In Italy, natural sinkhole phenomena in alluvial plain areas which are not directly connected to fluvial or karst processes are relatively uncommon. This paper deals with poorly understood processes and related landforms, known since few decades in the Po Plain. These processes have periodically produced shallow sinks, up to 2 m wide and deep, that disrupted agricultural activities, requiring in some cases costly remedial measures. On the basis of previous works, as well as available geological, geomorphological and geotechnical data, the possible triggering factors and the evolution of these phenomena are described, also in the light of the recent Emilia earthquakes of May 2012. In the proposed conceptual model, the predisposing factors of these phenomena depend on the stratigraphy and grain size distribution of recent loose alluvial sediments at shallow depth, and on specific hydraulic conditions of shallow semi-confined aquifers in the distal sectors of alluvial ridges. In susceptible areas, ground shaking can cause liquefaction of these susceptible soils and subsequent upsurge and eventual ejection of sands along pre-existing or newly-formed ground cracks. With time, at shallow depth, in clastic planar dikes and sub-horizontal sills, resedimentation and packing can occur and small-scale proto-chambers may evolve, by successive collapses and enlargement, also caused by the erosion of infiltrated water. The ultimate triggering factor for the formation of the sinkhole, also after a relatively long time-lag, may be related to local accidents that cause the final collapse of roofs. Known historical earthquakes could have been the original triggering factor of liquefaction of susceptible soils and formation of dikes and sills. The 2012 Emilia seismic sequence has shed light on this processes. Previously, liquefaction phenomena have been seldom observed in this area, mainly because moderate seismicity. During the Emilia earthquakes widespread coseismic effects were observed, as soil liquefaction and ground cracks. On the basis of the data gathered so far it is clear that the sinkhole evolution model is confirmed and that these landforms may be considered a secondary medium-term effect of earthquake-induced liquefaction.
BORGATTI L., BIANCHI E., BONAGA G., GOTTARDI G., LANDUZZI A., VITTUARI L., et al. (2013). Sinkholes in the Po Plain as evidence of paleoliquefaction events. Roma : Società Geologica Italiana.
Sinkholes in the Po Plain as evidence of paleoliquefaction events
BORGATTI, LISA;BIANCHI, ELIO;BONAGA, GILBERTO;GOTTARDI, GUIDO;LANDUZZI, ALBERTO;VITTUARI, LUCA;
2013
Abstract
In Italy, natural sinkhole phenomena in alluvial plain areas which are not directly connected to fluvial or karst processes are relatively uncommon. This paper deals with poorly understood processes and related landforms, known since few decades in the Po Plain. These processes have periodically produced shallow sinks, up to 2 m wide and deep, that disrupted agricultural activities, requiring in some cases costly remedial measures. On the basis of previous works, as well as available geological, geomorphological and geotechnical data, the possible triggering factors and the evolution of these phenomena are described, also in the light of the recent Emilia earthquakes of May 2012. In the proposed conceptual model, the predisposing factors of these phenomena depend on the stratigraphy and grain size distribution of recent loose alluvial sediments at shallow depth, and on specific hydraulic conditions of shallow semi-confined aquifers in the distal sectors of alluvial ridges. In susceptible areas, ground shaking can cause liquefaction of these susceptible soils and subsequent upsurge and eventual ejection of sands along pre-existing or newly-formed ground cracks. With time, at shallow depth, in clastic planar dikes and sub-horizontal sills, resedimentation and packing can occur and small-scale proto-chambers may evolve, by successive collapses and enlargement, also caused by the erosion of infiltrated water. The ultimate triggering factor for the formation of the sinkhole, also after a relatively long time-lag, may be related to local accidents that cause the final collapse of roofs. Known historical earthquakes could have been the original triggering factor of liquefaction of susceptible soils and formation of dikes and sills. The 2012 Emilia seismic sequence has shed light on this processes. Previously, liquefaction phenomena have been seldom observed in this area, mainly because moderate seismicity. During the Emilia earthquakes widespread coseismic effects were observed, as soil liquefaction and ground cracks. On the basis of the data gathered so far it is clear that the sinkhole evolution model is confirmed and that these landforms may be considered a secondary medium-term effect of earthquake-induced liquefaction.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
