Issue 49

M. L. Puppio et alii, Frattura ed Integrità Strutturale, 49 (2019) 725-738; DOI: 10.3221/IGF-ESIS.49.65 733 2.5 m from the base level. This observation can result useful because it can make it possible to know which the most dangerous area is needed to be checked and monitored for possible interventions.  Case 2. For higher values of tensile strength f t > f t.lim ), in the initial phase the crack propagation is similar, but once the inner core is completely broken, the external wall resists in all its height. This time, however, the formation of a fracture line is not noticed. In this situation the wall collapses due to the sliding mechanism. It is clear that the maximum displacement attained for the wall, of the order of some tenths of cm, is not realistic, since local cracks can occur and propagate through the wall thickness. These aspects, part of the fracture mechanics, cannot be gathered from the model, where masonry is homogenized. Observing what really happened during the actual retaining wall failure, it is possible to guess that case 1 (f t < f t.lim ) is the one that took place. Indeed, after the collapse the lower part of the wall remained in place. The collapse mechanism probably was due to a slit that takes place in the lower part of the external wall face and that probably propagates inward following the material friction angle, according to a scheme very close to what is represented in Fig. 7. In practice the increasing of the horizontal action produces a partialisation of the resisting section with the progressive reduction of the compressed section. The only compressed section offer shear resistance and the collapse is obtained for the overcame of the shear resistance of this section section. It can be concluded that the maximum masonry tensile strength is around a value of f t = 15 [kN/m 2 ]. (c) Figure 7: Stress distribution (a), corresponding cracking (b) and schematic view of the most likely collapse (c) The non-linear static analysis carried out considers the only effect of the increase of hydrostatic head at the upstream section of the retaining wall. This is valuated considering a progressive growth of the thrust. The presence of moisture, particularly in a long-term scenario, also produce a degradation in the capacity both of masonry that on the soil [33]. This aspect is not considered in this work. The safety evaluation of the historical retaining wall have to be done considering the effects of drain obstruction. This is a recurring condition in the historical wall and can produce a sudden increase of the hydraulic action in case of rainstorm. In the case of Volterra is possible to notice a double origin of the hydraulic head. If from one hand the hydraulic head came from a loss of water of a sewer line that produce a slow saturation scenario, the occurrence of the severe rainstorm the day before the collapse had a crucial role in the collapse of Volterra. In the common practical case that there is a clay or impermeable layer on the back of the wall, there can be an accumulation of water at a depth different from the foundation height. In this case the hydrostatic thrust has a greater lever arm and therefore a higher destabilizing moment with respect to the foundation. This can cause a different collapse mechanisms with crack in any cross section and therefore that aspect should be carefully considered.