Geomechanical modelling and stability analysis of the shallow underground water reservoir 'Palombaro Lungo' (Matera-Italy)

The city of Matera, as several other ancient historical cities and towns, is recovering a large part of its ancient parts developing a policy of urban regeneration. In the early 90s, during the restoration works of the main square, Piazza Vittorio Veneto, it was re-discovered a huge underground space used as water reservoir named Palombaro Lungo. It was built linking ancient pre-existing smaller caves and completed at the end of the nineteenth century making it impermeable with the cocciopesto (opus signinum) technique. The hypogeum is dug in a Plio-Pleistocene rock called Gravina Calcarenite. It is a calcareous sandstone weak rock with good mechanical properties, low permeability and easy to be dug. The internal geometry of the hypogeum and the rock thicknesses above the caves was reconstructed by the integration of topographic and GPR surveys. The paper presents a geo-mechanical model of Palombaro Lungo underground reservoir and evaluation about the stability of this structure. The stability analysis were developed using the numerical code UDEC for several transect of Palombaro Lungo assuming the calcareous sandstone blocks as deformable, trying to address the evolution of the stress-strain conditions. The case study can be considered as an example of general interest for the study of rupestrian underground reservoir of a longed form, where one planimetric dimension strictly prevails over the other. Numerical simulations showed a stress-strain state compatible with the fractures detected in situ and confirmed the absence of instability problems in the groundwater reservoir.