Side view

Monocacy Aqueduct,
C&O Canal

  • 2005 article on stabilization of the aqueduct for Society for Industrial Archaeology Newsletter

  • 1996 article on the aqueduct for The Chronicle of Higher Education

  • Aqueducts of the Chesapeake and Ohio Canal, in order from west to east

  • Downstream view
    View of the aqueduct from downstream, August 2005

    The towpath wall, seen from the west end

    Water level
    The towpath side, from the river bank on the west end

    Berm wall
    The berm, or upstream, side of the aqueduct, from the west end

    A section of new railing stands alongside a length of original railing bent out of place by floodwaters

    Monocacy Aqueduct Stabilization

    By Lawrence Biemiller

    Dickerson, Md. -- After 30 years in a steel girdle that held its deteriorating walls and arches together, the Chesapeake and Ohio Canal's landmark Monocacy Aqueduct has undergone a long anticipated stabilization. Thanks to a $6.4-million appropriation from Congress, its stonework was extensively repaired and reinforced, a new railing modeled on the original was installed, and the entire structure was sealed in a way that should prevent water and ice from working to pry it apart.

    The aqueduct, 516 ft. from end to end, is the longest and best preserved on the canal, which runs 184 miles between Washington and Cumberland, MD. The aqueduct was constructed of white stone between 1829 and 1833 to a design by the canal's chief engineer, Benjamin Wright. Its seven shallow arches marked off by handsome pilasters immediately made it one of Maryland's most noted structures. Although the canal closed in 1924, a victim of repeated Potomac River floods and changing transportation needs, its towpath and most of its historic structures remain intact, and today the towpath is popular with bikers, hikers, runners, and nature lovers. It has been a National Historical Park since 1971.

    But all of its aqueducts have suffered damage over the years. Denis J. McMullan, president of McMullan & Associates, served as engineer for both the Monocacy Aqueduct stabilization and for an earlier stabilization of the canal's Conococheague Aqueduct, which in 1920 lost its berm wall (the berm is the side of a canal opposite the towpath). McMullan says the canal's 11 stone aqueducts all face essentially the same two major problems.

    The first is that while the canal was in use, the structures all leaked badly, as old photographs document. In warm weather, the leaks tunneled through the rubble that fills the interior space between the bottom of the canal prism and the upper surface of the stone arches. In cold weather, water that had leaked into the structures froze and expanded, loosening stones -- particularly in the berm walls, which were about two ft. narrower than the towpath walls. Once the canal was drained for good, the leaking decreased, but the structures are still vulnerable to damage by whatever rainwater or snow melt gets inside.

    The second problem is flooding, which brought not only water but also trees and other debris crashing down the streams that the aqueducts crossed. The canal's designers had put the towpath on the Potomac side of the waterway, which made sense for other reasons but means that the aqueducts' weaker sides are upstream, where they bear the brunt of the assaults from floods.

    The two problems together, McMullan says, mean that the aqueducts are weakest just inside of their berm walls. Indeed, only three of the structures still have their berm walls and upstream spandrels completely intact -- the Monocacy, Antietam, and Fifteen Mile Creek aqueducts. The Catoctin Aqueduct, all but one arch of which collapsed in 1973, had previously lost its berm wall and upstream spandrels as well as much of its rubble fill and upstream portions of its arches.

    While the Monocacy Aqueduct has remained intact, a walk under the arch at either end reveals long cracks in the stonework beneath the berm wall. In places the upsteam spandrels bulge so dramatically that they might have been drawn in by a cartoonist.

    Hence the steel girdle, designed by Federal Highway Administration engineers, who also put hidden rods through arch stones to tie them together and partly filled the canal prism with dirt, to give the structure more weight and help it survive floods. The bracing not only surrounded and braced the aqueduct's exterior, but did the job that water in the prism once took care of, holding the berm and towpath walls apart.

    Although some skeptics doubted that the bracing did much more than spoil the structure's beauty and make it miserable for bikers to cross, McMullan says the bracing was "a very good idea to hold it all together." Some other aqueducts on the canal sport similar bracing, although not as extensive as that fitted to the Monocacy. In any event, the Monocacy's unappealing steelwork encouraged members of the all-volunteer C&O Canal Association and the park's former superintendent, Douglas D. Faris, who died in 2004, to press for the stabilization project.

    When McMullan and NPS officials began planning the stabilization, "we realized we needed something to hold the aqueduct together without its being visible," he says. Fortunately, the tie rods inserted in the arches in the 1970s were found to be in good condition and could remain. That was a start. New rods were inserted into the aqueduct's piers to hold them together, and their bases were grouted where the river had slightly undermined them.

    The next step was dealing with the rubble fill. McMullan's plan, injecting grout into the voids, proved difficult and time-consuming. The project's contractor, Corman Construction, came up with an easier approach -- removing most of the rubble and replacing it with a flowable, low-strength concrete mix.

    But the stabilization's showpiece is a new, well-drained concrete slab poured in the canal prism. It adds weight to the structure and provides a tight seal to keep water out of the interior. It also gave McMullan a tough layer into which he could drill a new set of hidden tie rods that help brace the entire structure. The surface of the concrete is scored in a pattern resembling wood planks, which McMullan surmises may have formed the prism's original floor.

    In addition to repairing and remortaring all of the structure's exterior stones, the stabilization brought a new railing to match the few sections of original iron railing that survive. But the new railing can be swiftly removed if flooding is predicted, instead of remaining in place to catch passing trees. "We want to keep this aqueduct another 150 years," McMullan says.

    The aqueduct was formally rededicated May 21, 2005.

    -- Society for Industrial Archaeology Newsletter, Vol. 34, No. 4, Fall 2005