Preservation of historic towers in Venice: the instructive monitoring-driven intervention on the foundations of the Frari bell tower

Historical documents report that Venice used to have more than one hundred bell towers. The peculiar environmental conditions in which foundations had to be built, however, have typically generated over the centuries substantial differential settlements and potential in-stability of such structures. Also thanks to the advanced skill reached in the foundation con-struction technique, about 80 towers are still presently standing, obviously requiring an increasing attention and awareness of their specific vulnerability from public authorities. Special control and maintenance programs have been thus devoted to bell towers and preservation interventions have been carried out in full respect of their historic, architectural and structural features. An emblematic example of such an approach is represented by the mul-tiphase intervention carried out on the bell tower of the Frari Basilica, the second tallest in Venice. The tower was affected by a slow but constant differential settlement with respect to the adjacent masonry structures of the Basilica. Starting some 15 years ago, modern re-medial measures were therefore implemented, first on the foundations - using a ground improvement intervention by fracture grouting - and then on the elevation structure. This chapter is aiming at presenting the well-documented Frari case study, from the preliminary crucial and accurate site investigations to highlighting both the strategy of improving the overall safety - without altering the original structure and without 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.