Streets of Washington is the brilliant blog covering some of DC’s most interesting buildings and history written by John DeFerrari. John is also the author of the equally brilliant Lost Washington DC. ‘Streets of Washington Presents…’ will feature some fascinating buildings and history from around PoPville.
The Aqueduct Bridge, Gateway to Georgetown
Before the magnificent Francis Scott Key Bridge was completed in 1923, a far homelier structure linked Georgetown to Rosslyn. Known as the Potomac Aqueduct or Aqueduct Bridge, it was born of Alexandria’s aspirations to rival Georgetown as a commercial hub. A remarkable engineering achievement, the bridge served as a vital Potomac crossing for 80 years.
The Potomac Aqueduct, c. 1865. Source: Library of Congress
It all began with construction of the Chesapeake & Ohio Canal in the 1820s. The canal project was a long, complex, and expensive effort originally intended to spur commercial trade with Georgetown (and Washington) by establishing an economical transportation link to the vast and fertile Ohio Valley. It turned out to be too expensive to build it all the way across the mountains to the Midwest, and it never lived up to its investors’ early hopes, but in the 1820s it seemed like the next big thing for the city. Alexandria merchants sorely wanted to get in on this expected action. It would have been too expensive to unload canal boats arriving in Georgetown and reload them on river boats to take them down to Alexandria, so a non-stop method was needed to get the canal boats to Alexandria.
The solution was to build an aqueduct bridge over the Potomac and connect it to a canal on the Virginia side to carry boat traffic down to Alexandria. Congress granted a charter to the Alexandria Canal Company in 1830 and pitched in $100,000 to support the project, which was to be privately owned. Work began in 1833 on both the bridge and the Alexandria canal and lasted a full decade. (All that’s left of the canal is a recreated lock at the privately-owned Tidal Lock Park on the Alexandria Waterfront.)
When Congress stepped in, it put the U.S. Topographical Engineers, predecessors of the Army Crops of Engineers, in charge of the bridge work. Captain William Turnbull (1800-1857) headed this daunting task. Building the bridge’s piers was the biggest challenge. The plan was to construct cofferdams at appropriate spots in the river, pump the water out and then build the piers inside them. However, they had to be built at an incredible depth—through 18 feet of water and 17 feet of silt—to reach a solid bedrock foundation. River cofferdams had never been built so deep before. The first ones erected leaked mercilessly and had to be completely replaced. The second set were little better, filling with water after an hour or so and with mud oozing in from below.
As recounted by Pamela Scott, Turnbull was clearly concerned that the deep and unproven cofferdams—even when finally watertight—might not hold up while the bridge piers were being constructed inside them. In his journal, he observed that the spectacle of “men busily at work so far below the surface of the river, seemed to interest the public exceedingly; but to the engineer, whatever might be his confidence in the ability of the dam to resist the immense weight which he knew to be constantly pressing upon it in the most insidious form, the sight was one which filled him with anxiety, and urged him to the most unceasing watchfulness.”
Continues after the jump. (more…)
