FREE ESSAY ON ROMAN AQUEDUCTS

ROMAN AQUEDUCTS

Ryan Gaddis
Pozzuolana and Roman Aqueducts
Western Civilization to 1660
Roman engineering was mainly of the civil type. The Romans built roads, bridges, baths,
stadiums, and other public buildings. One of the most amazing feats of engineering that
the Romans achieved was the building of the aqueducts. An aqueduct is an artificial
channel through which water is guided to the place where it is used. Most aqueducts of
ancient times were built of stone, brick or pozzuolana, a mixture of limestone and
volcanic dust. Rome itself had eleven aqueducts, ranging in length from 10 to 60 miles.
They were all built between 312 BC and 226 AD. Rome was the only ancient city reasonably
supplied with water. By A.D. 97, nine aqueducts brought about 85 million gallons of water
per day from mountain springs. Frontinus, Roman was ardent concerning Rome's aqueducts:
With such an array of indispensable structures carrying so many waters, compare, if you
will, the idle Pyramids or the useless, though famous, works of the Greeks!
While the Romans added almost nothing to pure science, they were masters of applied
science. They had no understanding of formulas or tensile strength, but they could still
figure out what worked and what didn't. Mostly, this was achieved by trial and error. The
invention of pozzuolana was one such example. The early Roman cement was made of a wet
mixture of lime and sand, with small pieces of stone, pottery, or tile embedded in it.
This type of cement was used mainly for mortared joints. Such cement crumbled easily.
Later, Romans began adding volcanic ash from the nearby Vesuvius. This new cement was
much stronger and harder than any previous cement, and it also dried underwater. When
dry, it had all of the properties of strong stone. The Smithsonian Institute has a
section of Roman aqueduct made from pozzuolana that was laid about 80 AD. When it was dug
up eighteen centuries later, it was discovered that it still behaved like stone. It did
not crumble when drilled or chipped, and in fact it was stronger than many stones would
have been after being subjected to the centuries.
While pozzuolana was a great discovery in ancient times, it was not used for building
much of the aqueduct structure. Huge and elaborate forms would have been needed to cast
the pozzuolana into the supporting columns or arches. The only thing needed to build of
stone is a strong footing. Also, pozzuolana is strongest under compression. Like modern
concrete, it has a very low tensile strength. This means that conduits made of pozzuolana
that had to carry water pressure would easily burst. Because of these factors, pozzuolana
was used mainly only to build the actual channel that carried the water.
Most of the Roman aqueducts were not made with pipe or pressurized closed conduits.
Instead the Romans built their water delivery system non-pressurized. There were multiple
reasons for this. First of all, large metal conduits were not able to be built at the
time. The largest metal conduits mentioned in Roman history were about 100 inches in
circumference. This is not nearly enough to supply the amount of water that the Romans
needed. Secondly, it was very hard to control leakage in the free-flowing channels, let
alone pressurized conduits. It was estimated that half of the water disappeared through
leakage and theft from its source to final destination.
The water ran through channels that were on average about 3 ?' wide by 6' deep. To keep
the water moving, the channels had to have a grade of at least two or three feet to every
mile. Because these were channels and not pressurized conduits, the Romans had to cut
through every ridge that was too high and build bridges over every valley that was too
low. The arcades or aqueduct bridges were all constructed in about the same manner.
First, a row of tall piers were set in. On top of these pier, a series of small arches
was placed. Above these arches was the actual water channel. This channel was constructed
of pozzuolana, with an arched roof. When an aqueduct crossed a very deep valley or gorge,
two or even three sets of arches were constructed. When people think of an aqueduct,
usually these arch bridges come to mind because they are the most noticeable part of the
Roman aqueduct system. However, Rome had over 260 miles of aqueducts, and less than 30 of
them were on arches. The length of the arcades was even less because often more than one
aqueduct would share the same arcade. The Romans never mixed the waters when they shared
an arcade because the water from each aqueduct was considered best for certain purposes.
Some of the aqueducts carried water that tasted so bad, it was mainly used for watering
gardens. Because of this, aqueducts were stacked one on top of another when they shared
the same arcade.
The pressurized conduits that the Romans did use were made of lead. These were called
siphons and were used to connect the main aqueducts to a specific place such as a king's
palace or a bathhouse. There were many different materials that the Romans could have
used for conduits. By this time, the Romans knew how to make pipes of lead, bronze, wood,
tile, and concrete. However, each of these materials has its own drawbacks. Wood will
crack and split. Bronze is hard to work and was so valuable at the time that unguarded
sections might be stolen. Tile and concrete have too low tensile strength to hold
pressure. A conduit that was built at Alatri (50 miles east of Rome) had a total vertical
drop of 340 ft. This resulted in water pressure of over 150 lbs. per square inch at the
bottom. Generally, the conduits were rolled from ? thick lead sheets. The seams were
closed with solder, and lengths of the pipe were joined together by overlapping and
soldering them in the same manner. Recently, many feet of this lead pipe was brought to
the surface of the Rhone River by ship's anchor.
Vitruvius warns of lead poisoning:
Water is much more wholesome from earthenware pipes than from lead pipes. For it seems to
be made injurious by lead, because white lead paint is produced from it; and this is said
to be harmful to the human body ... We can take example by the workers in lead who have
complexions affected by pallor. For when, in casting, the lead receives the current of
air, the fumes from it occupy the members of the body, and burning them thereupon, rob
the limbs of the virtues of the blood. Therefore it seems that water should not be
brought in lead pipes if we desire to have it wholesome.
However, this is one of the many good suggestions in ancient literature that was ignored
and forgotten. A true diagnoses of lead poisoning was not made until Benjamin Franklin
wrote on it in 1768.
When the aqueduct reached its destination, it was distributed through a fairly elaborate
system. The first step was a holding tank which kept the water for a time to let the dirt
and pebbles settle out. Next, the water was channeled to a water tower called a
castellum, or little castle. From here it was distributed by lead pipes to smaller
holding tanks and then to its final destination of a public bath, fountain, or the ground
floor of some wealthy private individuals.