100 Years of Geodetic Measurements in the Piazza Del Duomo (Pisa, Italy): Reference Systems, Data Comparability and Geotechnical Monitoring

4th Joint International Symposium on Deformation Monitoring (JISDM), 15-17 May 2019, Athens, Greece 100 Years of Geodetic Measurements in the Piazza del Duomo (Pisa, Italy): Reference Systems, Data Comparability and Geotechnical Monitoring Gabriella Caroti1, Andrea Piemonte2, Nunziante Squeglia3 1 Dipartimento di Ingegneria Civile e Industriale - Laboratorio A.S.T.R.O. - Università di Pisa – Largo Lucio Lazzarino 1 – Pisa (Italia), ([email protected]) 2 Dipartimento di Ingegneria Civile e Industriale - Laboratorio A.S.T.R.O. - Università di Pisa – Largo Lucio Lazzarino 1 – Pisa (Italia), ([email protected]) 3 Dipartimento di Ingegneria Civile e Industriale- Università di Pisa – Largo Lucio Lazzarino 1 – Pisa (Italia), ([email protected]) Key words: Levelling; Historical Survey; Measure Comparability; Geotechnical Monitoring; Structural Monitoring ABSTRACT The entire Pisa plains is subjected to subsidence movements, including the world-renowned Piazza dei Miracoli. This phenomenon, as a whole, has long been investigated, but over time the value of the site has commanded specific and steady focus. Early geodetic observations, dating back to 1908-1912, included the monitoring of the inclination of the Torre Pendente (Leaning Tower). Only in the 1960s, geodetic and topographical measures were extended to the entire site, using as altimetric reference a benchmark, named ', placed at the Baptistery. In 1989, a specific committee for the consolidation and restoration of the Leaning Tower was established, and in 1990 the Tower was closed to public access. The ensuing reconnaissance and executive planning provided the restoration and redesign of the levelling network for monitoring vertical movements of the site, also revealing that reference benchmark' was in fact subjected to subsidence. This issue was overcome by instating benchmark #999, which was anchored to the deep sands layer and acted as altimetric reference for any successive survey. Since 2012, monitoring of the vertical movements in the Piazza has been entrusted to ASTRO Laboratory of Pisa University. Lately, some interest has developed for investigating the evolution over time of subsidence in the site, thus requiring the use of a shared reference for different data sets. These uncertainties have had relevant consequences on monitoring of monuments, in particular about the tower, which has been subjected to important stabilization measures during the last decade of XX century. Since the observational method has been applied to the tower, levelling surveys became mandatory; a thorough discussion about reference points has also been carried out. The present paper has primarily historic connotations and describes the procedures followed to attune the diverse data sources in order to ensure their usability after roughly 100 years. I. INTRODUCTION The elevation of this benchmark has been defined as that measured by I.G.M. in 1886 at 6.7917m above The levelling surveys carried out during the past 100 mean sea level. years in the Piazza dei Miracoli have been targeted, Explanatory notes accompanying the technical until the 1990s, to the assessment and investigation of reports for the 1965-1966 (Istituto Geografico Militare, relative vertical shifts among the benchmarks laid out 1965: Istituto Geografico Militare, 1966) altimetric over the Piazza, rather than monitoring how monitoring surveys state that “l’interesse è rivolto ai subsidence, which involves the entire Pisa plains, is confronti tra dislivelli e per questo scopo il riferimento related with the Piazza and its reference benchmark. altimetrico ha poca importanza” (“the focus lies in The earliest surveys aimed at the analysis of vertical comparing elevation differences, and to this purpose shifts among benchmarks date back to 1911. In that altimetrical references are of little interest”), but “… al occasion, the reference benchmark, i.e. the one whose fine di legare i raffronti relativi alla realtà della elevation is assumed constant between successive situazione più probabile…” (“in order to link relative surveys, was provided by a line, signed by the initials comparisons to the actual, most probable situation”) “L.P.”, etched on the right-hand jamb of the main door IGM-CV-1886 was preserved as altimetrical reference of the Baptistery. The benchmark was included in the with the absolute elevation of 1886. National Geodetic Levelling Network of the Italian Levelling surveys performed in 1965 and 1966 Geographical Military Institute (I.G.M.), and in the allowed to link vertical benchmark IGM-CV-1886 to present paper will be referred to as IGM-CV-1886 horizontal benchmark φ’, set up in 1928 on the (Figure 1). threshold of the main