Curiously enough, the fill material used to create a level surface for the roadbed of a stone arch bridge is a surprisingly important topic. The typical stone arch bridge consists of an arch resting on abutments (which may or may not extend up above the arch springing, depending on how much weight is required to resist any horizontal thrust of the arch) with a pair of walls running along the entire length of the bridge, forming a sort of hollow box, if you will, which is then filled in to form a surface suitable to drive on.
Types of Fill
One can simply use dirt as fill, of course, and this can work quite well. However, there are some factors to keep in mind. First, dirt and loose gravel is easily washed out in a flood if the water goes over the top of the bridge, which, under the correct circumstances, can threaten various parts of the bridge, particularly the spandrel walls.
Also, dirt gets waterlogged rather easily, greatly increasing the pressure of the fill material on the bridge.
While the arch itself should be able to handle this extra weight with impunity, the fill-retaining walls can be forced out by the horizontal pressure of the fill, which, in the case of dirt, gets “runny” when very saturated. Occasionally, this shoving out of the spandrels will even force out portions of the arch with them, especially when poorly overlapped joints in the arch were used, creating longitudinal cracking of the arch of the bridge.
The use of “spalds,” i.e. fragments of rock as fill instead of straight up dirt provides some advantages, as this rocky fill is better drained and less easily washed out. Mortarless masonry “fill,” even if the masonry is of poor quality, is a superb fill material as it makes the stone arch bridge a much more solid mass, and results in minimal pressure on the spandrel walls.
Occasionally, concrete was poured to form a very solid fill in the bridge, this fill being impossible to wash out and greatly increasing the durability of the bridge.
Advantages of Solid Fill
An advantage of these solid fills like stone and concrete is that, besides being impossible to wash out, they tend to provide excellent counteraction of any tendency of arch stone to shove out at any time. In other words, if an exceptionally heavy weight on the bridge, it is sometimes possible for the line of thrust to exit the arch, resulting in a tendency for portions of the arch to be forced into the fill area. If the fill is dirt, the sections of the arch where the thrust line left the arch ring proper can successfully be pushed out into the fill, which is relatively malleable, and can cause failure of the arch. If the fill is stone or concrete, however, this solid mass tends to resist any distorting of the arch ring, resulting in a much sturdier bridge, as the arch cannot “flex” readily even in overloaded situations, as the fill material is much more solid than dirt ever can be. (Of course, in a perfect world the thrust line should be decidedly within the arch ring and certainly should never exit the arch, period. As an unfortunate aside, do to the shape of a Roman arch, bridges employing this type of arch often times do not have thrust line safely within the arch proper, meaning that the weight of the fill is being used to help “shove” all the arch stones together to keep the bridge stable.) In short, the use of stone or concrete as fill adds a wonderfully solid weight behind the arch that helps push it solidly together.
A common problem related to the fill in stone arch bridges is waterlogging of the fill. This can cause excess pressure forcing out the spandrel walls, and can also lead to deterioration of the masonry of the bridge as a whole, the masonry soaking up this water and crumbling over time during freeze/thaw cycles. The mortar joints will tend to fail, and this all can add up over time to a surprising amount of deterioration to the stone arch bridge. Fortunately, there are some basic solutions for stopping and preventing this continuing damage to the bridge.
Solving Fill Problems
One simple solution to most fill problems is to add weep holes in the bridge so as to allow water to drain out of the bridge, to prevent the fill from becoming waterlogged. A more complicated solution is replacing poor fill materials (like dirt) with other, better materials. Whenever removing fill from a stone arch bridge, it is important to be aware of the possibility that the fill is critical to holding the bridge together, like in the case of the Roman arch. In the case of a Roman arch where the fill is critical to the stability of the bridge, a temporary form as is used in building the arch in the first place will have to be utilized to hold the arch while the fill is being removed.