For their historical significance, special uniqueness in design and construction, and technical characteristics, ASCE’s Board of Direction has designated the Bonnet Carré Spillway in St. Charles Parrish, Louisiana, as a National Historic Civil Engineering Landmark, and the Gladesville Bridge, Sydney, Australia, as an International Historic Civil Engineering Landmark.
At a future date, ASCE will hold a ceremony and mount a 13″ x 19″ bronze plaque at each site, signifying the project as an ASCE Historical Landmark. Presently there are 265 projects in the ASCE Historic Civil Engineering Landmark Program, which began in 1964. The projects cover the categories of aviation, bridges, buildings, dams, power generators, research and development, roads and rails, surveys and maps, tunnels, urban planning, and water supply and transportation. Landmark nominations are reviewed and recommended for approval by the ASCE History and Heritage Committee.
The Bonnet Carré Spillway
Completed in 1931, the Bonnet Carré Spillway is a flood control operation in the Lower Mississippi Valley about 12 miles west of New Orleans. It allows floodwaters from the Mississippi River to flow into Lake Pontchartrain and thence into the Gulf of Mexico.
Nominated by the Louisiana Section, the Bonnet Carré Spillway represented a dramatic change in the U.S. Army Corps of Engineers flood control policy, from the traditional “levees only” approach to a new concept utilizing not only levees but floodways for a more comprehensive flood management approach.
Built in response to the Great Mississippi Flood of 1927, which inundated much of the Mississippi River basin, the Bonnet Carré Spillway consists of 2 basic components: a control structure along the east bank of the Mississippi River and a floodway that transfers the diverted flood waters to the lake.
The concrete structure was originally designed to reduce settling into the riverbank. The opening and closing of the spillway is done by hoisting 7,000 wooden “needles” (8″ x 12″ wooden beams) arranged in 350 linear bays. This is done by 2 rail-mounted gantry cranes atop the spillway. The number of needles removed determines the water flow through the floodway. Gaps built into each bay allow limited seepage anytime the river level reaches the bottom of the bays. Removal of all 7,000 needles takes about 36 hours and is usually done over a period of several days. Since opening, the Bonnet Carré Spillway has opened only 10 times, the last occasion being 2011.
The Gladesville Bridge
Celebrating its 50th anniversary last year, the Gladesville Bridge was the longest concrete arch bridge in the world when completed in 1964 and remained so for 16 years. It set new standards for bridge design and construction using concrete, and included a number of innovations that were subsequently widely adopted as standard practice.
The largest of a 3-bridge complex, including the Fig Tree Bridge and the Tarban Creek Bridge, the Gladesville Bridge, which spans the Parramatta River west of central Sydney, Australia, was intended to be the first phase of the freeway project as part of the North Western Expressway. However, because of community opposition, the freeway project was abandoned in 1977, leaving the Gladesville Bridge connecting the existing arterial roads.
Considered daring and untried at the time, the design of the bridge echoed the Roman method of building arches using segmented units built over a temporary formwork. In the case of the Gladesville Bridge, these were hollow precast concrete blocks which were hoisted up from barges on the river, then moved down a railway on the top of the formwork into position. Every few blocks, special inflatable rubber gaskets were inserted between the blocks. When all of the blocks in the arch were in place, the gaskets were “inflated” using synthetic hydraulic fluid, expanding the entire arch and lifting it away from the formwork to support its own weight.
Once adjusted to the correct position, the gaskets were filled with liquid concrete, driving out the oil and setting to form a permanent solid arch. The formwork was then moved sideways and the next arch constructed in the same fashion. Once all 4 arches were erected, the precast deck panels were placed on top of the arch. The arches were anchored into solid sandstone bedrock on either side of the river.