Investigating a damaging buried sinkhole cluster in an urban area (Zaragoza city, NE Spain) integrating multiple techniques: Geomorphological surveys, DInSAR, DEMs, GPR, ERT, and trenching

Abstract

This contribution analyses a complex sinkhole cluster buried by urban elements in the mantled evaporite karst of Zaragoza city, NE Spain, where active subsidence has caused significant economic losses (~0.3million Euro). The investigation, conducted after the development of the area, has involved the application of multiple surface and subsurface techniques. A detailed map of modern surface deformation indicates two active coalescing sinkholes, whereas the interpretation of old aerial photographs reveals the presence of two additional dormant sinkholes beneath human structures that might reactivate in the near future. DInSAR (Differential Interferometry Synthetic Aperture Radar) displacement data have limited spatial coverage mainly due to high subsidence rates and surface changes (re-pavement), and the Electrical Resistivity Tomography (ERT) and trenching investigations were severely restricted by the presence of urban elements. Nonetheless, the three techniques consistently indicate that the area affected by subsidence is larger than that defined by surface deformation features. The performance of the Ground Penetrating Radar (GPR) technique was adversely affected by the presence of highly conductive and massive anthropogenic deposits, but some profiles reveal that subsidence in the central sector of one of the sinkholes is mainly accommodated by sagging. The stratigraphic and structural relationships observed in a trench dug across the topographic margin of one of the sinkholes may be alternatively interpreted by three collapse events of around 0.6m that occurred after 290yr BP, or by progressive fault displacement combined with episodic anthropogenic excavation and fill. Average subsidence rates of >6.6mm/yr and 40mm/yr have been calculated using stratigraphic markers dated by the radiocarbon method and historical information, respectively. This case study illustrates the need of conducting thorough investigations in sinkhole areas during the pre-planning stage including a geomorphic approach. A sound geomorphic model is essential for the proper design of the site investigation, the interpretation of the data and application of effective mitigation measures. Once sinkhole areas are developed, urban elements largely restrict the applicability and performance of multiple techniques, substantially decreasing the feasibility and benefit/effort ratio of the investigations.