Analysis of a Subsidence Process by Integrating Geological and Hydrogeological Modeling with Satellite InSAR Data
Bozzano, Francesca1; Esposito, Carlo1; Franchi, Stefania1; Mazzanti, Paolo2; Rocca, Alfredo1; Romano, Emanuele3
1Earth Science Department - Sapienza University of Rome, ITALY; 2NHAZCA S.r.l. Spin-off company of "Sapienza University of Rome, ITALY; 3CNR-IRSA, ITALY

This paper focuses on the preliminary results of a study aimed at analyzing the subsidence process affecting an urban area, through the integration of geological and hydrogeological data and modeling with measurements of soil displacements derived from satellite SAR Interferometric data. The study area is the Acque Albule Basin, located 15 km East of Rome (Lazio, Italy), a tectonic depression filled by travertine deposits (Lapis Tiburtinus), that overlay Pliocene sedimentary deposits locally covered by marshy and lacustrine sediments. The travertine deposit hosts a main aquifer which is partially fed by a deep thermal aquifer partially mixed with the superficial aquifer, thus causing a complex groundwater circulation. This basin has experienced an intense and progressive urbanization since the end of the World War II. Beyond the widespread urbanization, the study area is also characterized by the presence of travertine quarries, industrial plants and thermal resorts activities. In particular, the gLapis Tiburtinush has been quarried since Roman Age. However, after the World War II, the dewatering methods has caused a significant drop in the water table of the travertine aquifer (originally placed approximately at ground level), thus changing the hydraulic flow-path of the entire basin. Since 80fs, significant damages to more than 140 buildings have been recorded, mainly caused by differential settlements in turn connected to a generalized subsidence in the study area. Based on the analysis of several stratigraphic logs the stratigraphic succession of the basin have been divided in three main units. The first unit consists of clayey-silty deposits with a high organic matter content and peat layers, very compressible, with thickness ranging from few centimeters to 13 meters (Late Pleistocene-Holocene). The second unit consists of a sandy]silty travertine deposit, usually loose with thickness ranging from few centimeters to about 18 meters (Upper Pleistocene). The third unit consists of strongly cemented travertine deposits, stratified in metric banks, locally jointed with an average thickness of about 60 m (deposited between 115k and 30k years). Cavities and clayey interlayers also characterize this unit. On the basis of the geological and geotechnical model described above, a finite difference numerical model of the groundwater circulation at the basin scale was set up by using Visual MODFLOW 4.2 numeric code, calibrated under both stationary and transient fluxes. This model allowed to reconstruct the evolution of the water table in the period 1954 to 2008 and to simulate the impact on groundwater of dewatering mainly due to quarry activities over time. In order to investigate and quantify the relationships between changes in piezometric levels and ground subsidence rates, SAR satellite data (using ERS, ENVISAT and RADARSAT data) have been processed and analyzed through advanced differential interferometry techniques (A-DInSAR) for the period 1992-2010, thus achieving the temporal subsidence evolution. The evolutionary trends were analyzed in relation to groundwater level and thickness of compressible deposits. A-DInSAR data shows that the subsidence rates vary widely in the area in relation to the thickness of peat and clayey deposits and of travertine in sandy-silty facies. The analysis confirmed the strong correlation between the extent and rate of displacements and the thickness of the compressible deposits observed: the greater the thickness, the greater the measured displacement. As a matter of fact, the areas where travertine outcrops are basically stable. In addition, it was possible to point out a correlation between the time-space distribution of subsidence and the evolution of water table lowering induced by withdrawals: data show an acceleration of the subsidence process in the areas progressively involved in the widening of the groundwater table cone of depression. Indeed, it was found that in the time interval 1992-2000, areas relatively far from the main pumping stations showed negligible displacements, despite the presence of compressible deposits overlying travertine. The same areas were progressively reached by the cone of depression over the period 2003-2008: significant displacements were recorded in the same period. Present researches focus in a more detailed comprehension of the links between the above mentioned geological and hydrogeological factors. To this aim an experimental site has been set up close the most subsiding areas. Apart from stratigraphic boreholes and geotechnical on site tests (CPTU), on site instruments to monitor the groundwater levels (electric cluster piezometers) and in depth ground deformation (assestimeter) have been installed. The transition to a seasonal time scale could improve the comprehension of the relationship governing the delay between changes in ground water levels and induced displacements. In this way ground displacement prediction based on the monitoring of ground water level could be set up. This study has been carried out in the frame of the CAT-1 project ''Geological reconstruction and monitoring in recently urbanized areas affected by subsidence.'' (ID: 13097) and it is financially supported by the Lazio Region and the PRIN Project 2009-2013 ''Analysis, monitoring and control of surface geological instability processes induced by human stress'' (Coordinator: Prof. Francesca Bozzano) financed by the Italian Ministry of University and Research.