Despite its name, rubble masonry can actually result in a high-quality, well-built structure. Though considered the lowest class of masonry, many rubble bridges built centuries ago are still in use.
Variety in Rubble Masonry
There are several varieties of rubble masonry. The highest forms of rubble masonry are scarcely different from squared-stone masonry. Other types of rubble masonry often feature a heavy dose of so-called “field stones,” which are simply random-sized stones found lying on the ground that have been gathered and utilized. Furthermore, some rubble masonry will still have neatly regimented courses, while other forms will have no courses whatsoever. Rubble masonry, more so than ashlar and squared-stone masonry, often reflects the local geology and what was readily available. So, for instance, in places where slabs of limestone are readily available from hillsides, one will often see neatly coursed stone structures, as the limestone tends to come out of the ground in even layers. On the other hand, locales with landscapes dotted with granite boulders will often feature bridges made out of a startling array of randomly sized and shaped rocks. Both results can be quite enduring if carefully constructed.
That said, rubble work is peculiarly suited for use with stratified stones such as limestone or sandstone, as the structure will inherently be of high quality with little labor.
Building Rubble Masonry
The stones in rubble masonry will not usually be heavily prepared. Generally, the builder will have obnoxiously protruding sections of the stones knocked off and let it go at that. This means that the quality of rubble masonry will reflect heavily on the skill of the mason, as well as the quality of the rocks available. This is why even a rubble limestone bridge may tend to fit the category of squared-stone work, simply because limestone naturally tends to come in the form of nicely shaped rocks. As stated above, rubble masonry may or may not be coursed. That said, it is generally recommended that courses be added to the masonry every couple feet of height or less, at least if the stones are not massive. Even if mortared, the joints in rubble masonry will often have a heavy quantity of stone fragments inserted between stones, ensuring a tight fit. The ends and corners of a rubble wall will usually have specially cut stones of at least squared-stone grade in order to ensure that the ends of the structure are stable. Occasionally, the ends of a wall may be buried instead. Rubble structures will usually need to be capped to prevent smaller stones below from being dislodged.
There are several ways to do this, and we have discussed the topic in some length here. Generally, however, a rubble wall is capped with large, flat stones.
Mortar
If rubble work is mortared, the amount of mortar that will be required varies considerably, depending on the quality of the stones used. By filling in the joints with chips of stone, the amount of mortar used can be reduced considerably. Broadly, approximately one fifth to one third of a rubble structure will consist of mortar.
Stone Arch Bridges 