Reinforced Repointing with RETICOLA system for the Church of San Nicolò – Treviso

Built starting from the 14th century by the Dominican order, on the initiative of Pope Benedict XI (Nicolò Boccassino from S.Vito di Valdobbiadene, in the Treviso area), the imposing Church of San Nicolò replaced a previous chapel dedicated to the same saint. Over the seven centuries of its history, it has undergone numerous changes and transformations, and in recent times, periodic maintenance interventions have been necessary to ensure its good preservation.

In recent years, an external crack was observed in the second southern sector of the main apse, adjacent to a masonry area that had been repaired in the past. The crack extended from above the mullioned window, reaching the rose window, arches, and friezes of the frame, and by 2010, it had also become visible on the interior. This prompted a study that revealed other significant fractures on the upper masonry of the apses and transept, requiring a major restoration intervention.

DESCRIPTION OF THE STRUCTURAL DAMAGE

A system of wooden staircases allows access, through the bell tower, to the vaults of the chapels and large attic spaces, partially equipped with floors. In these areas, the exposed masonry showed a complex pattern of cracks, with extensive lesions that, in several cases, exceeded one centimeter and, in the case of the wall above the triumphal arch of the transept, reached over ten centimeters.

The main cracks were monitored with a first intervention in 1960 and more extensively at the end of the 1980s to study their evolution. It was observed that the largest crack in the transept had widened by an additional 5 millimeters after 50 years. The repairs carried out in 1989 on the cracks in the vault of the central chapel, using metal bars, had not experienced further movement. However, a dozen new cracks had appeared after 25 years, with an average degeneration of around 5-6 mm. In particular, the crack in the main apse had widened by a significant 13 mm, and more recent measurements, taken with more refined instruments, recorded movements on the order of a tenth of a millimeter in six months. To understand the ongoing movements, some measurements were taken to check for verticality, given that several singular situations inside the building were visible to an observant eye. Penetration tests along the perimeter revealed a soil with poor mechanical characteristics and, near the bell tower, the likely presence of piles. In addition to the numerous collected data, numerous pieces of information emerged regarding the interventions carried out in the 19th century, which caused significant damage to the structure and led to the abandonment of the building. The diagnostic phase confirmed the movements responsible for the deterioration of the apse: the heavy tympana, 80 cm thick, that were placed above the triumphal arches until 1850 exerted significant forces on the upper masonry, both on the transept side (higher) and on the apse side (lower).

Furthermore, the structure of the chapel’s body is made up of six masonry shoulders, over 20 meters high and 90 cm thick, all arranged longitudinally in the temple, while in the orthogonal direction (the one of thrust), due to the presence of arches and tall windows, there is no element that can effectively counteract the thrust. Thanks to their thickness and compression due to their weight, the walls have withstood the shear stresses produced by the arches’ thrust. However, the soft ground has caused, especially in the northern sector, the rotation responsible for the loss of verticality of the shoulders of the central chapel. This progressive rotation caused the collapse of the internal triumphal arch, with the lowering of its keystone and the subsequent dramatic damage to the main apse two centuries ago.

INTERVENTIONS

In order to eliminate the thrust of the triumphal arches, after an initial design proposal that involved the insertion of stainless steel tie rods at the springing of the vault, the executed intervention consisted of a series of specific activities:

• Introduction of an effective chain system for the 19th-century arch constructed above the internal triumphal arch.
• Insertion of another chain system at the top of the transept wall, below the impingement plane of the roof trusses.
• Reconstruction of the most damaged masonry sections, particularly the wall and ribs of the second southern sector of the apse, using a stitch-and-sew technique.
• Injection of lime-based fibrous mortars into cracks in the vaults and minor fissures.
• Hooping of the upper masonry section of the apse, above the windows, with stainless steel cables placed below the surface in the mortar joints between one course of bricks and another (RETICULATE SYSTEM).

FIBER NET SYSTEM The adopted hooping system, RETICULATE SYSTEM, designed for exposed stonework and brickwork, has demonstrated adaptability in the presence of walls with specific characteristics. The implementation of the system involves creating grooves, 5-6 cm deep, along the mortar joints on both sides, smoothing the bottom of the groove with a first layer of high-strength fibrous mortar, drilling Ø12 through-holes, laying 3 or 5 mm stainless steel AISI 316 wire meshes on the hardened bottom of the grooves, inserting threaded Ø8 stainless steel bars with heads or end anchors, tensioning the wires from the heads, and further tightening by screwing the end nuts of the Ø8 passing connectors. The final pointing with mortar restores the original appearance of the masonry without revealing the intervention. For accuracy, some points were left visible in the attic to allow recognition of the trajectory of all the hoops. In the case of San Nicolò, the upper masonry of the apse has a thickness ranging from 1.05 to 1.20 meters and is completely bonded (it does not have a sack core). Furthermore, it features 60 cm pilasters on the exterior that extend the piers of the windows. The nine masonry panels show stitch-and-sew repairs in multiple areas, especially the area of the current damage, which is even 15 cm wider than the others. In the application to the apse, the Reticulate System was adapted to the specific case, given the need to limit the number of mortar courses to be removed. Therefore, three 5 mm wire meshes were inserted for each groove. The connectors are anchored with head plates in the upper courses, related to the attic, while they are dovetailed into the lower courses corresponding to the internal plastered panels. Special end anchors were designed to secure the three wire meshes, and the passage of the pilasters was obtained by drilling holes and inserting stainless steel tubes capable of accommodating the wire meshes.

The tension cables were extended along the entire length of the masonry shoulders of the main chapel and anchored with head plates or dovetailing from the opposite side of the apse. This provided an improvement in shear resistance to the walls, especially at the most damaged face.

CONSTRUCTION SHEET PROJECT_Consolidation intervention of the masonry of the apse and transept at the Church of San Nicolò

LOCATION_Treviso

DESIGNERS and PROJECT MANAGERS_arch. Cesare Gasperetto, engineer Bruno Barcati, with the collaboration of engineer Conte and engineer Alessandro Simionato

MAIN CONTRACTOR_ Gobbo Bruno s.r.l.

COMPLETION_2013