Analysis of subsidence in the metropolitan area of Naples based on SAR data

We present an analysis of subsidence phenomena and mechanisms affecting urban areas developed on soft volcanic rock where sinkholes frequently occur. The study focuses on the metropolitan area of Naples (Southern Italy), an important example of an urbanized area affected by instability issues. The sub-surface of Naples is characterised by tunnels and cavities excavated in Neapolitan Yellow Tuff (NYT) through history for aqueducts and sewer systems, as places of worship or to extract building materials. The study was carried out in the UNESCO area (about 31 km²) considering ground surface measurements acquired by C-band radar sensors on board the ESA platforms ERS-1/2 and ENVISAT, as well as the X-band sensors of the COSMO-SkyMed (CSK) constellation and the TerraSAR-X/Tandem-X (TSX) satellites (processed by TRE Altamira). SAR data show different wavelengths, spatial/temporal resolution, revisit time and monitored period. ERS-1/2 and ENVISAT are both characterized by revisit time of 35 days and spatial resolution of 5x20m, while second-generation X-band sensors determine an extremely high resolution and PS (Persistent Scatterer) density distribution (TSX PS density is 26769 PS/km²). Data from CSK and TSX show spatial resolution of few km² and reduced revisit time (8 days for CSK and 11 days for TSX). SAR data are capable of detecting ground subsidence or uplift deformations on urban areas. The available cavities and sinkholes (Guarino et al., 2018) inventories were considered as well as available thematic maps (piezometric level, NYT roof depth, water supply, sewerage, waterwork and historical buildings). The cavity dataset, counting 888 polygons, was related to the PS mean velocities to detect possible correlations between them. Finite Element Analysis (FEA) for three-dimensional modelling were performed using MIDAS GTS NX code to simulate failure mechanisms of real cavities. Numerical results highlight that the cavity planimetry and its height, the overburden thickness and the mechanical properties of the tuff material are the most influencing parameters. Saturation effect and tuff degradation were evaluated computing the safety factor by means of the strength reduction method. The role played by pillars in complex cavities in terms of stress distribution and stability conditions was investigated. Numerical results and InSAR measurements of subsiding areas are in agreement, although some differences due to local effects are encountered, variation in properties and the assumptions of a constant length of the cavities. Finally, an example of a structural collapse occurred on 8th January 2021 affecting the Ospedale del Mare parking lot, in the Ponticelli district, was examined. Ground displacement pattern and time series comprised between January 2016 and December 2019 obtained with the TSX data display downward trends, clearly showing that the area experienced “subsidence” over at least the past two years. This study demonstrates the usefulness of numerical analysis combined with InSAR measurement technology to assess cavity stability conditions and the study of subsidence phenomena in urban areas.

Guarino, P. M., Santo, A., Forte, G., De Falco, M., Niceforo, D. M. A. (2018). Analysis of a database for anthropogenic sinkhole triggering and zonation in the Naples hinterland (Southern Italy). Natural Hazards, 91(1), 173-192.