Dan Cruickshank’s Bridges: Heroic Designs that Changed the World. Dan Cruickshank

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      An intimate view of Roman precision engineering: a vista along the conduit on top of the Pont du Gard. This is the only place in the structure where mortar was used – necessary to ensure water did not leak through the joints in the masonry.

      The aqueduct has an overall length of about 262 metres and comprises three distinct tiers of structure. The lower tier is formed by six wide arches (each spanning a distance of something between 15.2 and 24.3 metres) which support a roadway. From this roadway rise arches similar in size to, and with their piers set over, those below. But this second tier is made up of 11 arches because the cliff faces forming the river valley taper dramatically as they rise higher. On top of the second tier sits the prime purpose for this astonishing structure: a stone-roofed conduit that carries the water to Nîmes. This conduit is supported on narrow-span round-headed arches – three to each wide arch on which they sit – of which thirty-five still survive. The rhythm and elegance of these arches is immediately striking. It was usual in Roman arched structures of this type for the piers to be one third the width of the span of the arch, a width that led to strong piers that could, if one arch collapsed, act as abutments to prevent the whole structure collapsing like a row of dominoes. But here the piers are spectacularly slender, just one fifth of the arch width. Clearly the men who designed and built the Pont du Gard had great confidence in their abilities, and in their creation. And they were right, through their daring they made not only a thing of strength but also of intense beauty. The Pont is incredibly pleasing to look at, its arches reading up from the water, one to one to three, are like a harmonic ratio. The pattern of arches gives an immensely satisfying appearance and feeling of strength, of an object built for eternity.

      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 СКАЧАТЬ