Issue 49

M. L. Puppio et alii, Frattura ed Integrità Strutturale, 49 (2019) 725-738; DOI: 10.3221/IGF-ESIS.49.65 727 Causes of the collapse The causes of the collapse are multiple and of complex interpretation. It is necessary to observe that, as documents demonstrated (Fig. 2a), even in the early 20th century the stretch of wall under examination was highly vulnerable. The picture clearly shows a different masonry texture (Fig. 2), probably due to previous partial collapses and reconstruction. In addition, the moisture content, visible before collapse (Fig. 2) and proved by the presence of local vegetation, could have played a crucial role over time. This effect was likely exacerbated after the extreme event of rainfalls occurred during the days preceding the collapse. No proper drainage holes were visible in the collapsed part. The lack of a drainage system (or a non-adequate system) behind the wall probably caused filtration motions behind the retaining wall. This phenomenon plays a crucial role in the stability of a retaining wall, as it will be discussed in the third section (“Stability of masonry retaining wall”). The filtration phenomenon could have determined stresses not allowable for masonry, leading to its failure. Also, the evidence of pipes (Fig. 2c) for water and gas services could have jeopardized the stability of the wall by reducing its resistance. However, the consequences could have been sensitively worse, due to the proximity of the buildings. It is worthy to observe that the landslide slope reached the foot of the rear structures (Fig. 2c), which surprisingly not only did not collapse, but also were not even damaged. The reason is perhaps due to the fact that part of the foundations is made of the ancient Etruscan walls, whose presence is proved by the pictures taken during the reconstruction (Fig. 2d) and by the results of standard penetration tests carried out for the soil identification. Possible consolidation strategies to increase the safety level of this structure are related to an efficient drainage system, a global reinforcement of the stretch of wall through tie-rods avoiding out of plane mechanisms and surface coating of the wall front face to limit the detachment of stones. (a) (b) (c) (d) Figure 2: Picture of the wall in the early 20th century (a), moisture evidence (b), top-down view of the collapsed zone that shows the presence of pipes (c), down-top view of the collapsed zone that highlights the texture of the existing foundation of the buildings (d).