Problems with fill in a stone arch bridge tend to fall into one of two categories. Either spalling and ice-related damage appears on portions of the bridge, or, more commonly, parts of the bridge start bulging out or sliding off. Both of these problems are, in fact, almost always a sign of water infiltration. In the end, fill problems and water infiltration go hand in hand. The primary exception to this general rule is damage caused by very loose fill pressing excessively outward on the approach and spandrel walls under a heavy load, causing damage.
For fill-related damages repair or even a rebuilding of the damaged bridge piece is likely necessary, but, to ensure a permanent fix, it is important to address the underlying problem of the fill itself.
Proper drainage can fix many fill-related problems in a stone arch bridge. Since most fill-related problems result from waterlogged fill, it follows that allowing the water to drain off will help prevent damage. True drainage problems are mostly a concern with mortared bridges only. A mortarless bridge can and does drain neatly through the joints, preventing water buildup, while the mortar in the joints of a mortared bridge tends to trap water in the fill. To aid in draining a mortared stone bridge, builders sometimes would add weep-holes. Most stone bridges, however, were not built with weep-holes. For this reason, it may be desirable to add weep-holes unobtrusively to the bridge.
Bracing the Bridge
Methods of bracing bulging components of the bridge, such as adding tie-rods and buttresses, can help stabilize things, but may not fix the root cause of the problem. These methods do not address waterlogged fill nor ensure drainage, and, unless carefully executed, negatively affect the appearance of the bridge. Hence, problems may continue to occur. On the other hand, if the spandrel walls and approaches were not originally built strong enough to hold the fill or modern traffic loads are overloading the spandrels, reinforcing the walls is a good option.
Also, tie-rods may still be a good or even necessary solution for situations where pressure from the fill and loads has delaminated a section of the arch ring from the rest of the arch. In severe cases, rebuilding the failing bridge components will probably be necessary. When rebuilding, it may be possible to strengthen the walls by increasing their thickness on the inside of the bridge; perhaps another wall, hidden inside the bridge, can be built to effectively increase the thickness of the exterior stone facing wall.
Replacing the Fill
In most serious cases of fill-type problems leading to shifting walls and partial collapse, it may be best to simply replace the fill altogether. Gravel can be used, as it does not become waterlogged like soil, though it is important to realize that gravel will still exert a sizable horizontal force on the spandrel walls, particularly under loads. Stones, either loose or laid, can be used as fill as well. Lightweight concrete is typically used to replace the fill in major bridge rehabs, and has the advantage of relieving horizontal pressure from the spandrel walls.
Fill problems have been a common difficulty historically, and replacing loose dirt with solid material often is the best solution for eliminating the problem. For two interesting historic examples of fill-related problems and the recommended and/or implemented repair procedures on two real stone arch bridges, see A Cowley County Stone Arch Bridge Before Its Time: The Island Park Bridge and The Hill Bridge on Dry Creek, Augusta, Butler County, Kansas.