Localizzazione e problemi di stabilità dei siti rupestri dell'area pugliese

Localization and stability problems in apulian rupestrian settlements ABSTRACT: The large number of rupestrian settlement sited in Apulia region have had the possibility to born and to grow up also in order to the special geological and geomorphological habitat of the sites where are localized. Only in few area it is possible to have a rock that is at the same time compact, impermeable, ma easy to be dug and modelled in architectural forms Among the different Apulian sandstone only the Gravina Sandstone (Lower Pleistocene) has this sort of characters, so that the localization of large rupestrian settlement is strictly conditioned by the outcropping of this lithotype and by the geomorphological characters of the site. While some settlements appear to represent a sort of troglodytic metropolis, which extend for several kilometres and are articulated in a compact succession of overlapping levels (from 3 to 12) of numerous caves and cavities, other settlements resemble ‘villages’, made up of a limited number of caves located on the same subhorizontal level, sometimes only one or two caves. The first of these settlements, located in correspondence to the steep slopes of the gorges, are influenced by the fact that in Puglia these are located only there. The most simple settlements were founded on the ‘lame’ and on modest, subvertical fault planes and/or palaeocoastlines. These sites since their origin they have suffered for an intrinsic fragility related to their constructive technique and to the distribution of the empty zone dug in the rock masses. In this way the actual stability condition are strongly conditioned by the continuous modification of the state of stress in the sandstone rock masses; due to complex distribution and concentration of stresses in the rock as an effect of the excavation activity and of the load transmitted by the buildings. The sandstone rock has locally a low diagenesis degree and it is characterized by an high imbibitions coefficient. The detrimental micro-climate conditions has favoured chemical processes as decalcification of rock and biochemical corrosion processes; favouring loss of strength of sandstone and rock collapse and mutilations to the decorative and architectural elements. An important destabilization role is also played by rock joints due rarely to tectonic activity and more often to stress relief due the valley o to digging activity or to the applied loads. The relative movements between rocky blocks induce detrimental effect on the sites. An other detrimental effect is induced by the morphogenetic dynamics of the sub-vertical valley banks of the sites. These banks are affected by falls and toppling of rocky blocks and secondary by translation slide.