Il rischio geomorfologico indotto dall’attività estrattiva nei Bacini Marmiferi Apuani (Alpi Apuane, Toscana)

The new quarrying techniques and the reject stone recovery system induce new geomorphological hazard scenarios in the Carrara Marble Basin. This work analyses the recent landslides and debris flows occurred in the Marble Basins, stressing their relationships with the quarrying activity. In the open space quarries, the progressive lowering of the working base level determines the formation of steep and more than 100 m high slopes carved in densely fractured rocks. In correspondence of these cliffs, several landslides occurred in the last years, someone mobilizing more than 30,000 m3. The triggering causes were the rainfall hourly intensity (60 mm/1h in a specific case) and the antecedent rainfalls (80 mm in the 34 antecedent hours in a specific case). The main landslides cinematic analysis highlighted that they are a structural collapse of high rocky bench, with rupture phenomena at the base, differential slides and local toppling. As regard the quarry dump deposits (locally called ravaneti), the temporal evolution of quarrying techniques determined the overlapping along the slopes of two layers with different granulometric and permeability characteristics. The deepest layer, with multi-decimetres blocks and scarce fine matrix, corresponds to the typical ravaneto of the 19th-20th centuries crossing period. The active surface ravaneto, with decimetres blocks and abundant fine matrix, is the consequence of the new diamond wire cutting methods, introduced in the 70s. Nowadays, fine material is added to the ravaneti also by the reject stones sieving, in order to produce CaCO3. In the last years, several debris flows affected the ravaneti. The rainfall recordings analysis shows a relative low threshold for debris flows triggering, with events occurred after 60 mm/5 h of rainfall and with a maximum intensity of only 14 mm/1 h. The mobilization occurs by means of initial soil slip that, with a progressive fluidification, becomes a debris flow. The increased frequency of the events in the last years can be referred to fine material adding to the ravaneti, forcing the saturation processes. In the Carrara Marble Basins, the highest geomorphological hazards for the personal and the infrastructures are represented by the abandoned sub-vertical and densely fractured slopes, and the increased concentration of fine fractions in the ravaneti, induced by the new quarrying techniques and of reject stone recovery.