Issue 49

M. L. Puppio et alii, Frattura ed Integrità Strutturale, 49 (2019) 725-738; DOI: 10.3221/IGF-ESIS.49.65 726 techniques can be used. In particular, out-of-plane modes represent the most vulnerable modes, regardless of the applied load, and therefore they should be analyzed with proper modelling approaches (kinematic [3–5], finite element [6–8] and rocking [9] analyses). A particular class of historic constructions are the masonry urban walls acting as defense system of the inner historic town. Their vulnerability to external actions, such as the degradation and flood events, needs to be properly considered. Several researchers analyzed collapses occurred to this typology of masonry structures [10] and analyzed retrofitting techniques to guarantee adequate safety levels [11,12]. This paper illustrates the numerical modelling of a stretch of urban walls in the well-known Volterra town located in Tuscany (Italy). The flood scenarios are here examined, in particular the effect of a monotonically increasing thrust due to water on the upstream wall section. A first purpose is to identify mechanical parameters of the historic masonry by considering the collapse of the walls occurred in 2014 after an extreme rainfall event. The structural features of the wall and the collapse are described in Section 3. The most important aspects related to the stability of retaining walls is illustrated in the Section 3, whereas Section 4 presents the methodology adopted to estimate the mechanical parameters and to simulate the effects of the flood on the walls. Section 5 includes the assumptions, the method and the results of the model in the as-built state, whereas the retrofitted state is discussed in Section 6. The last Section contains the results of simplified analyses on a 3-dim model of the historic walls [13,14]. T HE V OLTERRA ’ S URBAN WALLS : DESCRIPTION AND COLLAPSE olterra is a walled mountaintop town in Tuscany (Italy), with structures from the Etruscan, Roman, and Medieval periods. The city walls are a remarkable Etruscan work that was gradually modified over the centuries. The primitive walls were built on IV century BC and remained intact up to the High Middle Ages, when the municipality decided to build a new defense system. The ancient walls measure 7 km whereas the actual medieval walls measure only 2.6 km, encompassing an area of 26 hectares. The main building material is the panchino , a stone typical of the west Tuscany region, fine grained and white sandstone, easily workable and porous. It normally acquires hardness and strength with prolonged exposure to external factors and demonstrated to be durable. On January, 31 st 2014, after that an extreme rainfall occurred, 30 meters of the urban walls collapsed (Fig. 1a) during the night. Luckily, there were no casualties, and the building close to the wall (at a distance of few meters) was not damaged at all. The part of collapsed walls is located between Porta all’Arco and Piazza dei Fornelli, one of the most panoramic points of the town. The collapsed wall is dated back to the second half of the XIII century and differs in technical features from the Etruscan walls, being mainly characterized by smaller blocks (Fig. 1b). The collapsed portion of the wall can be defined as a retaining wall, the back part is completely filled with soil and behaves as a retainer for the rear two/four-storey buildings (Fig. 1a). (a) (b) Figure 1: Historic urban walls of Volterra, the portion of wall right after the collapse (a) and the view from the base of the walls (b). In the background of figure (b) is also possible to see the typical section of the Urban walls of Volterra. V