habitat rupestre.pdf - Archeomedia

S. Del Prete, M. Parise
Geo-lithological and lithotecnical aspects
Mechanic characteristics of rocks and excavation section play
a primary role for self-bearing vaults, as they determined their
compression strength and fracturing grade, two important parameters
for the coeffi cient of safety on tensions in vaults. All
of these factors directly affect the use of an underground cavity
in time.
Tuff rocks are associated to the Quaternary volcanic activity
in the Tyrrhenian coast (Tuscany, Latium, Campania). They
were important for the development of human settlements in
antiquity. These territories had volcanic deposits constituted
by a limited lava layer, and extended tephra (pozzolana) and
pyroclastic rock (tuffs and piperno). Those are classifi ed as
weak rocks, but they are suitable building materials with good
physical and mechanical properties, easily workable and good
heat insulators (VV. AA., 1967). More, the elevated quantity
of easily reachable material made them the most common
building materials in the Greek and Roman periods (Zevi,
1994; Albertini et al., 1997; Piciocchi & Piciocchi, 2005), as
the monuments testify.
On the other hand, sedimentary rocks were used to be excavated.
Variously cemented and porous Pliocene - Pleistocene
Calcarenite with fragments of Bryozoans, Echinids, Crustacea
and Mollusks have primary role in subtractive architecture.
Those rocks are improperly known as calcareous tuffs; as the
above described volcano tuffs, they have suitable physical and
mechanical characteristics, and they are easy to be excavated
and suffi ciently porous (Cotecchia et al., 1985; Cherubini et
al., 1993a, 1993b).
Other lithotypes are easy to be excavated, and can self-sustain
vaults. The presence of volcano tuffs, sandstones or calcarenite,
in combination with the morphological assessment and
sub vertical walls granting for a strategic position of the settlements,
constituted the fi rst element for the choice of locations.
Mines are slightly different: the fundamental geological element
here is the stratigraphic and structural setting, which determined
the presence of a mineral vein in a direction (Fig. 3).
Underground mines generally followed the main strike. This
is evident especially in prehistoric caves, where technological
limits brought to narrow galleries in compact rocks. The fl int
mines of Defensola (5.500-2.500 b.C.) in Gargano (Galiberti,
2005; Tarantini, 2007) are characterized by sub horizontal
excavations of two or three calcareous layers in galleries 60
centimetres high, enough to extract fl int stones. The copper
mine in Monte Loreto (3.500-3.100 a.C.) has a copper vein in a
0,4 to 1 m wide fracture (Bixio et al., 1999).
Static problems in walls and vaults rise when a work is excavated
through materials with different mechanical characteristic
(Bieniawski, 1979). In this situation, a solution could be
represented by different kind of coverings (Fig. 4). This could
happen also in the same lithology, due to the presence of water
veins, draining fractures, or frequent discontinuities (Fig. 5).
Hydrogeological and hydraulic factors
Water availability is another fundamental factor for human
settlements, as it granted solutions for sanitary needs. For this
reason, very long channels were realized, which branched in
underground tunnels and cisterns as they reached the settlement.
Ancient aqueducts were exclusively open air; the choice
of the springs to tap, and the path of the aqueduct as well, were
Fig. 5 The supporting structure for the vault in a bauxite mine (Cusano
Mutri, Monti del Matese - Campania) is realized in correspondence with a
karstic fracture (photo: N. Russo). The drip of the drainage through the same
fracture is evident.
Fig. 6 Floods during the works for a railway tunnel, Nocera - Salerno (from
Celico et al., 1977).
Fig. 7 The induced subsidence of the aquifer allows exploiting the mineralized
mass and the consequent realization of deeper mine tunnels (from Forti,
2006, modifi ed by Lang, 1995). At the end of the extraction works, the aquifer
will return at its normal piezometric level, and the tunnels will be fl ooded.
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