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BOTTINI
OF SIENA, ITALY: A MONUMENTAL UNDERGROUND MEDIEVAL AQUEDUCT
A. COSTANTINI1,
I.P. MARTINI2
1Dipartimento di Scienze della Terra, Universit¨¤
di Siena, Italy ˇˇ
Most
people are familiar with the impressive Roman and Medieval aqueducts which
locally span deep valleys with elegant, yet solid, multistorey arches. Less well
known are equally impressive Medieval underground works done to carry waters to
isolated walled towns. Siena is rediscovering and partially refurbishing one
magnificent, 25 km long system of tunnels (called bottini, from the
semicircular shape of the roof which resembles the shape of a half barrel; that
is, a small botte), which was built during Medieval times. These tunnels
collect water from the surrounding hill sides and carry it to the city. This
system consists of tributary small, secondary tunnels leading to two large
principal ones (Fonte Gaia and Fontebranda) which in turn
discharge into public monumental fountains.
Furthermore, numerous distributary tunnels carry water from the main
tunnels to the various houses (utenze). The whole constitutes not only an
engineering feat for the times, but also a complex, enduring, architectural art
work, and everything took advantage of the particular geological setting of the
area.
Why an underground aqueduct? Perhaps this was dictated by two main
things. (1) Siena is located in a Neogene rifted basin, on a hill of Pliocene
sandstones and conglomerates underlain by silty clays. The hill is bounded on
three sides by deeply dissected flanks, and on one side by topographically
irregular highlands underlain by sandstones and substrate rocks. No major river
flows nearby, and only few seasonal streams run in deep valleys, thus their
water cannot be brought by gravity to the city. This lack of water may have been
one of the reasons why Siena was never an important Etruscan or Roman
settlement. (2) However, during Medieval times, Siena grew to reach a dense
urban population of more than 80,000 inhabitants over an area of 2 km2
in 1300 AD. The thriving city needed water, a lot of it. Bringing it from the
hills surrounding the rifted basin by gravity would have required major
constructions of subaerial aqueducts, extremely expensive and difficult to
defend. Thus, it was decided to seek water in the immediate vicinities of the
town, collect and transport it inside the walled city through a more readily
defendable underground aqueduct. The construction of such an aqueduct was
feasible because the local sandstone is readily excavated and maintain vertical
walls. The collecting tributary tunnels were dug to near surface along the upper
and middle slopes of hillsides to intercept the surficial aquifers. The aquifers
are Pliocene bioturbated shoreface sandstones and conglomerates intelayered with
aquifuge lagoonal to lower shoreface and offshore silty shale. The recurrence of
up to four such shoreface sequences is associated with eustatic movements during
the overall transgressive Pliocene phase of that region. They are well exposed
on the tunnel walls.
Systematic construction of the tunnels started about 1200 AD and
continued, on and off, until 1466 AD. The underground aqueduct was excavated by
hand at various depths (2 to 18 m) below the hilly surface to maintain the
gradient necessary for water to flow. No particular rock bed was followed.
Locally, hard concretionary layers were encountered, which made the excavation
difficult, and the tunnels from an average width of 80 cm and height of 1.80 m
were reduced to the minimum necessary for a person to go by. To make sure the
pre-established direction was maintained during construction, to aerate the
tunnels, and to remove the extracted material, vertical wells (smiragli)
to the surface were dug as the tunnel construction progressed.
Parts of the tunnels were covered by bricks, primarily where there was infiltration of waters at the contact between sandstone and silty clays, and near open fractures. A small (15 cm wide; 20 cm deep) open channel (gorello) in the floor carried the water. Out of the distributary tunnels under the city, reservoirs were built which could be accessed from the surface through wells. To ensure that a sufficient amount of water was delivered to the clients, the main gorello was either partially dammed or was forced to bend thus slowing the flow and deepening it at the intersection with distributaries. The water could then flow into the secondary channels through plates perforated with holes (dadi) of different size, which determined the quantity of fluid that could pass through. The clients were charged according to the number and size of the dadi they required. |
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