Rubble Arch Building: Choosing the Stone

Tagged:
Design, Quality of Construction, Stones

Though some care is used in selecting and placing the stones in a rubble arch, rubble masonry can be readily used to build a strong arch for a stone arch bridge.

Building an arch of rubble masonry is a great DIY project, as rubble masonry is easy to build and, even if mortarless, can be built durable enough to last for centuries, and carry cars or even the occasional bulldozer.

Mortarless Stone Bridge — Just because a stone arch bridge is built of rubble masonry does not mean it is not strong.

Arch Width and Stone Size

The beauty of rubble masonry is how simply it can fit together using a wide variety of sizes and shapes of stones. However, for optimal stability, a rubble arch should use reasonably regular stones that have adequate thickness that only one or at most two layers are needed to achieve the total arch thickness. It is possible to use more layers than this, but to ensure durability it is always best to build a single-ringed arch if it can be done.

Dry-laid arch under construction — A variety of stone sizes are used on this arch, but the key was to make sure they were at least as long as the arch was thick. Occasionally a second arch stone was added behind the first layer for increased thickness, but generally the stones chosen were more than adequate to achieve the desired thickness on their own.

The Shape of the Stones

The arch stones need not be rectangles, though rectangular blocks are easier to work with. The key is to place the stones such that they interlock to fill in the gaps between stones of a given masonry course while ensuring the thickest part of the stones is up and the thinnest part down on the form. Almost all stones are wedge-shaped to some degree; so the key is to use this natural wedge to advantage. Very triangularly shaped stones are rare but can be found, but these are best saved for skewbacks rather than actual arch stones.

DIY Arch Bridge Under Construction — Though these arch stones are not perfectly rectangular, their natural shapes could be interlocked into each other creating a solid arch.

Making the Angles

The angles of most rubble arches are not perfect. There will often be gaps between the stones at the top, especially for smaller spans. These gaps are usually filled in with small stones and mortar. Breaking up some of the less-usable stones is a great way to obtain these stone fragments. Do not underestimate the use of stone fragments, so save them as you find them. Their use strengthens the arch and reduces the amount of mortar needed.

Rubble Arch Detail — Numerous tiny stones make the angles for this mortarless rubble arch. Mortared masonry uses the same principle, only the stones are added after or at least with the mortar to ensure the mortar fills all the nooks and crannies.