Geo-Archaeological sites in the Campi Flegrei volcanic Complex: Historical, Geological, and Geotechnical Analysis for their preservation

The Campi Flegrei volcanic complex provides a unique setting where archaeological remains coexist with active geological processes. Archaeological research has revealed that the current coastline lies far inland compared to the ancient shorelines, with submerged remains clearly showing the original extent of Roman settlements. Processes of ground lowering and raising have shaped a striking coastal landscape, where modern buildings stand on Roman ruins atop cliffs, with submerged structures extending into the sea. Frequent slope instabilities, mainly triggered by rainfall and seismic events, threaten both modern structures and valuable archaeological sites. This study investigates the mechanical behaviour of collapsible unsaturated pyroclastic deposits and tuff formations that influence cliff stability in the area. The textural and petrographic characterization of the unsaturated pyroclastic deposits was performed to clarify the origin of their collapse-prone behaviour. Laboratory analyses, including oedometer and direct shear tests on intact samples, demonstrate the collapsible nature of the Intracaldera Phlegraean Deposits (IPD) and quantify their collapse potential under wetting conditions. The results reveal a marked reduction in strength and volume stability upon saturation, confirming the key role of collapsible pyroclastic covers in triggering cliff failures and generating discontinuities within the rock mass. Finally, to assess cliff stability, a preliminary numerical modeling process was developed. Simulations show that failure mechanisms localize within the collapsible IPD layer, where safety factors approach critical values due to wetting. The results provide new insight into the geotechnical controls on slope stability in volcanic–archaeological contexts and supports strategies for preserving the geo-cultural heritage of the Campi Flegrei area.