habitat rupestre.pdf - Società Friulana di Archeologia

THE INFLUENCE OF GEOLOGICAL AND GEOMORPHOLOGIC ASPECT IN THE REALIZATION OF ARTIFICIAL CAVESCRHIMA-CINP projectFig. 13 Gas wreaths in the Dog’s Cave on the bank of the Lake of Agnano, inthe Phlegraean volcanic area, Campania, Italy (photo: R. Varriale).Fig. 14 Some collapses in a trachyte mine (left photo: S. Del Prete) and in atuff mine (right photos: B. Bocchino). The dimension of the rocks depends ontheir technical characteristics and their level of fracturing.Fig. 15 Ani, Eastern Turkey. Thinning of pillars in an obsidian mine (photo:R. Bixio - Centro Studi Sotterranei). The typical splintering along the pillarsis due to the vertical loads.the removal of the fine matrix, leaving rough incoherent elements.These fumaroles are subvertical and irregularly shaped(Di Girolamo, 1968, Del Prete & Bocchino, 1999). Their granulometryis similar to melted gravelly deposits, which hasvery different load characteristics from the tuff layers.Bradyseism is also associated to internal dynamics of theEarth, and it affects the use of an artificial hypogeum in time.The artificial cavities on the coasts of a volcanic area may beunder sea level because of negative movements of the Earth’scrust. So they may be confused with natural cavities, whichwere generated by the sea erosion.In the Phlegraean volcanic area, many ancient Greek and Romanvillages are now under the sea level. The same happenedto some tunnels from the Greek era near Castel dell’Ovo inNaples (Cilek et al., 1992) and to the cave called Spuntatore orVarule, in Ischia island (Buchner, 1943, Monti, 1980), whichin 2000 years has sunk some meters under the sea level.Stability of subterranean worksArtificial cavities are often abandoned, but the necessary precautionsabout their preservation are not taken (Fig. 17). This iswhy cavities suffer with precarious stability, creating risks forthe above territory. The closure of the original entrances hasaggravated the conditions, as well as the loss of memory andinformation about the spatial distributions of the hypogea hasincreased the risks.The more frequent failures in cavities are detachments of blocks(up to dozens square meters) from the vaults and/or from thepillars (Fig. 14). These partial collapses are frequently suddenand without premonition, and may occur even hundreds yearsafter the excavation.The instability of slopes where cavities are present, due to thethinning of pillars (Fig. 15) is particularly risky when they endangerarchaeological sites, as the gravine in Puglia and Basilicata.In many cases, the external walls of cavities partiallycollapsed (Bertucci et al., 1995; Bixio et al., 2002; Pecorellaet al., 2004; Parise, 2007; Fig. 16), and the stability of manycavities is seriously compromised by open cracks in pillars andvaults (Fig. 19).Excepting the above mentioned situations, the general effect ofcollapses is localized to the underground cavity and its nearbyareas, with moderate damage to people and things; even so, thealteration of the static conditions can be extremely dangerousand can cause a general collapse, with severe consequences onthe structures at the ground surface. This kind of situationsoccurs in areas where the intense subterranean excavation hascaused slow subsidence or sinkholes (anthropogenic sinkhole;Fig. 18). This phenomenon occurs in the extractive areas ofGallipoli (Fiorito & Onorato, 2004) and Cutrofiano (Cherubini& Sgobba, 1997; Bruno & Cherubini, 2005) in Salento.In Cutrofiano, the intensive extraction of local calcarenite andof overhanging clay has caused serious instability in the lastdecades (Parise et al., 2007). Analoguous situations occur inthe urban area of Naples and in the surrounding plain (VV.AA, 1967; Evangelista, 1991; Vallario, 1992; Evangelista et al.,1980; 2005; Hall et al., 2004).The discontinuities of a hypogeum can be of different nature:they can be pre-existing and strictly connected to the genesisof the rock formation (such as syngenetic fractures in a tuffformation, which can be caused by rapid cooling of melted deposits);others can be successive, due to the tensional redistri-volumeRicerca_OK_2012-11-15.indd 24 16/11/2012 15:00:47