Investigating a damaging buried sinkhole cluster in an urban area integrating geomorphological surveys, geophysics and trenching

This work analyses a damaging complex sinkhole cluster within the mantled evaporite karst of Zaragoza city, NE Spain. A detailed map of modern surface deformation indicates two active coalescing sinkholes, whereas two additional dormant sinkholes, currently beneath human structures, are revealed by old aerial photographs. ERT data and a trench excavated in the topographic margin of one of the sinkholes consistently indicate a larger subsiding area. GPR data reveal that subsidence in the central sector of one of the sinkholes is mainly accommodated by sagging. The stratigraphic and structural relationships observed in the trench may be alternatively interpreted by three collapse events, or by progressive fault displacement combined with episodic anthropogenic excavation and fill. Average subsidence rates of >6.6 mm/yr and 40 mm/yr have been calculated using different techniques. This case study illustrates the need of conducting thorough investigations in sinkhole areas during the pre-planning stage including a geomorphic approach.