vast drums open at top and bottom – are wrought, towed into position, sunk so that their bottom edges sink into, and are sealed by, soft mud or sand. Then the tops, standing well above high water mark, are sealed, and compressed air pumped in to keep water and mud from entering. To retain air pressure inside the caisson, workmen were to enter and exit the sealed chamber by means of an airlock.
A lenticular truss of the type used by Isambard Kingdom Brunel for his Royal Albert Bridge, Saltash, England, completed in 1859. The large wrought-iron tube forming the top arch of the truss is tied, or restrained, by the lower, inverted, arch made of a chain fabricated of wrought-iron bars. This design means that the arched truss exerts no outward thrust.
There was even a system devised for removing material – ‘muck’ as it was called – to the surface by means of a pit, filled with water to act as an airlock, via a ‘muck tube’ that was cleared by a crane fitted with a clamshell bucket. The water-filled ‘muck tube’ also helped to regulate pressure in the working chamber. Excess air pressure could escape through it and the tube could be used to prevent water pressure dropping – which was essential if water and mud were to be prevented from flowing into the caisson.
It was all very ingenious – the only problem was that the system killed or crippled people. The engineers didn’t know anything about what happens to the human body when it toils within a compressed atmosphere, and they had no concept that bubbles of gas can build up in the blood, and expand when a normal – decompressed – atmosphere is re-entered too quickly, causing not only great pain but serious, possibly terminal, physical damage. This distressing condition is also suffered by divers who ascend rapidly after spending too long a time at a great depth and has become known as ‘the bends’.
Although this mysterious ailment was quickly associated with pneumatic caissons, indeed it became known as ‘caisson disease’, ambitious engineers did not abandon the use of this useful time-saving invention and in 1872 it even caused the disablement of the engineer Washington Roebling when supervising the construction of New York’s Brooklyn Bridge (see page 287). When James Eads started construction of his epic railway bridge across the Mississippi at St Louis in 1867, he was under intense time pressure with loans only becoming available when specific construction targets had been achieved. So, naturally pneumatic caissons were the preferred option which, with men working up to 28 metres below water level, resulted in the deaths of fifteen men, the crippling of two, and serious injury to a further seventy-seven.
Due in part to these woeful sacrifices, Eads completed construction on time, and as it happens created one of the most significant bridges ever built. It crossed the river in three mighty arches, the longest with a span of 158 metres, and all constructed out of steel – which was the first time this material had been used so extensively in any major building. It was also the first time that a cantilever support system was used to aid bridge construction, with the arches being built outward from the piers, secured by guy wires, and used as platforms from which to continue construction until the two halves of the arch met.19 This method reduced the need for expensive scaffolding but, more important, was – to all practical intents and purposes – the only way to build a large bridge across the wide, deep and fast-flowing Mississippi.
The opening of the bridge on 4 July 1874 was a gala event. The poet Walt Whitman was present and soon declared the bridge ‘perfection and beauty unsurpassable’. A few days later, having had the aesthetics of his bridge so pleasingly praised, Eads demonstrated the solidity of the bridge to a wondering public by leading an elephant over its spans and then, realising this was perhaps not quite convincing enough, sent fourteen locomotives across the bridge, one after the other. The bridge (now known as Eads Bridge) was indeed a modern wonder. Popularly regarded as beautiful, evidently strong, and a record-breaker because its length of 1,964 metres made it the longest arch bridge in the world. Eads’ bridge was in many significant ways a model for, and portent of, things to come.
The Eads Railway Bridge across the Mississippi at St Louis. Started in 1867 to the designs of James Eads, the bridge pioneered the large-scale use of steel in construction and when completed in 1874 was the longest arch bridge in the world.
CHAPTER ONE
EMPIRE
The Pont du Gard, near Nimes, France: an aqueduct built around 2,000 years ago and one of the greatest surviving monuments from the Roman world. The stone blocks projecting from the pier serves as supports for timber scaffolding and centering during construction and maintenance.
ROME, MORE THAN ANY EARTHLY POWER BEFORE or since, expressed its might and its aspirations through architecture and engineering. It took the architecture of the past – of the Egyptians, the Greeks and the Etruscans – and transformed it to suit its own needs and to realize the demands of its growing empire. New functions such as roadways and water supplies emerged, demanding new types of buildings, and these could only be achieved through a radically evolved understanding of the potential of engineering. A series of spectacular developments took place, notably the rapid refinement of structural systems incorporating round-headed arches and domes, giving rise to buildings of unprecedented scale and complexity in which new materials such as concrete (see page 312) were used in major roles for the first time. Roman bridges are a perfect illustration of how awe-inspiring beauty can emerge from innovative engineering.
Rome’s breathtaking and groundbreaking contribution to the development of architecture was characterized by an ever-growing appreciation of the potential of engineered structures in which the forces of nature were harnessed and tamed to complete projects that would have been beyond even the imagination, let alone the practical grasp, of earlier generations. The resulting structures combined the cardinal architectural virtues identified by the Roman architect Vitruvius over 2,000 years ago – ‘commodity, firmness and delight’ – by which he meant an architecture that simultaneously fulfils its functional requirements and is stable, while also being poetic and imbued with a power to inflame and engage the intellect.
The aqueduct in Segovia, Spain, constructed during the 1st century AD, snakes through the town, its conduit supported on increasingly tall tiers of arches as the ground level falls away.
Roman architecture, at its best combining sublime beauty with function, often played a vital role in consolidating and spreading Roman power and maintaining its civilization. This fascinating combination of characteristics is perhaps best expressed by the bridges and aqueducts that Rome created throughout its empire. Their roles as routes of communication and the means of supply were of vital importance to the well-being of the Roman world. Aqueducts brought a plentiful supply of water – essential for the Roman concept of civilized life – and roads transported goods and luxuries over great distances, allowed wealth creation through trade, and security through speedy troop movements. In addition to being functional objects, bridges and aqueducts were also intended, in their design and solidity, to express Rome’s cultural aspirations and the longevity of its vision. Together, these intentions produced structures of intense beauty – a beauty that comes from the pure and powerful realization of functional demands and of the way in which the potential of available building material can be enhanced through design.
A remarkably large number of Roman bridges survive, in whole or in part, still fulfilling their original function within the former empire. They continue to astonish, inspire and delight, through their scale,