door of the Baptistery near its 1 4th Joint International Symposium on Deformation Monitoring (JISDM), 15-17 May 2019, Athens, Greece right-hand jamb, below IGM-CV-1886 (Figure 1). Elevation of φ’ was measured at 4.28361m and was held as reference for later surveys. Figure 2. The subsoil of the Tower. In recent years some interest arose in extending the investigations of subsidence in the Piazza area also prior to 1993. Achievement of this goal required tracking, from the late XIX century to date, of vertical shifts of both IGM-CV-1886 and φ’≡100 benchmarks, by researching historic data, and homogenization of elevation references. Figure 1. Location of IGM-CV-1886 and φ’ benchmarks. II. MATERIALS A. Elevation monitoring from early 1900s to the 1990s The statement contained in the explanatory notes is Piazza dei Miracoli and its monuments (Figure 3) have only partially shareable. The fact that level differences long been subject to measurements. The main focus has are the most important information is dependent on consistently been placed on the Tower, whose the continuity of the object structure. While the inclination has always provided a topic for investigation statement is true if referring to the surface on the entire and monitoring. Piazza, this is not the case when seeking information on Earliest data related to inclination measures of the the distortions which single building are subjected to, Tower come from E. Cresy and G. L. Taylor, which, in since level differences measured between different 1817, performed two overhang measures by dropping building include the effect of different concurring a plumb line from the VII order to the support plane of phenomena. As already discussed by Squeglia and the elevation walls, resulting in 12 feet, 6 inches ¼ Bentivoglio (Squeglia et al., 2015), movements of the (3.82m) and 12 feet, 7 inches (3.84m), respectively Piazza also definitely affect movements of the Tower, (Lancellotta et al., 2018; Lunardi, 1993). but referring to a benchmark placed on the Baptistery Official levelling benchmarks set up in the Piazza dei would also add interactions between Baptistery and soil Miracoli area date back to late XIX century. of little or no relevance for the Tower to the level From 1878 to 1900, I.G.M. set up and measured the difference between Baptistery and Tower. foundation National Altimetric Network on the entire Also for this reason, in view of the works for the national territory. securing of the Tower in the 1990s, members of the International Committee for the safeguard of the Pisa Tower recognised the importance of assessing local subsidence and investigating its effects, along the ongoing monitoring of relative shifts. To this purpose, deep benchmark 999 was established in 1992 by anchoring a 60m invar rod to the deep sand layer (Figure 2); the following year, its elevation was measured at 3.57232m. Benchmark 999 and its elevation provide the current reference and constraint for all levelling surveys. Figure 3. Pisa, Piazza dei Miracoli, 1931 (Historic archive, Family Gen. Enrico Pezzi) 2 4th Joint International Symposium on Deformation Monitoring (JISDM), 15-17 May 2019, Athens, Greece and Taylor (Cresy et al., 1829), he demonstrated that Within these proceedings, in 1886 two benchmarks until 1911 the inclination had increased by 5.5 were set up by the main entrance of the Baptistery and millimeters for each meter of Tower axis. subsequently measured (Regia Commissione Geodetica Professor Pizzetti reported on "Studi Geodetici” Italiana, 1911): (geodetical investigations), declaring that, around the - A circle, etched on the last step by the left-hand Tower, 6 interred benchmarks – labelled with the jamb of the door facing the Cathedral, acting as letters A to F – had been set up in protective cockpits, horizontal benchmark with absolute elevation of providing for robustness and durability. For each 4.2930m. This benchmark, referred to as IGM-CO- benchmark, the reference was provided by the flat top 1886 in the paper, was destroyed after 1943. of a bronze pin embedded in the foundations, on which - Vertical benchmark IGM-CV-1886, made up by a the corresponding letter was etched, along with the line, captioned with the letters “L” and “P”, etched wording “Studi Campanile 1910-11”, Figures 5 and 6. on the right-hand jamb of the above mentioned door (Figure 1; Figure 4), with absolute elevation equal to 6.7917m. Figure 4. Detail of benchmark IGM-CV-1886 Figure 5. Layout of the 1911 benchmarks in Piazza dei Miracoli Archive researches show that around the same time other benchmarks, set up on the floor of the christening font in the Baptistery and on the ground floor by the Tower entrance, were measured. These benchmarks have all been destroyed upon reconstruction works. To the best of the Authors’ knowledge, until

Load more