One of the most spectacular legacies of Roman civil engineering still stands in the Spanish town of Segovia, 70km north west of Madrid. Constructed in 80AD, the elevated structure is the final section of an 18km aqueduct built to carry clean water from the nearby mountains into the town.
The structure is typical of the Romans' ambitious plans to bring fresh water into town centres. Although not the oldest elevated aqueduct - the Aqua Marcia, built near Rome in 144BC and the 273m long Pont du Gard, built near Nimes in southern France in 20BC, both preceded it - Segovia is remarkable for its size.
With a maximum height of 31m, it is 18m lower than the Pont du Gard, but with a 700m long arcade - as the elevated section is known - the overall structure is much more impressive.
Segovia's aqueduct was built to carry water across a depression just south of the town. As it approaches the centre its gradient decreases from 3.3% to 0.3%, slowing up the flow. This is thought to have helped to prevent erosion in the difficult to maintain elevated section of the conduit, as well as producing a relatively easy to control supply for the town centre.
The aqueduct turns a sharp 53degrees corner towards its downstream end to follow the contours of the depression. Some Roman aqueducts, like the one at Aspendos in Turkey, had buttresses at such points because engineers feared that the force exerted by water changing direction could have caused them to collapse. But historians believe that the relatively flat gradient on the Segovia structure minimised these forces, and persuaded its engineers to dispense with the extra structural support.
Aqueducts are the most impressive evidence of the Romans' skill at building stone arch bridges. At Segovia the Roman engineers decided on an arcade structure, effectively a double deck bridge with two tiers of arches resting on 117 piers, to cross the shallow depression leading up to the hill on which the town stands.
'The Romans' use of repeated arches - in effect a modular system of construction - meant that they could build an aqueduct bridge of almost infinite length,' wrote Trevor Hodge in his book Roman aqueducts and water supply. At Segovia these arches are made from 20,400 locally quarried granite blocks held in compression without cement or mortar.
The main limitation on short span stone arches was their height. Taller arches needed longer columns, but these would have needed to be extremely chunky if they were to withstand horizontal forces.
For Segovia the engineers decided to give the arcade extra horizontal stability by building two levels of arches as the structure approached its highest point. Columns for the second tier stand on top of those for the first so that loads are transferred directly to the foundations. Arches at the first level then provide lateral support while those on the top tier distribute loads from the water conduit through the columns.
Even then the height of some parts of the Segovia arcade appears to have bothered the Romans. At its highest point it has a thick parapet wall built across the first tier. This effectively ties the three tallest arches together, giving them extra horizontal support at what would otherwise have been one of the aqueduct's weakest points. The extra support presumably also enabled the bridge's engineers to avoid incorporating a third tier.
Roman policy was to establish their infrastructure before they colonised a new part of Europe. At Segovia the plan was probably to get the water supply set up before populating the town.
Unfortunately the town does not seem to have lived up to early expectations. The aqueduct had the capacity to deliver water at a rate of about 20 litres per second, enough to serve a population of 35,000. But Segovia's population did not reach 33,000 until 1960.