Progressive rock failure in coastal cliffs: analysis of preparatory and triggering actions in a field laboratory at the Island of Ventotene (Italy)

Sea-wave impacts and near-surface thermal fluctuations are periodic stressors capable of exerting long-term mechanical effects that drive progressive rock mass failure and are regarded as a preparatory process for landslides. These processes pose significant hazards in coastal areas, which are expected to increase due to climate change. Despite scientific consensus identifying sea waves as a causative factor for slope instability, the mechanisms that govern subcritical crack growth and the transition to critical failures remain poorly understood. Failures often occur without any precursory signs and may happen even in the absence of major destabilizing forces.

To investigate the fatigue processes experienced by rock masses under periodic loading and to understand the mechanisms driving coastal cliff failure with a goal in hazard assessment, a coastal sector was instrumented, monitoring both subaerial and underwater environments. This monitoring focuses on the effects of stressors and corresponding rock mass deformations. As part of the TRIQUETRA Horizon EU project, the Ventotene Field Laboratory (VFL) was established and inaugurated in May 2024. The VFL was located in a portion of the tuffaceous sea cliff of the Punta Eolo promontory, where a thick succession of ignimbrite deposits (ascribable to the Parata Grande geological unit) experienced large instabilities in the past tens of years.  The cliff is still evolving with rockfall and rock toppling mechanisms, that are threatening the archaeological excavation of the Roman Villa of Giulia (of the 1^st century A.D.) and the Cemetery where Altiero Spinelli, the father of European thought, lies.

The monitoring system includes a fully equipped weather station, conventional geotechnical sensors, and specialized devices for measuring sea-wave characteristics. These devices include a sea-wave and currents Doppler profiler, dynamic titanium water-tight pressure gauges to assess wave impacts at the cliff base, and instruments to measure elastic and plastic deformation of the fractured rock mass, such as crack meters, a biaxial tiltmeter, thermocouples, and load cells.

Additionally, laboratory mechanical investigations were carried out to evaluate the strength and stiffness of the intact rock while examining the roles of water saturation and salt crystallization in rock weathering.

Findings from the first year of monitoring revealed notable responses of the fractured rock mass to intense rainfall events, which caused sharp and partially reversible fracture openings. Cyclical deformation, including dilation and block tilting, was observed in response to daily and seasonal temperature fluctuations. Data collected from the monitoring system have been used to inform a stress-strain finite difference numerical model to analyse the static influence of basal notches on slope predisposition, as well as the preparatory effects on slope stability of combined thermal and marine actions. Ongoing numerical and laboratory geomechanical analyses aim to provide a more comprehensive understanding of the progressive rock failure processes driving the evolution of these complex systems.