Monitoring leaning towers by geodetic approaches: effects of subsidence and earthquake to the Ghirlandina Tower

Abstract

The research focuses on structural monitoring and movements identification applied to cultural heritage protection. The final purpose is the integration among different and independent technologies for analyzing and investigating the geometry changing over time of ancient leaning towers. The paper deals with a novel strategy implemented to compute differential vertical displacements starting from results obtained by repeated high-precision leveling network adjustments. These results usually aim at monitoring the subsidence phenomenon, while their use in engineering applications is more or less absent in literature. Moreover, the multidisciplinary approach is also able to analyze subsidence gradients in order to extrapolate the trend of the vertical axis and compute structure's rotations. The approach is applied to the Ghirlandina Tower, Modena (Italy), in order to identify the leaning and the subsidence trend. About 30 years of leveling campaigns provide a useful dataset to test the methodology, which is finally validated by the independent observations collected by a pendulum. The approach allows to compute the mean total displacement since 1984 of about 4.7 cm with 30% occurring over the last 6 years. In the same period, the total overhang of the tower (1.30 m in 2007) increased by about 19.1 and 10.4 mm towards southwest. The approach is also able to identify anomalous behavior of the tower such as the reversal tilting trend due to the scaffolding in the years of restoration and the permanent deformation suffered after the 2012 Emilia Romagna earthquake (failure of 4 mm in 6 months).

Geodetic monitoring and geotechnical analyses of subsidence induced settlements of historic structures

“…As can be inferred from these data, the apses of the Cathedral and the Tower result to be the portions characterized by the greater additional subsidence with respect to the one experienced by the whole city center (mean values are 48 mm for the Tower and 18 mm for the apse of the Cathedral over 32 years). [14] A careful analysis of the trend shown in Figure 8 also highlights that several benchmarks report an apparent uplift starting from November 2011 to 2012, particularly marked for benchmark n.3. The apparent anomaly is explained by remembering that the results are not absolute elevations, but they are referred to the reference benchmark n.12, which is conventionally considered as a stable point, even if it is not.…”

“…Long-term measurements of settlement and tilting help to capture what can be considered a "physiological behavior" of the structure and to detect possible deviation from this trend, by identifying causes of potential damage of instability mechanisms. [14] In addition, dynamic measurements to identify the actual behavior of the structure and the influence of soil-structure interaction are of great value to forecast its response in presence of seismic events. [13] However, a deep knowledge of the construction history is also essential to develop any reliable model to be used to forecast future trends.…”

“…[34] The network was adjusted by constraining all benchmarks to the reference one, numbered as 12, which is conventionally fixed to an elevation of 10 m. This choice, according to adjusted elevations obtained in 1991, does not influence the interpretation of results, because a relative analysis is performed and the attention is paid to differences of elevation that are the most interesting concerning the potential dangerous consequences on the stability of the structures. [14] The purpose of this local network, indeed, is to highlight differential effects for the structural safeguard, leaving the investigation of the absolute subsidence effect of the whole town to the regional network. It should be noted that it is always possible to obtain the absolute elevation of each benchmark of the local network through the connection of the reference point to a remarkable benchmark belonging to the regional network for the subsidence monitoring and located in the Apennines nearby Modena, considered not affected by the investigated phenomenon.…”

“…Many papers restrict in fact the use of high-precision leveling to the study of geophysical phenomena [5][6][7][8] forgetting the great impact of the induced subsidence in structural analysis, a topic that has been investigated only in few occasions. [9][10][11][12][13][14] The methodology, quite often used for analyzing risk and instability scenarios in environmental contexts, [15,16] can be summarized as follows: implementation of a 3D finite element numerical model on the basis of geotechnical characteristics; data collection of significant movements with respect to the investigated phenomenon; and optimization and validation of the numerical model by means of the real dataset.…”

Critical Issues and Key Points From the Survey to the Creation of the Historical Building Information Model: The Case of Santo Stefano Basilica

“…The network was adjusted by constraining all benchmarks to a reference point that is installed in St. Stefano Square, outside the monument, and belongs to the Municipality network for monitoring the subsidence phenomenon. The data preprocessing lies in controlling the closure of leveling rings before proceeding to the network adjustment (Castagnetti et al, 2016). The survey of the 21 rings lasted two working days; the network geometry and the number of measurements are sufficiently redundant to allow a high-quality computation, thus obtaining an average accuracy of 0.4 mm (95% level of confidence) for the final elevations.…”