The term subsidence refers to the phenomenon in which the ground surface sinks measurably and significantly in some areas. In some cases this phenomenon has natural origins (meaning causes not resulting from human actions) and in others stems from mankind. Natural causes include the compacting of the most geologically recent sediments, the collapse of underground cavities, settling after earthquakes and tremors. The most significant intervention of human origin is extraction of fluids from the subsoil.

In some areas of the Emilia-Romagna Region, the extraction of fluids from the subsoil has produced significant subsidence, to varying degrees. The areas most affected are:

• Ravenna, where the phenomenon is essentially due to the historical extraction of hydrocarbons (methanogenic water);
• Bologna, where the phenomenon is primarily related to the extraction of water for civil, industrial and agricoltural use.

The physical phenomenon may be briefly described as follows: Lowering the water table translates into lower hydrostatic pressure in the interstices between granules. The result is increased pressure on the granules on which the consolidation process depends.

The phenomenon is virtually irreversible, and is most obvious where there are greater piezometric level sinking and thicker layers of compressible sediments.

Page updated 22 August 2011