There landslide occurred yesterday in Niscemiin the province of Caltanissetta in Sicily, is still underway. Second Except Cocinadirector of the Sicilian Civil Protection, it is a landslide “unprecedented” for Sicily. The situation worsened starting from late yesterday afternoon, with others sagging which caused a widening of the landslide front and an increase in vertical shear which reached up to 50-55 metersas confirmed by the mayor Massimiliano Conti. The safety zone has been widened up to aa 150 meters from the front, bringing the number of evacuees to 1500 (over 300 families). Two of the four access roads to the town, the provincial SP10 and SP12, were closed, leaving the almost isolated town.
The alert remains maximum: as declared by the president of the Italian Geological Society Rodolfo Carosi the retreat of the landslide crown is gradually continuing, therefore the buildings on the edge of the slope are at high risk. According to Cocina, houses within a distance of 50-70 meters from the front would be destined to collapse due to the progressive subsidence of the sandy soils exposed following the landslide.
The geological reason why the landslide does not stop lies in the stratigraphy of the area affected by the landslide. A geological report presented to the Sicily Region in 2019 confirms that under the soil there is in fact a first layer of alluvial deposits due to the waterways present there, made of pebbles immersed in a poorly cemented matrix. Below we find a layer of sands and one of limits before arriving at a substrate of marly clays. Higher states are porous, while clays are much less permeable.
With the intense rainfall recently fallen with Storm Harry, the porous materials allowed rainwater to infiltrate down to the clay substrate. At this point the clays became soaked in waterand this caused them to lose resistance, causing them to slide towards the valley. The overlying layers, poorly cemented and weighed down by the absorbed water, are at that point also collapsed due to the formation of a interstitial surface of water accumulated on the upper edge of the clay layer, which reduced the friction between the two masses. Consequentially the landslide crown began to retreatbecause the sandy and silty layers do not have enough consistency to sustain themselves over time without support. This process, once started, does not stop with the end of rainfall.
However, we must not “blame” the tempered Harry. Although it may have played a role in triggering this landslide event, the process was already underway. In fact, let us remember that this is one of the areas with the highest landslide risk in Italy, already classified by ISPRA as P4 (very high risk) and the risk to buildings in the vicinity of the crown was indicated as R3-R4 (high-very high). The same area had been hit by a large landslide in 1997which was partially reactivated by yesterday’s event, as well as by a landslide just about ten days ago. In short, it was not a sudden and unexpected event: the area was fragile, already partially compromised by previous subsidence and was considered high risk.
