Risk assessment of the impact of pyroclastic currents on the towns located around Vesuvio: a non-linear structural inverse analysis

摘要

In a.d. 79, the catastrophic eruption of Vesuvio, which later was described in two famous letters by Pliny the Younger to Tacitus the Historian, destroyed Pompeii, Hercolaneum, Oplontis and Stabiae, resulting in many thousand of victims. After a few hours of the eruption, the several-kilometre-high volcanic column began to collapse, provoking strong air shocks as well as destructive pyroclastic density currents, which travelled down the volcano slopes. In 2000, an archaeological excavation survey, which was performed on the east slope of the volcano in the Terzigno–Vesuvio area at a distance of about 5 km from the vent, brought to light the ruins of several Roman villas that were completely destroyed by these currents during the a.d. 79 eruption. The present paper proposes a new structural analysis, which starts from the study of the damage produced on partially collapsed masonry walls, and determines the dynamic pressures of the currents that overran this site. The non-linear structural analysis, which is based on strength values obtained by means of experimental tests, is of the ''inverse type'' and takes into account the limit behaviour of the ancient Roman masonry. The values of the dynamic pressures that were capable of producing the collapse of the masonry walls were obtained by utilising a modern limit analysis theory. The obtained results show that dynamic pressures of a few kPa (1–5) were able to cause masonry buildings to collapse. These values are consistent with those proposed in some of the latest volcanological studies made by numerical simulations of pyroclastic flow propagation. It is shown here that these dynamic pressures are even able to determine the collapse of both modern reinforced concrete and masonry wall buildings that are largely present in the area. Therefore, in possible future eruptions, dynamic pressures of this magnitude would flatten a large urbanised area, where ～700,000 people are currently living. The obtained results give a better definition of both the risk to pyroclastic currents in possible Vesuvio eruptions and provide new guidelines for construction in the neighbouring zones.