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Building Stone Arch Bridges: Masonry Basics

Building Stone Arch Bridges: Masonry Basics
For the DIY enthusiast, building a stone bridge is a great project. Building with stone is actually is not so very hard once one acquires the knack; practice makes perfect. The best way to learn masonry is to do it. All you need to know are some masonry basics, and then you are ready to practice.

Rules of Masonry

  • The main rule of masonry is that any joint between two stones meeting end to end needs to be spanned by the layer of stone above it. This helps bond the stonework together. An occasional exception to this is allowable; but you should never have more than two joints atop one another.
  • Stones should generally be laid with the long side horizontal; otherwise they have an inclination to tip out. They should be laid in a fashion that makes them the most stable.
  • When using mortar, it is important to remember that mortar is not glue. The best way to think of mortar is as a weak stone that takes on the shape of the stones above it and below it for maximum surface contact (and thus friction). Any mortared stonework must be able to stand without mortar. The “gluing” properties of mortar should never be relied on (mortar decays over time). These gluing properties just waterproof the joints and help keep the stonework from being dislodged by impacts.
  • With stonework laid without mortar, every stone should already be a fairly good and stable fit. The use of stone chips and shims helps maximize surface contact and keeps the masonry from wobbling. Mortarless stonework is best laid with a slight tendency to point in on itself. The heaving of frost action then can have no permanent effect on the masonry; the stones will be inclined to push together, which helps hold the structure up. Certainly the stones should not have an inclination to slide out or the project will be reduced to rubble over the years. For the top course of masonry, since there is no mortar to help prevent stones from being dislodged, the best bet is to use massive, though relatively thin, stones to help keep everything in place.
  • An arch is built like a wall that is curved along the line of the arch. Any natural angling of the stone should be utilized with the tapered end pointing towards the arch form. The use of stone chips to fill in any gaps is advisable, though it is even more critical in an arch than in a wall that the stones be a good fit.
  • One final method of stone building that is more labor intensive is to precisely cut each stone to fit perfectly on its neighbor. Such stonework is very strong, and is also the premium method of building an arch, although it is entirely possible to build enduring arches even if the stones are not precision fit.