Issue 49

M. L. Puppio et alii, Frattura ed Integrità Strutturale, 49 (2019) 725-738; DOI: 10.3221/IGF-ESIS.49.65 728 Geometrical and mechanical features of the walls The Volterra’s historic Walls tend to show a variety of different styles. However, the collapsed wall section can be considered sufficiently homogeneous; the cross-section, evaluated through visual inspections after the collapse and check of original drawings, is shown in Fig. 3. The structure is 9 m high, 3,6 m thick at the bottom and 2,3 m thick at the top, with an average slenderness of about 3. The inclination of the internal wall (10°) is greater than that of the external one (4,5°). The external and the internal wall develop along all the height and turn out to be about 75 cm thick, keeping it constant from the base to the top. The base is constrained to the ground; indeed, it lays at a depth of about 90 cm with respect to the valley slope. This aspect certainly generates a minimum of soil passive thrust that helps to constrain the retaining wall at its base but there are many uncertainties due to this aspect in particular related to the downstream slope. The wall is multi-leaf with two square stone external walls with fairly resistant lime mortar and a central filling core. This inner core has the only purpose of providing weight, and therefore stability, to the gravity wall, but it shows scarce traces of mortar, with a behaviour that is halfway between a poor masonry and a coarse-grained soil. (a) (b) Figure 3: Wall structure with dimensions in m (a) and cross section view of the collapsed wall [12] (b). The main problem in the collapse assessment is due to the fact that the material that makes up the masonry is strongly differentiated. Being an ancient wall, it is difficult to identify a precise masonry texture scheme. It follows that the material mechanical features are hardly identifiable, and it is only possible to make assumptions validated by the analysis of physical models [15]. Unfortunately, any experimental test on the remaining debris after the collapse was made to assess the mechanical properties of the wall. The method followed to perform the numerical analysis is expressed in the fourth Section (“Methodology for the estimation of wall mechanical parameters from numerical models”). S TABILITY OF MASONRY RETAINING WALLS he stability of a masonry retaining wall regards the stability of the system wall+backfill and that of the foundation. According to Chan [10], the stability of an old retaining wall is related to four parameters: (1) the retained soil strength, (2) the retaining wall geometry, (3) the slope inclination and (4) the groundwater level behind the wall. Among them, the soil resistance plays a non-fundamental role in the structural stability, with an incidence by only 10% in the studies performed by the cited author, whereas the main influence is given by the groundwater level and by the backfill T