Risks Associated with Subsidence-Collapse of Sinkholes. A Case Study in the Town of Alcalá de Ebro (Zaragoza, Spain)

Alcalá de Ebro is located 35 kms northwest of the city of Zaragoza. On the banks of the river Ebro itself. And more specifically at the outlet, towards the river, of a ravine (Juan Gastón), with a basin of reception of contributions of more than 230 km2. Over time, urbanisation and agricultural development have eliminated the final stretch of the drainage channel, and these water inputs have become sub-surface, filtering through the ground. This section of the Ebro River valley is located on a Tertiary substratum, marliferous, which favours dissolution-socavation processes that, in turn, cause sinkholes. The land tends to sink progressively or generates sudden collapses in the ground. Many studies have been carried out to try to understand not only the origin of the phenomenon, but also its geometry and the area affected by it in the town of Alcalá de Ebro. In this sense, it has been possible to model an area around the main access road, where numerous collapsing sinkholes have been found, blocking the way and affecting houses. It also affects the mote that protects the village from the floods of the river Ebro. Based on these studies, specific knowledge has been obtained, which has allowed us to evaluate measures to consolidate the ground, which have been implemented with apparent success up to now. Several injection campaigns have been carried out; first using expansive resins and finally with columnar development, with special low-mobility mortars, to fill-consolidate the undermined areas and prevent new subsidence. The actions have produced satisfactory results, but it is considered necessary to continue with the observation of the situation, extending the attention to a wider area to prevent, as far as possible, new problems of subsidence-collapse.