habitat rupestre.pdf - Società Friulana di Archeologia

S. Del Prete, M. PariseGeo-lithological and lithotecnical aspectsMechanic characteristics of rocks and excavation section playa primary role for self-bearing vaults, as they determined theircompression strength and fracturing grade, two important parametersfor the coefficient of safety on tensions in vaults. Allof these factors directly affect the use of an underground cavityin time.Tuff rocks are associated to the Quaternary volcanic activityin the Tyrrhenian coast (Tuscany, Latium, Campania). Theywere important for the development of human settlements inantiquity. These territories had volcanic deposits constitutedby a limited lava layer, and extended tephra (pozzolana) andpyroclastic rock (tuffs and piperno). Those are classified asweak rocks, but they are suitable building materials with goodphysical and mechanical properties, easily workable and goodheat insulators (VV. AA., 1967). More, the elevated quantityof easily reachable material made them the most commonbuilding materials in the Greek and Roman periods (Zevi,1994; Albertini et al., 1997; Piciocchi & Piciocchi, 2005), asthe monuments testify.On the other hand, sedimentary rocks were used to be excavated.Variously cemented and porous Pliocene - PleistoceneCalcarenite with fragments of Bryozoans, Echinids, Crustaceaand Mollusks have primary role in subtractive architecture.Those rocks are improperly known as calcareous tuffs; as theabove described volcano tuffs, they have suitable physical andmechanical characteristics, and they are easy to be excavatedand sufficiently porous (Cotecchia et al., 1985; Cherubini etal., 1993a, 1993b).Other lithotypes are easy to be excavated, and can self-sustainvaults. The presence of volcano tuffs, sandstones or calcarenite,in combination with the morphological assessment andsub vertical walls granting for a strategic position of the settlements,constituted the first element for the choice of locations.Mines are slightly different: the fundamental geological elementhere is the stratigraphic and structural setting, which determinedthe presence of a mineral vein in a direction (Fig. 3).Underground mines generally followed the main strike. Thisis evident especially in prehistoric caves, where technologicallimits brought to narrow galleries in compact rocks. The flintmines of Defensola (5.500-2.500 b.C.) in Gargano (Galiberti,2005; Tarantini, 2007) are characterized by sub horizontalexcavations of two or three calcareous layers in galleries 60centimetres high, enough to extract flint stones. The coppermine in Monte Loreto (3.500-3.100 a.C.) has a copper vein in a0,4 to 1 m wide fracture (Bixio et al., 1999).Static problems in walls and vaults rise when a work is excavatedthrough materials with different mechanical characteristic(Bieniawski, 1979). In this situation, a solution could berepresented by different kind of coverings (Fig. 4). This couldhappen also in the same lithology, due to the presence of waterveins, draining fractures, or frequent discontinuities (Fig. 5).Fig. 5 The supporting structure for the vault in a bauxite mine (CusanoMutri, Monti del Matese - Campania) is realized in correspondence with akarstic fracture (photo: N. Russo). The drip of the drainage through the samefracture is evident.Fig. 6 Floods during the works for a railway tunnel, Nocera - Salerno (fromCelico et al., 1977).Fig. 7 The induced subsidence of the aquifer allows exploiting the mineralizedmass and the consequent realization of deeper mine tunnels (from Forti,2006, modifi ed by Lang, 1995). At the end of the extraction works, the aquiferwill return at its normal piezometric level, and the tunnels will be fl ooded.Hydrogeological and hydraulic factorsWater availability is another fundamental factor for humansettlements, as it granted solutions for sanitary needs. For thisreason, very long channels were realized, which branched inunderground tunnels and cisterns as they reached the settlement.Ancient aqueducts were exclusively open air; the choiceof the springs to tap, and the path of the aqueduct as well, were21volumeRicerca_OK_2012-11-15.indd 21 16/11/2012 15:00:45