ABSTRACT
The whole Pisa plains, including the world-renowned Piazza dei Miracoli, is subjected to subsidence. This phenomenon has long been investigated, using a benchmark placed on the Baptistery as vertical reference. Since the ‘90s, in sight of consolidation works on the Leaning Tower, main focus was placed on studying subsidence at site, rather than global, level. To this purpose, in 1993 a new benchmark, called 999, was installed in order to prevent shifts due to local and surface disruptions, and provide a vertical reference for subsequent investigations. Since instating benchmark 999, evolution over time of site-wide subsidence has also been investigated. This paper presents a detailed study for recovering historical information about vertical references used from early XX century to date and to homogenize the different data sources. The results have been used in geotechnical investigations to analyze shifts and distortions of each monument, along with each monument-soil interaction, for their appropriate preservation
