Analysis of the dynamic behaviour of an underground cavity roof via operational and numerical modal analysis

Underground cavities can pose significant hazards when located near urban areas or strategic infrastructure, but they can also represent heritage sites due to their natural or archaeological significance. The combination of geological and anthropogenic predisposing factors can lead to the formation of sinkholes, which can evolve toward subsidence and eventually generalized failure of the ground surface. Among all geological and environmental factors that can destabilize these natural structures, seismic loadings have seldom been investigated in depth, even in areas characterized by intense seismic activity. Understanding how underground cavities can modify near-surface seismic wavefields and sustain earthquake-induced stresses is paramount to designing reliable seismic risk management strategies in urban or heritage site areas. In this study, we present the preliminary results of an integrated approach that combines array-based ambient vibration measurements and eigenfrequency numerical modelling to investigate the dynamic behaviour of Il-Latnija cave, a shallow underground cavity in Malta. A drone-based photogrammetric survey was performed to reconstruct a detailed 3D model of the site, capturing its geometry, including the overhanging roof and single skylight. Ambient vibration datasets were collected using a linear array of five geophones and analyzed with the Frequency Domain Decomposition technique to identify the resonance frequencies and mode shapes of the cavity roof. The field experiment allowed us to identify four resonance frequencies, with the first mode characterized as a vertical bending motion used to calibrate a 3D eigenfrequency numerical model. The numerical model successfully replicated the first mode, but discrepancies were observed for higher modes due to site-specific structural complexities, such as fractures and heterogeneities, not accounted for in the simplified numerical configuration. This approach provided valuable preliminary insights into the mechanical and structural characteristics of the cave roof, while also highlighting the limitations of our simplified model and the need for further refinement to better account for geomechanical heterogeneities and structural complexities.