Investigation of a large collapse sinkhole affecting a multi-storey building by means of geophysics and the trenching technique (Zaragoza city, NE Spain)

An active sinkhole around 100 m long has been investigated in the city of Zaragoza (NE Spain). Subsidence activity on this depression, including the sudden occurrence of a collapse sinkhole 5 m across, led to the abandonment of a factory in the 1990s. At the present time, a building with 100 flats and shallow pad foundations partially built on the sinkhole, is affected by rapid differential settlement. The development of the sinkhole results from the karstification of the halite- and glauberite- bearing bedrock and the sagging and collapse of the overlying bedrock and alluvium, more than 30 m thick. GPR and electrical resistivity profiles have provided information on the distribution and geometry of the subsidence structure. The application of the trenching technique and geochronological methods (AMS and OSL dating) has allowed us to infer objective and practical data on the sinkhole including (1) Limits of the subsidence structure, (2) subsidence mechanisms, (3) cumulative subsidence (> 408 cm), (4) subsidence rates on specific failure planes (> 1.8 cm/year), (5) episodic displacement regime of some fault planes. The available information indicates that the progressive deformation recorded in the building will continue and might be punctuated by events of more rapid displacement. This work illustrates the practicality of the trenching technique for the study of sinkholes in mantled karst areas.