Context:
The Pont du Gard, is not a bridge as the name might suggest but part of an aqueduct. It was built over five years by the Romans in the 1st Century AD to carry water to Nemausus, modern day Nîmes and is the 3rd tallest Roman aqueduct in existence. The original aqueduct was originally 50km in length, however the segment of the aqueduct that was named the Pont du Gard is very short in comparison. The pont du Gard crosses the Gardon river near the town Vers-Pont-du-Gard - literally “Near the Pont du Gard” - and is now a popular tourist attraction. The Pont du Gard was in usage up till the 6th Century AD. and its upkeep, in comparison to the rest of the aqueduct which is now almost entirely dilapidated, was due to the funds given to support it from a nearby toll gate. However, having been classed as a world heritage Unesco site and attracting many tourists, the bridge is now self-sustaining.
Structure:
The Pont du Gard is a three tiered aqueduct, with semi-circular arches of “Vers and Castillon-du-Gard” limestone. Over the aqueducts’ entire original length of almost 50km, the drop in height is only of 12.3m, and along the length of the Pont du Gard, there is a drop of 2.5m.
The gradient of the aqueduct is not constant as it was necessary to vary the gradient in relation to the lay of the land in order to optimise water flow. The average gradient is of 24.8cm/km however for more than half of its length it is less than 10cm/km as there are sections with much larger gradients of around 4m/km.
The lowest tier of the Pont du Gard has 6 arches, is 142m long, and has piers that are 6m thick and at their tallest and 22m in height. The middle tier has 11 arches, is 242m in length and has piers of 4m in width with a height of 20m. The third - and final - tier has 35 arches, although it was originally 47, a length of 275m and piers with a width of 3m in width and 7m in height.
The canal through which water passed in the top tier is called the specus. The canal bottom is made of crushed limestone and lime mortar, and the vertical walls perpendicular to the canal bottom are covered in broken terra cotta to make the canal watertight. The top of the canal along the length of the Pont du Gard is a stone vault, however when the specus passed underground as it did for 90% of its course to Nemausus, the top of the specus was a stone slab. The vaults of the specus were made of large parallel arches, and not using alternating vertical joints.
Structural behaviour:
The semi-circular masonry voussoir arch of which all three tiers are constituted, was a staple of Roman design. It puts the voussoirs material into compression, which plays to its strengths as stone has good compressive strength poor tensile and shear strengths. In addition the use of semi-circular arches as oppose to segmented or elliptical arches meant that it was possible to construct each arch as a bridge unto itself, and simply link them to each other, without the need for additional buttressing support during construction.
Whilst the Pont du Gard is obviously very structurally stable, as shown by its longevity, critical analysis conducted by Steven E. Kreely from the European Institue of Technology on this part of the aqueduct has revealed that it is extremely inefficient. The stresses from the dead load of the aqueduct amount to less than 1,500 psi, and the aqueduct could have been built to half the extrusion without undermining its structural stability. This in itself is not a large problem however, the initial cost and time of construction could have been drastically reduced with this knowledge.
Construction:
The Pont du Gard was constructed by tier. Once the line of the aqueduct was traced, the wooden foundations for the bottom piers were lain, masonry piers were then constructed and the voussoirs were then lain upon semi-circular frames until the keystone of each arch was placed, making the arches self-supporting. This was repeated for all subsequent levels.
The aqueduct of which the Pont du Gard is a part of, is testament to the extreme accuracy with which the Romans calculated trajectories and angles, and the tools which they developed to do so. One of the most essential tools used to draw out the trajectory of the aqueduct was the groma, this is a wooden stick with a cross laying perpendicularly across its top, and plumb lines hanging freely at each of the ends of the cross. It was used by the surveyor to trace out the line to be taken by the aqueduct.
Along with the groma, the chorobate was used to calculate horizontal line trajectories. Because of the lack of modern technology to store data about the positioning of different elements, it is still possible to see instructions written on the stones themselves about their placement, however most instructions were recorded on wax tablets, which were discarded after use. Another part of the construction process which is still visible are the stones that jut out from the side of the Pont du Gard onto which the wooden scaffolding was anchored.
On the construction site, much of the machinery was wooden, however Roman ingenuity meant that many of the machines were able to lift many hundreds of times their own weight. One of such machines was known as the polyspaston, and was so efficient that the very same design was used in construction well into the middle ages. The polyspaston consisted of a large wheel in which 2 - 3 labourers would walk, and a series of pulleys and winches to multiply the force generated by the labourers, in order to lift blocks that weighed at times up to 6 tonnes.
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