Historic Stone Bridge Repairs – Final Report

The Hunterdon County Road Department gives some local Delaware news about the realization of its innovative repair of the historic stone culvert on Sandbrook-Headquarters Road. The original stone arch, sagging and partially collapsed, was lifted back into position without dismantling it. Parts of the intrados (the inner barrel of the arch, not the stones you see on the side walls) were lifted up four or five inches, and it is now just the shape it was when new. This is the first time such a technique has been tried around here. The engineers and repair crew are pleased with the result, and enthusiastic about having the new capability. John Glynn, Director of Roads, Bridges, and Engineering, told me: “I was pleasantly surprised, from a structural standpoint” with the success of the arch repositioning. “You and I won’t ever have to discuss this bridge again, in our lifetime.”

Mr. Glynn says this repair cost $42,000 in labor, material, and equipment. The road was closed for less than three months, with the actual repair work taking six weeks. Building a new two-lane (as now required) culvert in a similar situation, with stone veneer on its walls, recently cost about $100,000 and took a month or two longer. In that case, the old culvert’s foundations were very poor, and it would have been fruitless to attempt a repair like this one.

Asked about whether this kind of historically accurate repair would be done routinely from now on, Mr. Glynn pointed out that such repairs must be considered on a case by case basis, since no two are identical. He was pleased this time to be able to follow the recommendation of the Stone Arch Bridge Inventory report commissioned by the county, phase two dated November, 1998. Having commissioned and paid for the report, “We should use it as our guideline”. In the case of this culvert, Mr. Glynn points out that the report actually recommended closing the road altogether, if necessary to preserve the historic structure.

Repairs have been planned for the tall stone arch bridge on Pine Hill Road, near the covered bridge. First the arch will be entirely repointed, and then portions of the sidewalls and parapet walls will be dismantled and rebuilt. The rebuilding will be done like the original, but with somewhat higher parapet walls, since they are currently dangerously low. How much higher will depend partly on whether Delaware Township agrees to lower the level of the road leading up to the bridge, which has been built up thicker and thicker over the years. While the county is responsible for the bridge, the township owns Pine Hill Road.

We didn’t get photographs of the filling and paving process. The repaired arch barrel was covered with clean 3/4 inch stone, and that was covered with dense graded aggregate to support the asphalt pavement. Before paving, a waterproof barrier membrane was laid on the aggregate and turned up against the inside of the parapet walls to prevent water from penetrating into the bridge structure. Then a mat of asphalt was laid down, with a minimum thickness of one and a half inches.
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