GNGTS 2013 - Atti del 32° Convegno Nazionale

NEWDATA ABOUT THE SINKHOLE HAZARD AT CASALABATE (LECCE PROVINCE) M. Delle Rose 1 , L. De Giorgi 2 , G. Leucci 2 1 Istituto di Scienze dell’Atmosfera e del Clima – CNR, Lecce, Italy 2 Istituto per i Beni Archeologici e Monumentali – CNR, Lecce, Italy Introduction. Casalabate is a coastal small town frequently affected by sinkhole events. The first noticed event occurred on 1993 and caused several destructions of buildings. In the same year, a sinking of a small portion of emerged beach happened (Delle Rose and Federico, 2002). Four years later a collapse affected the submerged portion of beach and allowed the formations of a submarine spring, since then the bathing was forbidden for some weeks. Two episodes of sinkhole on the emerged beach occurred during 2000 and 2004, along a strike no longer than 30 meters. Each of they caused on surface short-lived elliptical depression quickly filled by sand. All the aforementioned episodes occurred inside a narrow area (about 250 m long and 100 m wide) close to the coast and oriented according to a north-north-east - south-south- west tectonic structures system (Delle Rose and Leucci, 2010). Further episodes occurred on March 2010, November 2010 and March 2011. The last noticed sinkhole happened on 7 January 2012. Usually the sinkholes occurred during or after rains or sea storms and each episode lasts some minutes at least. Due to geological and geotechnical uncertainties, Casalabate became an intriguing case study of sinkhole hazard in urban areas and three geological-geophysical models were recently proposed to explain the ground-instability and to asses the related hazard (Delle Rose and Leucci, 2010; Calò et al. , 2011; Margiotta et al. , 2012). 3D geological model and new geophysical data here shown can contribute to an efficient predictive model taking into account the hydrogeology context. Moreover, some epistemological considerations had performed to improve the interdisciplinary exchange between geologists and geophysicists. Geological setting. Casalabate is located on the Adriatic side of the Salento Peninsula (Southern Italy) on a coastal plain elevated a few meters above sea level (Fig. 1). The Salento Peninsula is the emerged southeastern portion of the Adriatic Carbonatic Plate (Auct.) which is formed by Jurassic-Cretaceous limestones and dolostones covered by Tertiary and Quaternary carbonates and marls. The bedrock of Casalabate is build up by Quaternary carbonate deposits, some ten of meters thick, lie on Cretaceous limestone (Rossi, 1969). Nevertheless, geological and geotechnical data suitable to civil engineering lacked until few years ago. As a consequence, to face the sinkhole hazard the municipal authority performed a extensive bedrock survey by means of boreholes and geophysical measures. The results of such a survey was used in Delle Rose and Leucci (2010) and in Calò et al. (2011). Casalabate bedrock resulted built up by “two overlap units the lower of which is a moderately cemented massive calcarenite made up mainly of marine bioclasts and medium-large in grain size. The upper units is formed by sets of variableness cemented thinly laminated and cross-stratified calcarenite [ancient dunes] whose grains were terrestrial or marine supplied. The latter is 5-10 m thick, whereas the base of the former are locally unknown but, on the base of the setting surrounding Casalabate, the thickness of the massive calcarenite doesn’t excess 20-25 m. Both the calcarenite units show a nearly omogeneous low-angle beds dip, which seems to be due to primary clinostratification as well as tectonic eastward tilting” (Delle Rose and Leucci, 2010). Four tectonic fracture systems (medium strikes N 5°E, N55°E, N100°E and N145°E) probably affect both the stratigraphic units. The shape and orientation of the landforms are clearly due to the spatial arrangement of the tectonic fractures. This structural control is particularly evident concern the shape of the depressions filled by clayey-silt marsh deposits. The calcarenite units were interested by various periods of dissolution karst processes to begin from the Middle Pleistocene. Usually such a process are named as “parakarstic” (Auct.) to emphasize the small dimension of the morphological forms respect those charactering the Jurassic-Cretaceous limestones and dolostones. 107 GNGTS 2013 S essione 3.2