534
Baratin, Bitelli, Bonnici, Unguendoli, Zanutta
2.3 Topographic Structural Monitoring
The evaluation of the structural stability of a building, as well as of its surrounding areas is important prior to any restoration
intervention. Structural instability can either be attributed to damages and/or deficiencies in the construction, or to the geological
characteristics of the area, in which case it is likely that more than one building is affected. In the case of the latter, an extensive
monitoring network, based on precise topographic levelling should ideally be set up. A similar exercise was recently launched in
Mdina, where a very precise altimetric grid network was installed to quantify differential movements throughout the medieval city,
and in particular in the proximity of Palazzo Vilhena (fig. 9).
Topographic surveying using high precision geometric levelling is presently considered as the most suited technique for this scope,
as it enables the monitoring of large areas and has an accuracy in the region of millimetres. A number of benchmarks are carefully
secured in strategic locations of the area being studied, and periodically levelled. The accuracy of this system is dependent upon the
effective distribution of the benchmarks, and their realisation, as well as on the correct measurement and data processing.
In the case of Mdina, around 80 (eighty) benchmarks were anchored in strategic locations to ensure a reliable monitoring of the
whole city. Around 12 (twelve) of these specifically address the monitoring of the vertical movements at Palazzo Vilhena.
In order to ensure the maximum possible accuracy, the network established is made up of a series of closed traverses. The first
readings where taken in July 2001. High precision digital levels with calibrated invar rods were used, and an average accuracy of one
millimetre was obtained for the heights.
(b)
Figure 9: (a) Example of a benchmark description;
(b) the Mdina levelling network, with densification of the benchmarks near the Palazzo Vilhena
