The most legendary French bridges: The Viaduct of Garabit
The engineering structures world has always tried to push the limits of what is possible. Build bridges or viaducts to connect two points above an obstacle that seemed unbridgeable. Stones, concrete, metal … the materials and techniques evolve over the centuries to meet each time with greater challenges.
Among the major French structures that have marked the history of civil engineering in France and beyond, we can mention the flamboyant Viaduct of Garabit, at 200 km from the Pont du Gard!
Historic Point on the Garabit Viaduct
In the midst of economic development, it is important for nineteenth-century France to make its most isolated regions benefit from the boom in major cities. In addition, the multiplication of railways offers new opportunities to remote areas.
A major project is therefore committed to connect Bezier to Paris through the Massif Central. This new line will enable wine merchants to make their products known more easily in Paris and across the country.
A major difficulty was, however, on this project. Indeed, the route that had been planned required the construction of a structure over the Truyère river, a natural boundary. The viaduct to build was therefore forced to overlook the valley on 122 meters. Never seen for the time!
Contrary to what one can hear, it is Leon Boyer who is at the origin of this unusual metallic structure. This road and bridge engineer studied and designed the entire project for the Garabit viaduct, then entrusted it to Gustave Eiffel.
At the start of the project in 1880, the construction site is a topic of conversation for engineers around the world! How to build a bridge as big in such difficult geographical conditions? It seems like an impossible bet, but the project leaders are determined to prove that France is the best country in the world on the creation of works of art!
Because of the strong gusts of wind that rush along the Truyère gorges, it is not conceivable to build a conventional suspension bridge. We must find a solution that opposes the least possible resistance to the wind so that the work does not oscillate dangerously. The Viaduct of Garabit is largely inspired by a work built by Eiffel in Portugal a few years earlier, the Maria Pia bridge.
At the launch of the project, the Garabit Viaduct is made to become the largest metal structure in the world!
Today, nearly 150 years after its construction, the majestic Viaduct of Garabit is besides candidate to be classified with the world inheritance of Unesco.
The Garabit viaduct in figures
564 m: deck length to cross the Truyère;
52 m: height of the arch in the center of the structure;
From 24 m to 60 m: height of the various pillars of the bridge;
3000 tons of iron needed for the realization of the structure;
122 m: high above the river Truyère;
400 workers participated in the project; all trades combined;
20,370 m3 of masonry make up the bases of the metal structure;
678 768 rivets make it possible to hold metal parts together;
April 24, 1884: the date on which the two half-arches of the work are assembled;
4 years: duration during which it was necessary to wait between the end of the work and its first use of the viaduct by a train (out of phase of test).
The construction secrets of the Garabit viaduct
The project of the viaduct of Garabit brings together 3 great engineers: Léon Boyer, Gustave Eiffel and Théophile Seyrig. But the construction of the structure is not simple for all that.
Indeed, at the launch of the site, there is no road to access the Truyère Gorges and precisely on the site of the site.
The various metal parts made in Levallois Perret had to be transported by train to Neussargues. Then they were transported to Garabit with horses and oxen. These logistical problems have therefore considerably lengthened the construction time of the viaduct.
To try to limit the delays, Eiffel takes the initiative to install a real village at the foot of the viaduct.
In January 1880, the site was officially launched. The work begins with the installation of the concrete abutments at the two ends of the structure as well as the 5 seats that will accommodate the piers of the bridge.
A temporary bridge is also mounted to build the viaduct at best. This ephemeral 33m-high bridge is equipped with 2 railway tracks. Small locomotives transport materials from one bank to another. This small structure is also very useful when it was necessary to place certain parts vertically and will be even essential when raising the central arch above the void.
Tense bars, compressed bars, crosses, thanks to the know-how of Eiffel and his colleagues, the structure is balanced and protected against deformations. The metal parts are they assembled thanks to rivets, a flagship technique of the nineteenth century.
Then it’s time to install the apron. For this step, it was decided to work in 2 stages. The 2 pieces of the deck had to be pushed from side to side of the shore before being assembled. It took 60 days to push the deck on the West side for 103m and 164 days for the East Bank deck to push it on 282m.
As the deck progressed, it was also necessary to build the large arch to support the deck of several tons. Thus, after a four-year project, the two pieces of the arch are assembled together with the deck. The calculations were very accurate because the two parts arrived at 0.5cm from their ideal position.
On the Viaduct of Garabit, everything has been designed so that the building has a long life and can be used for years in the best conditions!
Thanks to its very airy structure, the wind passes through the structure without putting too much stress on it and is also protected from rust despite the humidity of the place.
Gustave Eiffel also had the idea to install a special mechanism at the base of the structure to make it more flexible and help it to absorb the movement of load during train passes. It consists of 4 ball joints that can tilt up and down and up to support the slight movements of the bridge.
The Matière® company, specialized in the construction of bridges, has not yet had the opportunity to build such legendary structures. Nevertheless, its know-how is more and more recognized in France and throughout the world. Matière® has had the opportunity to build many railway bridges in France and abroad, such as the Souppes-sur-Loing railway bridge or the Dimbokro railway bridge.
For the construction of these railway works, Matière® offers modular solutions such as Unibridge® that meet the different international standards (Eurocodes, AASHTO, British Standards and Australian Standards).