PSI - Issue 78

Marco Peroni et al. / Procedia Structural Integrity 78 (2026) 2110–2117

2114

Now, the unprecedented worsening of the drying of the subsoil due to the unusual drought during the summer of 2022 has evidently brought the subsoil instability to an even deeper level in the subsoil, thus also affecting the deeper layers on which the foundations of the Collegiate Church probably rest. The damage observed is consistent with a “tilting out” of the plane of the masonry wedge at the outermost portion of the southern apse basin; the overturning thrust appears to derive both from a probable outward leaning of the masonry and the two outermost pilasters and from the pushing action of the semi-dome covering the apse which, having lost the counterbalance provided by the transept arch, tends to collapse and thus generate a radial thrust on the elevations. The coexistence of the forces described above with the loss of support of the outermost portions of the building (located downstream and therefore more subject to subsidence and foundation instability) has generated a kinematic process that is schematized graphically below for a simpler and more immediate understanding of the phenomenon.

Fig. 4. Particular of the vault cracking pattern.

The presence of a double order of openings (doors on the ground floor and aligned windows above) on both sides of the southern apse certainly did not help to contain the phenomenon and even amplified it, since, as is well known, the openings in the masonry acted as a ‘magnet’ for the course of the cracks, which in fact, in the case under examination, pass through all the openings from the top to reappear at their base. As we have said, even the slightest alteration of the balance between the contrasting thrusts of the vaults and arches that make up the church's attic can trigger chain phenomena and further exacerbate the cracking pattern: it is likely that the detachment of the southern semi-dome from the respective arch that connected it to the transept may have weakened the chain action that this portion of the building generated on the rest, and in particular on the arch itself (see Fig. 3 et 4). Moreover, the displacement suffered by the perimeter walls at the top with respect to the central body of the building most likely caused the loss of support (generally just a few centimeters) of some of the brick slabs that make up the roof, opening a breach in it and causing part of the roofing tiles to collapse on the vault below.