The Landslide of Agrigento Hill (Sicily, Italy)

Abstract

This paper illustrated the geological, morphological and hydrogeological studies performed for the analysis and monitoring of the landslide involving the northern side of the hill of Agrigento (335 m a.s.l.) on which the ancient Cathedral was built during the 11th Century. The hill is made of a typical Plio-Pleistocene transgressive succession made of clays (M. Narbone formation), calcarenites, sands and clayey soils (Agrigento formation). The area has been unstable since 1315, involving both the little-welded, very porous and fractured calcarenitic sections (E-W) from Pleistocene and the clay layers interstratified within these sections. Since 1924, from time to time, various typologies of disruption have occurred: falls, flows and more complex phenomena. Fractures of the calcarenitic sections of the hill, pointed out by Grappelli’s studies in 1966, have reactivated from March 7, 2005 and until now, thus triggering movements of the side of the hill and damages to the infrastructures. The landslide movement is very slow and it is located in the slope mainly from the bottom of the hill to XXV Aprile Street. The re-activation of the existing discontinuity represents the rotational sliding surface that passes along the sub-vertical crack and through the cracked calcarenites, the silty sands and the altered light-brown clays. The rotational sliding appears to be the main reason for anomalous behavior of the ground Cathedral system and probably the heavy rainfalls, from September 2004 to February 2005, could have caused the movement.