This has always been an exciting job for our survey crews; and, since it happens so infrequently, most of the surveyors involved have only had a chance to participate in the bridge survey once in their careers. But that one time provides a story to tell family members for the rest of their lives.
Of course, just because it is done by different people doesn’t mean it is done differently each time. We, at VanDemark & Lynch, have designed programs and detailed methods to complete these specialized bridge surveys over these many years; and our field crews do an excellent job of documenting their work so that we are able to duplicate our efforts, no matter how much time has elapsed, or how surveying technology has changed.
Our survey involves two primary tasks. First, we check to see if the bridge has settled vertically. We accomplished this by determining the elevation of permanent monitoring points set in each pier (the bridges stand on 21 piers in the Delaware River). It is a tricky process to survey across the river to each of the piers. It involves transporting surveyors by boat to each pier in order to leap-frog elevation measurements across the river and back again.
The second task is to determine if the bridge towers are showing any signs of leaning. To do this, we set a survey instrument on the pier at the bottom of each tower, and then a survey crew member goes to the top of each tower. (This time around, our crews were very happy to find that the DRBA had installed elevators for each of the towers. Up until this time, our surveyors had to climb each of the towers using the permanently installed ladders. That’s a tall ladder to climb 250 feet!) A sight reading is then taken straight up the tower, and a measurement is taken at the top and bottom of the tower. If the two measurements don’t match, it indicates that the tower is leaning. The DRBA really does not want these towers to show any sign of leaning; and, thankfully, over these many years, they have not.
This specialized type of survey requires extreme precision and accuracy, and all of our measurements are recorded to the nearest 0.01 feet (1/8th of an inch).
Of course, this type of work must be completed safely, especially when our survey crews are working up to 420 feet above the Delaware River. We have a detailed safety plan in place before we start any work, and maintain close communication with the DRBA. When necessary, certain traffic lanes on each bridge are shut down to accommodate our field crews.
All in all, we are proud of our affiliation with HNTB and the DRBA, and our history with the Twin Spans, and we hope we are given the opportunity to conduct this survey the next time it is scheduled, which should be around 2017.
The Delaware Memorial Bridge is truly an engineering marvel, and we are proud to be a part of its history.
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