The vast lower arches, one upon the other, carry a more delicate structure and it’s this delicate upper structure that is the business-end of this engineered marvel. Nothing here is for show: everything is designed and built to support the relatively small conduit running 48.7 metres above the river, constructed with a slight fall so that the water within it could flow serenely from Uzes to Nîmes.
This is indeed heroic architecture, the epitome of that produced in the ancient world. It’s a thing intended to last and is built on a heroic scale to fulfil a seemingly modest function. Yet it carried one of the blessings of civilization – a ready supply of water – to thousands of people. It made cities habitable, farms verdant and people clean, healthy and happy.
As if the form, scale and proportions of the Pont du Gard are not enough to impress, its materials and techniques of construction are almost as remarkable. From the huge blocks of stone from which it is wrought, there are, here and there and in regular fashion, strange protuberances. These tell of the way the structure was made. The stones were cut at a quarry on the riverbank and carted to the site as massive cubes – some weighing 6 tonnes or more – and then hoisted into position using one of the variety of lifting machines Roman builders had at their disposal. But as with all arched and domed structures, the construction process and maintaining stability are problems.
Arches and domes are immensely strong forms – especially when rendered in solid masonry – for their very shapes become stronger as they bear the downward force of gravity, the ‘dead’ weight of their materials and the ‘live’ weight of any loads they might carry. This is a very direct example of strength through design. But by their very nature, these curved forms, although immensely strong, exert some of their load in a lateral direction. Domes and arches want to spread, and so have to be restrained by adequate abutments or buttresses. On the Pont du Gard this is achieved by the piers, which are ultimately restrained and stabilized by the immovable cliff faces, onto which they pass the weight they carry, giving them stability.
But another problem with arches and domes – especially when being built high and on a large scale – is stability during the construction process, before all the forms are locked in equilibrium and before the final keystone is in place. The favoured way in the Roman building world to achieve stability during construction was to support the incomplete dome or arch on a timber scaffold, shaped and engineered to carry the structure until it was complete and could carry itself. This scaffolding was known as ‘false-work’ or ‘centering’. What’s fascinating about the Pont du Gard is that completed elements of its structure – notably the piers and the springings of the arches – served as part of the system of scaffolding that was used to help support those parts still under construction. So the stones protruding from the face of the piers supported timbers that formed part of a scaffold that allowed masons to work on the higher portions of the Pont. The same is true of the bold cornices at the springing level of the arches and the ribs that project from the ‘intrados’ or lower face of the arch. Both these details were not intended to be primarily ornamental but to provide a firm lodging-place for the timber centering required for the construction of the arches.20 A number of these strange projections could have been removed when construction was complete, but it seems that the engineers here saw no need to remove the evidence of the construction process and, more to the point, everything could be used again, in various ways, for any necessary repair work. So, at one level and among many things, the Pont is a permanent scaffold carrying fixing points for use in its own future maintenance. This is an astonishingly far-sighted and very modern concept.
Equally ingenious as this designed-in system of maintenance, is the way in which the stones used in the bridge were cut and fixed together. The engineers realized that maximum strength would result from maximum precision, for if the individual stones fitted tightly together movement would be minimal. Precision was difficult to ensure, but the masons here achieved it to such an extent that the stones are laid without mortar (except in the conduit at the top), which was a huge bonus in itself, because without mortar to be washed away by rain, or blown by wind, or cracked by frost, there would be no need for regular re-pointing to keep the structure sound. The Roman engineers really were creating buildings to last for eternity. The ambition is incredible and the achievement massive. The world that created this mighty work has long gone, but its vision endures in work as steady and timeless as nature itself.
The scale and quality of the construction that the engineers and masons achieved can only be properly appreciated by considering for a moment the tools and machines at their disposal. They had picks and chisels made from hardened iron for working stone, which were adequate, with skill, application and time, for cutting limestone and sandstone with precision. They also used water power to operate stone saws and lifting machines, though not to any great extent.’21 Construction work, from cutting and transporting stones to raising them into place, was a very important part of the economy of the Roman state. It was a way of creating work and jobs, keeping people employed and out of trouble, and of getting money to flow through society. For example, Emperor Vespasian (AD 69-79) refused to let builders use water-driven hoists ‘lest the poor should have no work.’22 So in place of mechanical power, the Romans preferred – almost as a matter of state policy – to achieve lifting largely by muscle power, operating cranes or systems of pulley blocks hung from legs or poles and worked by winch or capstan. Using these devices, heavy stones were lifted by slings, or gripped by pincers or a kind of ‘lewis’, a lifting tool comprising metal bars inserted and wedged into a dovetailed cavity cut in the top of a block of masonry.
Detail of the Pont du Gard showing piers rising from the lower arcade. The projecting stones were supports for timber scaffold during construction.
Metal was also used in construction, usually in the form of wrought-iron cramps or bars set in lead and then placed in cut recesses to bond stones together. As Vitruvius explains when discussing how to avoid the problem of crumbling mortar: ‘…leave a cavity behind the [facing wall]…on the inside build walls two feet thick…and bind them to the fronts by means of iron clamps and lead.’ Work executed in this way, Vitruvius claimed, ‘will be strong enough to last forever.’23
Vitruvius also had some specific things to say about aqueducts, reminding his readers of the importance of an adequate and reliable water supply and stating that for aqueducts or ‘conduits’ the masonry should be ‘as solid as possible and the bed of the channel have a gradient of not less than a quarter of an inch for every hundred feet, and let the masonry structure be arched over, so that the sun may not strike the water at all.’24 Vitruvius also recommended that, when the water carried in the conduit reaches the city it should be held in a ‘reservoir with a distribution tank in three compartments.’ The system was intended to segregate water used for different purposes and to prevent people tapping into the main flow and stealing public water for private use.25 Vitruvius also recognized that water quality was very important, so recommended that it was best to