Recent extensometric data for the monitoring of subsidence in Bologna (Italy)

During the past several decades underground fluid withdrawals induced severe cases of land subsidence in Bologna, Italy. The area features middle Pleistocene to Holocene alluvial deposits more than 400 m thick. Here, levelling measurements revealed a subsidence rate of more than 11 cm/year from 1974 to 1981. Subsidence monitoring was carried out using a levelling network and, recently, an interferometric analysis. In 2005 two pipe extensometers were installed in the northern part of the area, anchored at 100 and 200 m depth, respectively. The regional subsidence rate is now reduced; however, the monitoring performed by means of the two extensometers shows that the subsidence rate is cyclical with seasonal peaks.