The effect of pyroclastic flows of the 79 AD eruption of Mount Vesuvius on the Pompeii's city walls. The case study of the sector near the Tower XI

During the archaeological excavations carried out in the summer of 2017, it was immediately observed that the three uppermost courses of the two leaves of the city walls in the north area of Pompeii were staggered by 10 cm from the underlying ones. This arrangement cannot be attributed to a “building error” but, more probably, to a subsequent event. Ruling out the initial hypothesis of a damage due to the historical earthquake in 62 AD or, in any case, to a seismic event, the authors have performed some numerical analyses, in order to investigate the possible cause of that damage, that seems to be compatible with the action exerted by the pyroclastic flows provoked by the 79 AD eruption of Mount Vesuvius, that is the only other important event recorded in that period. This paper reports on the construction technique and the historical phases of Pompeii's city walls construction and presents the investigations of the crack pattern and the performed numerical assessments. The method used herein has allowed the timeline of the events that caused that damage to be retraced, as well as proving that a damage mechanism different from the translational one, that was observed directly on-site, could not have been caused by a thrust, provoked by the windy flows, in the magnitude that has roughly been computed by the authors. If the hypothesis of the block displacement of the city walls is corroborated also by other future investigations, this damage would be the second one attributable to the eruption of Vesuvius, the first one being the lack of the first floors of the buildings, a damage already attributed to the thrust action of the windy flows by volcanologists.