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Construction
Construction of Pont du Gard
ancient Roman aqueduct bridge that crosses the Gardon River in Vers-Pont-du-Gard, France
- 1st century – 1st century CE
- Europe
Construction
ancient Roman aqueduct bridge that crosses the Gardon River in Vers-Pont-du-Gard, France
Fetching the article from Wikipedia…
The Pont du Gard is an ancient Roman aqueduct bridge built in the first century AD to carry water over 50 km (31 mi) to the Roman colony of Nemausus (Nîmes). It crosses the river Gardon near the town of Vers-Pont-du-Gard in southern France. The Pont du Gard is one of the best preserved Roman aqueduct bridges. It was added to UNESCO's list of World Heritage sites in 1985 because of its exceptional preservation, historical importance, and architectural ingenuity.
The bridge has three tiers of arches made from Shelly limestone and stands 48.8 m (160 ft) high. The aqueduct formerly carried an estimated 40,000 m3 (8,800,000 imp gal; 11,000,000 US gal) of water a day over 50 km (31 mi) to the fountains, baths and homes of the citizens of Nemausus (Nîmes). The structure's precise construction allowed an average gradient of 1 cm (0.39 in) in 182.4 m (598 ft). It may have been in use as late as the 6th century, with some parts used for significantly longer, but lack of maintenance after the 4th century led to clogging by mineral deposits and debris that eventually stopped the flow of water.
After the Roman Empire collapsed and the aqueduct fell into disuse, the Pont du Gard remained largely intact with a secondary function as a toll bridge. For centuries the local lords and bishops were responsible for its upkeep, with a right to levy tolls on travellers using it to cross the river. Over time, some of its stone blocks were looted, and serious damage was inflicted in the 17th century. It attracted increasing attention starting in the 18th century, and became an important tourist destination. A series of renovations between the 18th and 21st centuries, commissioned by local authorities and the French state, culminated in 2000 with opening of a new visitor centre and removal of traffic and buildings from the bridge and area immediately around it. Today it is one of France's most popular tourist attractions, and has attracted the attention of a succession of literary and artistic visitors.
The location of Nemausus (Nîmes) was somewhat inconvenient when it came to providing a water supply. Plains lie to the city's south and east, where any sources of water would be at too low an altitude to be able to flow to the city, while the hills to the west made a water supply route too difficult from an engineering point of view. The only real alternative was to look to the north and in particular to the area around Ucetia (Uzès), where there are natural springs.
The Nîmes aqueduct was built to channel water from the springs of the Fontaine d'Eure near Uzès to the castellum divisorum (repartition basin) in Nemausus. From there, it was distributed to fountains, baths and private homes around the city. The straight-line distance between the two is only about 20 km (12 mi), but the aqueduct takes a winding route measuring around 50 km (31 mi). This was necessary to circumvent the southernmost foothills of the Massif Central, known as the Garrigues de Nîmes. They are difficult to cross, as they are covered in dense vegetation and garrigue and indented by deep valleys. It was impractical for the Romans to attempt to tunnel through the hills, as it would have required a tunnel of between 8 and 10 kilometres (5 and 6 mi), depending on the starting point. A roughly V-shaped course around the eastern end of the Garrigues de Nîmes was therefore the only practical way of transporting the water from the spring to the city.
The Fontaine d'Eure, at 76 m (249 ft) above sea level, is only 17 m (56 ft) higher than the repartition basin in Nîmes, but this provided a sufficient gradient to sustain a steady flow of water to the 50,000 inhabitants of the Roman city. The aqueduct's average gradient is only 1 in 3,000. It varies widely along its course, but is as little as 1 in 20,000 in some sections. The Pont du Gard itself descends 2.5 cm (0.98 in) in 456 m (1,496 ft), a gradient of 1 in 18,241. The average gradient between the start and end of the aqueduct is far shallower than was usual for Roman aqueducts – only about a tenth of the average gradient of some of the aqueducts in Rome.
The reason for the disparity in gradients along the aqueduct's route is that a uniform gradient would have meant that the Pont du Gard would have been infeasibly high, given the limitations of the technology of the time. By varying the gradient along the route, the aqueduct's engineers were able to lower the height of the bridge by 6 metres (20 ft) to 48.77 metres (160.0 ft) above the river – still exceptionally high by Roman standards, but within acceptable limits. This height limit governed the profile and gradients of the entire aqueduct, but it came at the price of creating a "sag" in the middle of the aqueduct. The gradient profile before the Pont du Gard is relatively steep, descending at 0.67 metres (2 ft 2 in) per kilometre, but thereafter it descends by only 6 metres (20 ft) over the remaining 25 kilometres (16 mi). In one section, the winding route between the Pont du Gard and St Bonnet required an extraordinary degree of accuracy from the Roman engineers, who had to allow for a fall of only 7 millimetres (0.28 in) per 100 metres (330 ft) of the conduit.
It is estimated that the aqueduct supplied the city with around 40,000 cubic metres (8,800,000 imp gal) of water a day that took nearly 27 hours to flow from the source to the city. The water arrived in the castellum divisorum at Nîmes – an open, shallow, circular basin 5.5 m in diameter by 1 m deep. It would have been surrounded by a balustrade within some sort of enclosure, probably under some kind of small but elaborate pavilion. When it was excavated, traces of a tiled roof, Corinthian columns and a fresco decorated with fish and dolphins were discovered in a fragmentary condition. The aqueduct water entered through an opening 1.2 metres (3 ft 11 in) wide, and ten large holes in the facing wall, each 40 centimetres (16 in) wide, directed the water into the city's main water pipes. Three large drains were also located in the floor, possibly to enable the nearby amphitheatre to be flooded rapidly to enable naumachia (mock naval battles) to be held.
