Fenomeni di sprofondamento del piano di campagna in pianura padana: il ruolo del contesto geologico, geomorfologico e geotecnico

In Italy, natural sinkhole phenomena which are not connected to karst processes are relatively frequent. In the plains of Veneto and Emilia-Romagna, for example, several small subcircular lakes are described, whose formation is thought to have been caused by evorsion (erosional processes related to vertical turbulence) and/or suffosion (piping) in alluvial sediments. This paper deals with particular phenomena and related landforms, recently developed in the Po Plain between the provinces of Modena and Bologna. These processes have periodically produced shallow sinks, up to 2 m wide and deep, that disrupted agricultural works and exposed farm equipment to hazard, requiring costly remedial measures. In some cases, the land productivity itself was impaired. On the basis of literature information, as well as available geological, geomorphological and geotechnical data, the possible triggering factors and the evolution of these phenomena are described. An inventory carried out some years ago shows that these phenomena tend to develop in different geological settings, from the apex of the alluvial fans down to the lower alluvial plain (from 64 to 6 m a.s.l.), generally within relatively short distances from streams and artificial channels. In most cases, the affected soils have silty-sandy textures, related to alluvial ridges. The latter are composed by sandy channel fills, bounded by levees of mixed sand and silt, which pass outwards to finer silts and clays of interchannel zones. As sediments accrete, distributary streams shift laterally, forming new channels. Old channels are then gradually buried under interchannel silts and clays. As a result, the siltyclay body of the alluvial plain is laced with intersecting strings of loose sands surrounded by less permeable sediments. These sand strings, which may be recharged by streams and channels, behave as artesian aquifers at their distal ends. On the basis of Cone Penetration Tests (CPTs), some of the affected areas have been characterized with reference to lithology, stratigraphy and relevant geotechnical parameters. Under an overconsolidated cohesive unit, with sufficient clay fraction to form and retain shrinkage cracks in the so-called active zone, at a depth in the order of 6 meters, a sandy unit can be typically found, that has shown to be susceptible of liquefaction as a consequence of natural or artificial causes (seismic shaking, water table sudden drawdown, heavy vehicles transit etc.). The liquefaction and subsequent packing of loose sediments leads to the development of proto-chambers, that reveal as sinkholes when their roofs collapse. In the proposed conceptual model, triggering and evolution of these phenomena depend on the grain size of sediments involved, and on specific hydraulic conditions related to the distal sectors of alluvial ridges in a recent alluvial plain.