The foundations of a stone arch bridge are the single most important factor in determining the life of the bridge. If the foundations are good, the bridge can last for centuries — or even a millennium! If the foundations are bad, the bridge may last but a few years, or even a few months. Failure of the foundations is by and large the most common cause of stone arch bridge failures.
Curiously, stone arch bridges frequently have but shallow foundations. Setting the bridges a short distance below the river bed seems to have been the frequently favored method of construction. To prevent settling, long wooden piles were often driven into the ground where the masonry was to rest and the piers and abutments set on the piles. The logic is that wood, when not exposed to air, can last indefinitely; therefore, wooden piles rammed into the mud under the river ought to last a long time. This may work fairly well on small streams that don’t flow heavily, but for bigger streams and rivers, this method is too easily undermined.
To prevent undercutting, a “box” made of piles (properly called a “starling”) is built around the pier and filled with something, such as gravel. The problem is that not only are the foundations still rather vulnerable — especially as the starlings decay — but the starlings themselves greatly constrict the waterway. This horrendous type of “foundation” was apparently used on the old London Bridge, and to such an advantage that we now have the song “London Bridge is Falling Down” for posterity.
A major problem with shallow foundations is that the river bed is lowered over time by erosion; this is a normal process for any self-respecting river. An object like a bridge pier or abutment in the river aggravates erosion by causing “scour,” or heavy localized erosion that can quickly lead to the undermining of the foundations.
What exactly causes scour is still a hazy subject. Piers are particularly vulnerable to scour on the downstream side. However, external factors can cause scour to occur around piers, which in and of themselves are apparently safe from this destructive erosion. An example would be the once-famous concrete arch bridge over the Kaw River in Topeka, Kansas. This bridge was considered to be a masterpiece in design and stood for decades (it is rather surprising how many mentions of the Kaw River Bridge you will find in old books discussing bridge works). It met up with an unexpected end, however. The deflection of the river current caused by the piers of a new bridge being built nearby caused scour to occur at the old bridge. This caused the bridge to unexpectedly partially collapse, tragically during rush-hour traffic. An effort at temporary repairs only caused the bridge to collapse further, and thus ended an engineering landmark.
Stone arch bridges are even more susceptible to scour than most bridges, as they consist of discreet blocks rather than large solid pieces, allowing for localized settling and collapse. This is why it is absolutely critical for stone arch bridges to have their foundations looked after.
Fortunately, there are several long-lasting and relatively simple solutions to prevent scour. First, paving the river bed with concrete under the bridge provides a fine, long-lasting scour barrier. More localized concrete structures, such as aprons around the abutments and “pontoons” around the piers are cost effective solutions. Many of the bridges of Cowley, Kansas, have been treated with aprons and pontoons.
A more complex solution is “underpinning,” where concrete is put under the bridge piers and abutments. Should the scour dig down below the bridge foundation level, the bridge will still stand, as it is now resting on a sort of concrete pillar. If, happily, the bridge is set on bedrock, there is much less of a threat of the bridge being undermined. If the bedrock is, say, granite, there is almost no scour risk. However, sedimentary rocks like limestone can still be damaged by the action of water. This is why, even though most of the Cowley bridges were set on bedrock — sedimentary in origin in this part of the country — many of them have concrete protection around the piers and abutments.
Furthermore, just like the bedrock they sit on, the bridge stones themselves are slowly damaged over time at the waterline. This is another reason for the use of concrete aprons. The action of freezing and thawing water around the stones has an adverse effect on the masonry.