This schematic of a cofferdam shows how the second pier of the aqueduct was constructed in the midst of the Potomac River’s deep water.
This photo from the Civil War period captures the Alexandria Aqueduct as it crosses from Georgetown to Virginia. In the foreground, a boat passes on the C&O Canal.
It was 1830, and Alexandria, Virginia, was jealous. The new Chesapeake & Ohio Canal was two years into its construction. When completed, it would bring cargo through Georgetown—Alexandria's neighbor across the river—and leave Alexandria without the economic benefits of this riverine trade. What was the city to do? [1]
An idea was hatched: Why not build an aqueduct across the Potomac so that boats could float off from the end of the C&O Canal and onto a bridge to Alexandria? Well … there were a few reasons why this was ill advised. For starters, at the point where the aqueduct would span the Potomac, the river is wide and deep. Multiple piers would be needed to hold up the bridge, and these would need to be built deep underwater. To complicate matters further, in 1830, there were only a few examples of deepwater pier construction for engineers to learn from. In the United States, only the Market Street Bridge over Philadelphia’s Schuylkill River was built on such deep bedrock—and those engineers hadn’t left good notes about how they got the job done. And finally, though nobody knew it at the time, railroads would quickly overtake canals as the primary mode of cargo and passenger transit—rendering this aqueduct relatively useless by the Civil War. [2]
Yet, it’s for these very reasons that the aqueduct is so historically significant. The ruins that remain on the Georgetown side are the abutment, but if you gaze across the river, near the end of the Francis Scott Key Bridge, you can spot one of the remaining piers of the old aqueduct. [3] Figuring out how to engineer and build bridges spanning deep water made it possible for the railroads to expand westward—and across many other rivers.
Congress printed reports on the aqueduct’s construction in 1838, 1841 and 1873, complete with detailed drawings, and according to historian Richard Kapsch, they “became a manual of construction on how to build railroad bridges across the wide and deep American rivers.” [4]
This schematic of a cofferdam shows how the second pier of the aqueduct was constructed in the midst of the Potomac River’s deep water.
The Alexandria Canal Company was incorporated in 1830 with the authorization of the U.S. Congress, which also gave $400,000 to construct the aqueduct. [5] The U.S. Army Corps of Engineers assigned Captain William Turnbull to oversee the project [*], and he was the one to discover that the bedrock on which they needed to build the bridge peers was an average of 28 feet below water. [6] Plans were drawn up for how to safely support the bridge, and in 1833, the Alexandria Canal Company hired Dr. John Martineau to be their contractor and begin the construction.
Turnbull was not pleased with the plan that Martineau came up with for cofferdams—an enclosure built in a body of water and then pumped dry so that workers can excavate and construct below the water line. Martineau’s design was circular, like a giant barrel, and it had no clay puddling sealing the cracks between its wooden beams. When he tried to pump the water out of the cofferdam, more flooded in to take its place. Before a good solution could be found, the cofferdam was destroyed by ice. Martineau quit! [7]
This schematic demonstrates how construction workers operated derricks to hoist and lower stones into the cofferdam for pier 1.
Turnbull took over, and he built cofferdams that were two nested rectangles, like a box within a box. Between the outer and inner rectangles, he used plenty of clay puddling to prevent water from seeping in. More importantly, he did a much better job than Martineau of driving the pilings of the cofferdam down into the mud so that water wouldn’t seep in from the bottom. Finally, by October 1834, the cofferdam was pumped dry. Workers climbed in and excavated six feet of mud, all of which was carried up in buckets by a steam-powered windlass.
In 1835, they began laying stones (weighing 3–4 tons each) and mortaring them together with hydraulic cement to build up the structure of the pier. How did they get those stones safely down into the cofferdam? First, they would be put on a boat called a scow and brought to the edge of the cofferdam. Then, workers would roll an A-shaped derrick with a winch at the top toward the boat and use it to crank the stone up off the boat floor. The derrick would then be rolled back down its tracks and over the cofferdam, where the winch would be cranked to lower it down to the bottom. This novel design was created specially for this project, and it made the work go quickly. [8] Turnbull and his team had figured out how to build deepwater piers! Now they had to repeat it a few more times.
By the time all the piers were constructed and the wooden span was built across them, it was 1843. The Baltimore & Ohio Railroad had already reached Cumberland, Maryland, beating the C&O Canal by eight years and siphoning off a good amount of its trade. [9] The canal aqueduct was used until 1888—with a brief interlude during the Civil War when it was drained so military vehicles could be driven across. In 1868, these two purposes were combined, with a vehicle deck added above the canal, but 20 years later the vehicular need was great enough that the span was replaced with a metal truss.
After the Key Bridge was completed in 1923, the aqueduct was rendered obsolete and closed to traffic. The above-water portions were deconstructed and removed in 1933, and in 1962, all the piers but one were deconstructed to 12 feet below the water line so boaters could more safely navigate the river. [10]
That last pier and the Georgetown abutments are all that remain of this engineering marvel.
Today you can visit the remaining aqueduct abutment on the Georgetown side of the Potomac River. If you peek at the river, beneath the second arch of the Key Bridge (counted from the Alexandria side), you can spot the only pier that remains from the old aqueduct.
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