When we observe an aqueduct there are two things which immediately spring to mind (no pun meant!) The first of these is their size which is rendered particularly imposing by the regular archways which in some cases reach as high as 110ft (30m). The second is the precision required to maintain a constant downward slope and flow in the absence of pumps. Pumps existed but they certainly weren’t capable of handling of the volumes of water in question. Excessive slope would mean sloshing water, lack of slope would bring points of stagnation.
Masonry of Aqueducts
The masonry used to construct the aqueducts could take many different forms of which the principal ones were bricks and mortar or stone. Local materials would be used wherever possible and this implies a great variety of architectural results. The mortar itself was quick drying and was probably made of pure chalk rather than common lime mortar which is slow to dry.
The famous aqueduct of Segovia in Spain is amazing not only because of its size but also because of the quality of the work. The stone used to construct it has been perfectly chiseled in order to create a perfect match with the masonry surrounding it. In this way no mortar was actually required to bind it and the aqueduct still stands. In other cases brickwork was mixed with stone. Foundations would be of stone, the pillars of either stone or brick, the arch of stone and the parapet possibly of brick. Availability of materials was all important. Brick tended to make the job easier and faster but the greater durability of (hard) rock was always borne in mind.
Sizing and construction of the supporting pillars of Aqueducts
The next question is how big should the pillars and arches be in order to support their own plus the weight of the water? The Roman engineers probably resorted to a mixture of experience coupled with graphical means which ensured that the thickness of supporting pillars matched the size of the arch above and sufficient to carry the weight channeled through it.
The pillars would be constructed on top of deep foundations which preferably rested on a solid rock or heavily compacted under-bed. It would not be quite common for the foundations to reach several meters under ground. They would normally be constructed out of solid rock blocks, possibly bound together by iron keys.
The greatest difficulty was met when such pillars would have to be constructed in order to cross rivers. In these cases wooden skirts rendered impermeable with clay and terracotta would allow dry access to the river bed in order for proper construction of the foundations (see section on bridges for further details).
Constructing the arches of Aqueducts
A cursory glance at an aqueduct will show that the supporting pillars often, if not always, have a collar of jutting masonry towards the top, before the arch section begins. The purpose of this collar was to provide a ledge on which to support wooden trusses around which the arch would then be built. Once the arch was complete on top of its supporting pillars the truss could be safely knocked down.
The trusses were made of wooden beams which on the outer edge of the curve (the one in contact with the masonry) actually fitted into holes in the masonry. This provided greater stability during construction. These holes in the masonry remain visible and could be reused should the arch section require scaffolding for repair work.
In areas where wood was scarce, such as northern Africa for example, there is evidence to suggest that the trusses could even be made of ceramic (terracotta) and reused. The ceramic sections were tubular and made rather like water pipes so that one section could be inserted into another, thus creating a longer section. These ceramic trusses were extremely versatile as they allowed many different forms and shapes to be assembled very rapidly.