The Impact Problem

Stone bridges are particularly vulnerable to impacts by traffic, as the individual stones are easily dislodged. In this post, we investigate ways to prevent this type of damage from happening.

Historic stone bridges are highly vulnerable to impacts from road traffic. The nut of the problem lies in the fact that individual stones are much more easily dislodged than, say, the cohesive concrete curb of historic concrete bridges.

Maintenance

Dislodged stones are best replaced as they are knocked off, and maintenance of the mortar (when used) helps hold the stones in place. Unfortunately, this maintenance does not make a stone bridge immune to damage from vehicular impacts. This means, for highly traveled structures, some modification may be required to prevent repeated damage.

Young Street Culvert — Dislodged stones are a common sight in the walls of historic stone culverts exposed to impacts. In this case, even some of the arch stones have been dislodged. Routine maintenance is advisable in cases like this to prevent further damage.

Concrete Curbs

Concrete curbs are the best bet for long-term stability, as the solid weight of the concrete curb is much harder to move than individual stones. Furthermore, the curb can be used as an anchorage point for traffic barriers as required; traffic barriers should not be bolted directly to stones, so if a traffic barrier is in order a concrete curb is pretty much a requirement.

Hill Bridge — The concrete curb on this bridge supports a modern traffic barrier. Where the concrete curb was used, the bridge has fared better than the sections where the traffic barrier was bolted directly to stones.

Concrete Decks

Concrete curbs can still be dislodged, especially on small structures where the curb is by necessity short. Probably the single most durable answer is a concrete slab deck poured on top of the stone bridge, with the concrete curbs built in. This type of structure is almost impossible to dislodge, and has the added advantage of substantially increasing the durability of the stone bridge by distributing the weight of loads. All in all, pouring a concrete deck where acceptable will result in a great improvement to a stone bridge.

NE 110th Street Double Arch Walnut River bridge — The curb and railing on this bridge are a part of the cantilevered slab added on top of the structure to widen it. This type of deck helps greatly to preserve the bridge, though it does obscure it somewhat.