The spring still exists and is now the site of a small modern pumping station. Its water is pure but high in dissolved calcium carbonate leached out of the surrounding limestone. This presented the Romans with significant problems in maintaining the aqueduct, as the carbonates precipitated out of the water during its journey through the conduit. This caused the flow of the aqueduct to become progressively reduced by deposits of calcareous sinter. Another threat was posed by vegetation penetrating the stone lid of the channel. As well as obstructing the flow of the water, dangling roots introduced algae and bacteria that decomposed in a process called biolithogenesis, producing concretions within the conduit. It required constant maintenance by circitores, workers responsible for the aqueduct's upkeep, who crawled along the conduit scrubbing the walls clean and removing any vegetation.
Much of the Nîmes aqueduct was built underground, as was typical of Roman aqueducts. It was constructed by digging a trench in which a stone channel was built and enclosed by an arched roof of stone slabs, which was then covered with earth. Some sections of the channel are tunnelled through solid rock. In all, 35 km (22 mi) of the aqueduct was constructed below the ground. The remainder had to be carried on the surface through conduits set on a wall or on arched bridges. Some substantial remains of the above-ground works can still be seen today, such as the so-called "Pont Rue" that stretches for hundreds of metres around Vers and still stands up to 7.5 m (25 ft) high. Other surviving parts include the Pont de Bornègre, three arches carrying the aqueduct 17 m (56 ft) across a stream; the Pont de Sartanette, near the Pont du Gard, which covers 32 m (105 ft) across a small valley; and three sections of aqueduct tunnel near Sernhac, measuring up to 66 m (217 ft) long. However, the Pont du Gard is by far the best-preserved section of the entire aqueduct.
Built on three levels, the Pont is 49 m (161 ft) high above the river at low water and 274 m (899 ft) long. Its width varies from 9 m (30 ft) at the bottom to 3 m (9.8 ft) at the top. The three levels of arches are recessed, with the main piers in line one above another. The span of the arches varies slightly, as each was constructed independently to provide flexibility to protect against subsidence. Each level has a differing number of arches:
The first level of the Pont du Gard is adjacent to a bridge that was added in the 18th century. The water conduit or specus, which is about 1.8 m (6 ft) high and 1.2 m (4 ft) wide, is carried at the top of the third level. The upper levels of the bridge are slightly curved in the upstream direction. It was long believed that the engineers had designed it this way deliberately to strengthen the bridge's structure against the flow of water, like a dam wall. However, a microtopographic survey carried out in 1989 showed that the bend is caused by the stone expanding and contracting by about 5 mm (0.20 in) a day under the heat of the sun. Over the centuries, this process has produced the current deformation.
The Pont du Gard was constructed largely without the use of mortar or clamps. It contains an estimated 50,400 tons of limestone with a volume of some 21,000 m3 (740,000 cu ft); some of the individual blocks weigh up to 6 tons. Most of the stone was extracted from the local quarry of Estel located approximately 700 metres (2,300 ft) downstream, on the banks of the Gardon River. The coarse-grained soft reddish shelly limestone, known locally as "Pierre de Vers", lends itself very well to dimension stone production. The blocks were precisely cut to fit perfectly together by friction and gravity, eliminating the need for mortar. The builders also left inscriptions on the stonework conveying various messages and instructions. Many blocks were numbered and inscribed with the required locations, such as fronte dextra or fronte sinistra (front right or front left), to guide the builders.
The method of construction is fairly well understood by historians. The patron of the aqueduct – a rich individual or the city of Nîmes itself – would have hired a large team of contractors and skilled labourers. A surveyor or mensor planned the route using a groma for sighting, the chorobates for levelling, and a set of measuring poles five or ten Roman feet long. His figures and perhaps diagrams were recorded on wax tablets, later to be written up on scrolls. The builders may have used templates to guide them with tasks that required a high degree of precision, such as carving the standardised blocks from which the water conduit was constructed.
The builders would have made extensive use of cranes and block and tackle pulleys to lift the stones into place. Much of the work could have been done using simple sheers operated by a windlass. For the largest blocks, a massive human-powered treadmill would have been used; such machines were still being used in the quarries of Provence until as late as the start of the 20th century. A complex scaffold was erected to support the bridge as it was being built. Large blocks were left protruding from the bridge to support the frames and scaffolds used during construction. The aqueduct as a whole would have been a very expensive undertaking; Émile Espérandieu estimated the cost to be over 30 million sesterces, equivalent to 50 years' pay for 500 new recruits in a Roman legion.
Features of the Pont du Gard
Although the exterior of the Pont du Gard is rough and relatively unfinished, the builders took care to ensure that the interior of the water conduit was as smooth as possible so that the flow of water would not be obstructed. The walls of the conduit were constructed from dressed masonry and the floor from concrete. Both were covered with a stucco incorporating minute shards of pottery and tile. It was painted with olive oil and covered with maltha, a mixture of slaked lime, pork grease and the viscous juice of unripe figs. This produced a surface that was both smooth and durable.
Text from the Wikipedia article “Pont du Gard”, available under CC BY-SA 4.0.