Monitoring-driven design of a multiphase intervention for the preservation of the Frari bell tower in Venice

Since the time of its construction in the 14th century, the Frari bell tower has been affected by a slow but constant differential settlement with respect to the adjacent masonry structures of the Basilica. About 15 years ago, the clearly evident signs of structure instabilities provided the impetus for the implementation of modern remedial measures, first in foundation and then on the elevation structure. A ground improvement intervention using soil fracturing (also known as fracture grouting) was carried out in order to improve the mechanical characteristics of the clayey layer underlying the tower. Once the aim of improving the stability of the soil-foundation system had been achieved, a new solution was required to reduce the damaging inter-action between the masonry structures, activated by the foundation settlements. A structural joint between the bell tower and the basilica was finally executed in order to accommodate the system deformability. The paper presents a well-documented case study, since the preliminary crucial and accurate site investiga-tion, highlighting both the approach aimed at improving the overall safety - without altering the original structure and substantially modifying the current stress distribution - and the innovative methodology, adopted throughout, of a gradual and modular design, constantly driven by the outcome of an extensive real-time monitoring system of the soil-structure interaction.