Building a stone wall is a straightforward proposition, once one has learned a few basic masonry skills and has done it a few times. However, to build a stone arch bridge, one must build an arch. It will be quickly found that an arch is not as simple to build as a wall, simply because of the arch’s curved shape. That said, there are simple ways to make this form of construction easier.
Sizing the Arch
One of the first things to consider is the size and rise of the arch. There are many options here, but ultimately it can be broken down simply:
- What is the span to be? For a stone arch bridge, study the stream to determine this. Be aware that many streams may look larger simply because of pooling backwater; if possible determine to what extent this is the case.
- What is the acceptable rise? Remember that the lower the rise of the arch, the more material required at the ends of the arch to resist thrust. It is recommended that one not build an arch lower than a 90-degree segment of a circle (rise over run approximately 1 to 5). If necessary build multiple spans.
- To calculate the required arch thickness for rounded arches 90 degrees or higher, you can use a simple formula: Take the square root of the span of the arch (in feet), and divide this number by 2.5 for your arch thickness in feet. This is intended as a minimum thickness; if your stones vary in height, just ensure none are smaller than this number.
Some Necessities
To build an arch you will need not only a temporary formwork to support the unfinished structure, but you will also need reasonably flat slabs of rock; rounded boulders won’t work. If you live in granite boulder country, probably the best bet is to find large boulders you can split slabs off with feathers and wedges. Sedimentary rocks usually can be found in flat slabs; this is especially true with sandstone, though depending on the sizes you may still need to split the stones into something more manageable. Schist is another excellent non-sedimentary stone to use that can be found in nice slabs. Shale should be avoided, as it is highly susceptible to the elements and has a nasty tendency to compress over time leading to arch distortion and collapse. Slate also can compress, though it is still superior to shale.
Basic Stonelaying
Unless you are planning on precision cutting every stone, it is best to lay arches in rubble masonry. To do this, place stones down on the form with the tapered end down; little trimming as long as you have fairly flat slabs. Rare is the stone without a natural taper, but if you’d like you can easily add a slight taper by knocking off the edges of the part of the stone to be placed downward on the form. The stones in the arch should still overlap like bricks in a wall to avoid running joints.
As a general rule, for a rubble arch the arch is easier to make with thinner stones; the thin stones tend to round the curve much better.
And Now for the Curve
To make the curve in rubble masonry there are several options. For a mortarless bridge, you will shim up the arch stones like a wall, only with much bigger shims, looking for naturally wedge-shaped slivers (see photo above). This isn’t as hard as it sounds, but it is time-consuming. Wedges for shimming can often be made simply by breaking up stone slabs or knocking off the edges of a flat slab of stone. If you are really ambitious, you can even cut crude wedges for between arch stones with a little practice; see Working With Stone Part 4: Cutting Wedges.
For mortared rubble arches, you can simply lay the stones in hard mortar to make up the angles, essentially building the arch like a wall with a generous dose of mortar to fill in the angles. This can work, but is hardly ideal, and is only suitable for small spans; mortar tends to be relatively weak and deteriorates over time, making total reliance on it a poor idea in the long term, though if your arch stones fit rather well naturally you can still get by with this. You could lay the arch like the morterless arch mentioned above, adding mortar in with the shims, but the best method is probably the traditional method. To do the traditional method, the formwork is rendered waterproof, or else the stones are placed on the form into a nice bed of mortar that will harden and waterproof the bottom joints. Once the arch is completed, the face joints are pointed and grout is poured into the top joints, then stone slivers poured in afterwards. These stone chips help hold the arch. Naturally, when pouring mortar into an arch, it is easier with a segmental arch than a Roman arch, as the mortar must somehow be forced into joints that are almost horizontal. Regardless of how the arch is built, it may be necessary to use a few strategically placed stone fragments to prop the stones up to the desired angles during construction until the arch is completed.
Conclusion
An arch need not be a difficult structure. A rubble arch can be built simply and is surprisingly strong. In fact, many medieval bridges remain in Europe that were built with rubble arches as outlined above. These relatively crude structures have stood the test of time, and many are still used, carrying loads far greater than their builders could have dreamed of. Probably the single most important key for success in rubble arch laying is to lay the stones with the naturally tapered end down, as this creates a strong start for the whole arch.
Stone Arch Bridges 