Thursday, August 14, 2008
Pont du Gard
The highest aqueduct the Romans ever built, described as “the most daring construction” of its day, supplied the provincial town of Nemausus (modern Nîmes) in Gaul (France). It delivered daily an estimated 44 million gallons (200 million liters) that were distributed through ten mains to the city’s baths, fountains, public buildings, and houses. The most spectacular part of it is now known as the Pont du Gard. The description on the UNESCO World Heritage List of this remarkable feat of engineering reads: “The hydraulic engineers and … architects who conceived this bridge created a technical as well as artistic masterpiece.”
Aqueducts had been employed more than two millennia earlier in the cities of the Indus valley and Mesopotamia, but the extensive systems that the Romans constructed both at home and abroad were the most sophisticated in the ancient world. The earliest, the Aqua Appia, was a 10-mile (16-kilometer) underground conduit built to serve the city of Rome in about 310 b.c. The Aqua Marcian, built 150 years later, carried water 56 miles (90 kilometers) to the capital; for about a fifth of its length it was supported on arches above the ground. Altogether, ancient Rome was supplied by eleven aqueducts, delivering an estimated 350 million gallons (1.6 billion liters) daily.
By the end of the first century b.c. Nemausus had become a key Roman settlement with a growing population, thought to be close to 50,000. Its water supply was inadequate, and following a visit by the emperor Augustus’s son-in-law Agrippa in 19 b.c., plans were put in hand to bring water about 30 miles (50 kilometers) from the Fontaine d’Eure springs at Uzès to a stone reservoir near the city. The aqueduct was probably completed during the reign of Trajan (a.d. 98–117); it took over eighty years to build. In all Roman aqueducts, water flowed from source to destination under gravity, so the scale of the project was daunting—mountains had to be tunneled, hollows filled and valleys crossed—and a high degree of precision also was critical: the Uzès spring stood only 57 feet (17 meters) above the reservoir, and the 12-mile (20-kilometer) direct route between them called for 30 miles (50 kilometers) of aqueduct winding through the hilly region. The average gradient was a mere 1 in 3,000; in some places it was as gentle as 1 in 20,000. It was made steeper just before it reached the Gardon Valley, in order to reduce the height of the awesome Pont du Gard across the river about 11 miles (18 kilometers) northeast of Nîmes.
The massive 155-foot-high (47.2-meter) three-tiered bridge spans 920 feet (275 meters) across the valley. It was constructed of locally quarried limestone, finely dressed into ashlar blocks, some weighing up to six tons. The lower two tiers are laid without mortar, the blocks being secured with iron clamps. The 473-foot-long (142-meter) bottom tier has six irregular voussoir arches, 73 feet (22 meters) high, set out to span between stable rock outcrops in the riverbed. The middle tier has eleven 67-foot -high (20-meter) arches; spanning just over 800 feet (242 meters), it carries a 23-foot-high (7-meter), 35-arch arcade that supports the specus, a covered rectangular water channel about three feet (1 meter) wide and 6 feet high. That the Romans considered the structure to be utilitarian and “ordinary” is evidenced by the projecting blocks that nobody bothered to dress after the centering that they supported was removed.
The bridge has proven to be extraordinarily strong. Despite the Gardon River being “one of the most treacherous and rapid” in France, the Pont du Gard has resisted its onslaught for two millennia. Sheer mass—the weight of stone has been estimated at 16,000 tons (14,500 tonnes)—is one reason for that, and another is the combined skills of the engineers who designed it and the masons who built it. Moreover, its slightly convex line and shaped piers combine to resist the current and whatever debris the river sweeps down, even when the entire bottom tier is entirely immersed, as sometimes happens; the foundations stand on bedrock: and its arches are much thicker than was then customary.
The simplicity of construction of the Pont du Gard provides an excellent demonstration of the Roman technological development par excellence: the voussoir arch. They learned much from their Etruscan forebears, including the exploitation of the arch, that enabled them to develop a new kind of architecture. When a flat stone beam (lintel) spans between upright supports (posts), the construction method is called trabeated. If the span is too great or the lintel too heavy, tensile or stretching forces will cause it to crack on the bottom. Posts needed to be closely placed to avoid such failure. The arch and its extensions—the vault (arches placed side by side) and the dome (intersecting arches)—allowed the use of compressive materials (the only durable kind available) to enclose space, spanning huge distances without the need for intermediate supports. That technique, known to the Greeks, Egyptians, and Mesopotamians, had not been exploited by them but the Romans applied it with genius to build such bridges as the Pont du Gard and to create vast interior spaces.
The wedge-shaped stones used to construct arches are called voussoirs. Normally, they were cut roughly to shape in the quarry and accurately dressed on the site. Centering (a temporary timber framework), itself strutted from the ground or from projections in the lower levels of the structure, carried the arch until it could support itself. The first voussoirs were placed at the springing (the part of the pier carrying the arch) and others added until a keystone locked the arch at the center.
Some scattered remains of the aqueduct survive: a 55-foot-long (17-meter) three-arched bridge near Bornègre; a few exposed sections of the subterranean channel; another raised section near Vers; sections of tunnel; and the Pont de Sartanette, spanning about 100 feet (32 meters) over a small valley. The Pont du Gard itself continued in use until the ninth century, after which it was abandoned. Some of its stones were subsequently plundered by builders with less noble projects in mind, but generally it has survived human intervention. The bottom tier has long been used as a pedestrian thoroughfare, and in the Middle Ages stones were removed from one side of the piers so that laden mules could pass. In 1702 local authorities began renovations, attempting to close cracks, filling in ruts and building corbels to help support the road. The removed stones were replaced, and in 1743 a new bridge was built beside the lowest tier, widening the “roadway” for vehicular traffic. Overawed by the majesty of the structure, Napoléon III ordered its restoration (1855–1858), and subsequent projects have consolidated the mighty piers and arches.
As noted, the Pont du Gard was given World Patrimony status by UNESCO in 1986, and two years later the Regional Council of Gard began a historical and ecological protection program. In 1991–1992, fretted stones in the bottom level were replaced and waterproofing was improved; similar work continues on the second tier. Currently, the monument attracts more than 1 million visitors annually. There is growing concern that the Pont du Gard will be further threatened as those numbers increase—an example of the tension that everywhere exists between conservation and tourism.