the full load.
Related terms
- Relieving arch
- Segmental arch
- Round arch
- Pier
- Pile foundation
- Cellar
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