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 each weigh up to six tons. They were assembled without the use of mortar
and therefore had to be perfectly shaped to fit 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, 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.
© John Welford
I can verify the width of the top tier water channel as 3m as I walked across it in 1978! Not all the water passage cover stones were intact requiring a few scary gaps to jump and one section you had to drop down into the water channel then climb back on to the top once the cover stones were available again. No handrails or barriers just a small notice saying 'Cross at your own risk' - those were the days!
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