Landslide-infrastructure interaction: slope gravitational deformation on a steep slope in stiff clay upstream of the Santo Stefano Viaduct (Matera – Southern Italy)

Slope gravitational deformations pose significant and complex geomorphological risks, with direct implications for the stability of road infrastructure, particularly for critical structures such as bridges. This study presents a critical analysis and an interpretation of a slope gravitational deformation on a quite steep slope in stiff clay of the Subapennine Clay Formation, characterized by thick interbedded sand levels. The slope is upstream of the Santo Stefano Viaduct (SS 655 "Bradanica") in Basilicata Region, Southern Italy. This infrastructure has recently experienced extensive structural displacement, necessitating deck repositioning. This research contributes to the understanding of the slope deformation activity in relation also to rainfall in the light of the management of engineering geological and geotechnical risk for infrastructure, particularly concerning bridge interaction. These findings were validated by a monitoring network, installed between 2022 and 2024, comprising two inclinometers and two piezometric verticals, instrumented to a depth of 60 meters. Inclinometer data interpretation identified active slip surfaces at depths ranging from 25 to 30 meters. The presence of elevated pore water pressures and their variations suggest a certain groundwater circulation in sand levels in stiff clay. A correlation was found between piezometric increases and accelerated displacements, highlighting a critical interaction between hydrological dynamics and the mechanical response of the slope. These results underscore the importance of integrated, long-term monitoring strategies for assessing and managing deep-seated slope instabilities, especially in areas with critical infrastructure.