Cracks in a stone arch bridge are clues to something going on in the structure. All cracks signify tension in the cracked region. This tension pulls apart the joints and possibly even the stones, leading to cracks. Cracks are not necessarily a problem in and of themselves, though it is recommended that they are pointed up as if they were intended joints to prevent water issues. What causes the cracks is of the most significance, and the underlying cause should be determined and addressed as needed.
An Overview of Cracks
Settlement Cracks
Cracks mean tension. Sometimes this tension is caused by settlement. This is obvious if there is a distinct tilt in the wall. Settlement cracks are often mostly vertical. Some settlement is harmless, but the cause of the settlement should be determined. Gradual settlement due to the weight of the bridge over the course of many years is one thing, but settlement due to undermining indicates a problem that must be rectified.
Bulging Walls
Cracks can also be caused by horizontal pressure pushing out walls. These are usually quite easy to spot, as a section of the wall will usually stick out visibly beyond the rest of the wall. Obviously, this soil pressure should be resolved, but these types of bulges are not usually a sign of imminent failure if slight.
Hinges
Cracks caused by hinges in the arch are very serious; they mean the arch itself is unstable and structurally failing. Typically, there are several obvious hinge points, creating a distinctive pattern clearly indicating failure.
Front Circle Cracks
A peculiar type of crack is the so-called “front circle crack,” and is caused by a section of the arch breaking free from the rest of it. Soil pressure pushing out spandrel walls can be a cause; in this case the spandrel wall was bonded well enough to the arch that, rather than merely bulging, the sliding wall took a piece of the arch with it. Another cause is in the case of a stone bridge with soft fill and hard spandrel walls. Heavy loads flexing the arch can cause the part of the arch below the spandrel wall to break free, as this part is stiffer than the center part of the arch thanks to the reinforcing effect of the spandrel walls. Front circle cracks are always a concern, as they affect the weight-limit of a bridge, and can lead to collapse of the arch facing, spandrel wall, etc. However, they are not necessarily a sign of a major overall problem, and can be repaired.
Conclusion
While there are plenty of cracking possibilities, the ones listed are some of the most common. In any crack, the crack itself is merely the tell of a different issue. The inspector’s job is to determine what caused the crack in the first place, to monitor the crack over time to see if it is worsening, and to advise repair of the underlying problem and possibly the cracked part of the masonry itself, depending on the level of damage.
Stone Arch Bridges 