The Pont du Gard, Nimes, France

The Pont du Gard is a remarkable survivor from a previous age. Built nearly 2,000 years ago it does not look very different today than it did when first built by the Romans during the first century of the Christian era.

The Pont du Gard was built as an aqueduct to cross the River Gardon near the Roman city of Nemausus (modern Nimes in southern France). It was part of a system of canals that brought water to the city from springs 50 kilometres away. The system was carefully calculated so that the water flowed downhill at a constant gradient of 1:3000, given that the springs were only 17 metres higher than the city.

This meant that the aqueduct crossing the River Gardon had to do so at a height of 49 metres (161 feet). The result was a three-decker structure that needed to be carefully designed and built. This would have been an engineering challenge even today, with all the modern materials, computers and techniques currently available. The fact that the Roman engineers were able to solve the problem without such aids makes the final result all the more amazing.

The construction was designed to minimise the strains caused by the massive weight of the masonry. The lowest level consists of six arches that are 22 metres high and six metres wide. Resting on top, but extending further due to the lie of the land, is the second level that has 11 arches that are four metres wide.

The highest level is the water channel itself. This comprises 35 small arches and is seven metres high and three metres wide. The length of this third level is 275 metres (900 feet).

The Pont du Gard was built from limestone blocks that weigh up to six tons apiece. They were assembled without the use of mortar and therefore had to be shaped with extreme accuracy so as to fit alongside their neighbours. One special technique the builders used was to alternate the courses of stone in terms of the direction in which the blocks were laid. A row of “headers” was laid on top of one of “footers” rather than headers and footers alternating within the same row. This meant that friction could be used to provide extra binding strength to the structure.

The aqueduct took about three years to build and would have needed a workforce of around 1,000 men using simple tools. The blocks would have been raised into place with a block and tackle system powered by treadmills.

When complete, the Pont du Gard would have transported around 20,000 cubic metres of water every day. It continued to do so for the next 300 years, but during the 4th century it began to fall into disrepair. Even so, water flowed across the bridge until the 9th century when it became completely blocked by silt.

The Pont du Gard functioned as a footbridge as well as an aqueduct, and in the late 17th century it was decided to widen the bridge by shearing some of the stone off the pillars of the middle level. This proved to be a mistake because it looked for a time as though the whole structure might collapse. Repairs had to be made and an extra footbridge was added alongside the original low level in 1747. Further restoration work was done during the 19th century.

Today’s visitors can cross the Pont du Gard via the 18th century footbridge and admire the engineering and workmanship of the original builders. It is hardly surprising that the Pont du Gard became a UNESCO World Heritage Site in 1985.