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Introduction
The Lake Pontchartrain Causeway holds the distinction as the longest over-water highway in the world. The 24-mile long bridge spans 1/1000th of the earth's circumference on its trek between Metairie and Mandeville, Louisiana. The twin spans are situated 84 feet apart with an estimated 30,000 vehicles per day (vpd) utilizing the crossings. The northbound span is free to motorists. The southbound span carries a $3 levy to passenger vehicles.
The 24-mile long causeway features an average height of 15 and 16 feet over the waters of Lake Pontchartrain. Incorporated into the design are three main ship passes, where the bridges elevate to a height of 25 feet. Larger vessels can pass through two marine spans, each with a 50 foot clearance. A drawbridge for the largest of ships exists eight miles south of the north shore. The highest point of the bridge overall is no more than 60 feet.
Seven turnouts are included along the route for breakdowns and other maintenance needs. Variable Message Signs (VMS) can be found at the crossovers to alert motorists of potential accidents and other hazards. A radio station and 102 call boxes also are part of the safety precautions incorporated into the overall bridge operations.1
History
As the 1718-founded city of New Orleans grew during the early 20th century, accessibility became an increasing problem. These issues culminated in the mid 1920's. During that time, proposals arose to address the need for improved access from the city of New Orleans to the North Shore of Lake Pontchartrain. Of these, one of the more popular at the time involved a north-south causeway across the middle of the lake. The lake's relatively low depth, an average of 10-16 feet, allowed the conceptualists the idea for dredging a series of islands from north to south, for an eventual roadway between the two coast lines. The envisioners planned to connect these new land bodies with a series of land bodies, thus creating not only a crossing, but developable land. In fact, several plots of land were sold before the project ever broke ground. It was at this time, that another proposal arose for a Lake Pontchartrain crossing. This concept involved the construction of a two-lane concrete bridge to the east. Then gubernatorial candidate Huey P. Long was in staunch opposition to this bridge proposal, as it was to include a $1.25 toll per car. His bid for office in 1924 was in vain, and the bridge dubbed the Watson Williams Bridge (U.S. 11) opened to traffic. By 1928 Huey P. Long ran successfully for the governorship of Louisiana. One of his first desires was to see a free bridge across the Pontchartrain. The realization of this came in the form of the Rigolets Bridge (U.S. 90) between Lakes Pontchartrain and Borgne to the east.
With the 1920's proposal to create islands for a causeway north-south abandoned and new crossings to the east. New schemes arose for a crossing over the middle of the lake. New technology and increased demand led to the mid 1950's idea for what would become the Lake Pontchartrain Causeway. The innovation that allowed for the new crossing involved pre-stressed concrete. The 1949 completed Walnut Street Bridge in Philadelphia, Pennsylvania was the first pre-stressed bridge built. The success of that project proved that the technique could work for much larger projects. For the Lake Pontchartrain Causeway, this granted the concept the ability to become reality.
The formation of the Louisiana Bridge Company in 1954 represents the formal beginning of the Lake Pontchartrain Causeway time line. Work began in 1955 on the first span, with the acquisition of 40 acres between Mandeville and Lewisburg for the operations that would create the pre cast bridge sections. The prefabricated bridge pieces created at the job site could be easily transported by barge to the bridge construction for assembly over water. The on-site location of these facilities allowed for quick construction of the bridges. The first pilings were sunk on May 23, 1955.
Pre-stressing the concrete piles allowed for hollow piles as opposed to solid piles. That meant that instead of having a maximum width of 24" for a support, engineers could now create 54" diameter piles. A revolutionary construction method allowed for the spinning of the new concrete supports at higher speeds. This compacted the concrete and removed excess water, thus allowing for a stronger cured concrete. These actions allowed for piles with a load support of 10,000 pounds per inch.
Each pile would be no longer than 16 feet in length. Holes bored within the concrete housed pre-stressed high strength steel wire. Assembling the piles involved stacking each pile on top of one another until the desired height of 88 feet was reached. This length allowed for each pile to submerge 70 feet within the sand strata below. A steam pile driver did the job, and the 4 and 5/8th inch wall supports were sawed off to a consistent height.
Above the piles, concrete caps are mounted via steel posts. This element of the construction allows for the support of 54 foot long concrete slabs. These slabs constitute the actual bridge deck and roadway surface.
The original design featured two separate drawbridges, each eight mile from shore. The $29 million 23.83 mile project opened four months ahead of schedule to much fanfare. It took 200 engineers and workmen to complete the job. The August 30, 1956 opening date culminated with the assembly of 2,243 piles, each supporting a 33 foot wide 56 foot section of roadway. The highway carried a $1 toll.
While the connection was welcomed by all, the convenience of the route and mileage it saved for travelers eventually overwhelmed the highway. Within a few years the bridge succumbed to 3,000 vpd. From this volume resulted mid 1960's preparations for a second lake crossing.
The parallel span to the 1956 bridge began during September of 1967. Breakthroughs in construction methods became evident with the design of the new bridge. Instead of two piles per support, designers decided to increase the number of piles to three. The increased strength allowed for an increased load support. Thus instead of 54' slabs as used in the first bridge, 84' slabs were used instead. Speedy construction resulted in the creation of 20 300-ton slabs within a seven day time period at times. This culminated in the construction of 1,600 feet of bridge per day. Additionally increased diameter of the pile walls themselves to 5" allowed for more cover over the steel reinforcement bars. This increase in size decreases the effects of the brackish water on the steel itself.
The construction of the new northbound bridge also brought about the creation of seven connecting spans/alleys. The $26 million project completed in 25 months with only two construction related deaths reported. Construction coincided at the time on the old bridge as well. This bridgework resulted in new bascules, widening of the navigational channel to 125 feet, and the replacement of southern drawbridge with a ship channel. On May 10, 1969, the causeway opened to traffic.
The northbound span is actually slightly longer than that of the southbound span. The 23.87 mile bridge holds the title of the largest over-water bridge in the world. Additions to the bridge vicinity include the placement of "spare parts" at various locations along side the route. These are in place in case a structural impairment occurs by way of boating collisions. In fact, there have been 16 collisions in the history of the causeway. 55 travelers have died since 1961 along the causeway, some in due part to the boating accidents. To help prevent such events from occurring, a radar system is in use to alert officials when a boat draws to within one mile of the spans. Additionally 80 police units and 30 maintenance trucks patrol the causeway at any given time. These operations in conjunction with yearly inspections allow for the a safe bridge system