NON-INVASIVE ANALYSES

AN OVERVIEW ON PECULIAR TECHNIQUES

  

In summary, diagnostics can be defined as a set of manual, visual, historical, instrumental and laboratory procedures that permit the formulation of a judgement, and thus of a comprehensive analytical diagnosis through a “stratification” of information.
What has a peculiar value, is the interdisciplinary nature and thus the mutual exchange of information between the different figures interacting in the restoration project, united by a common objective: the protection of Cultural Heritage through awareness, conservation and enhancement.
The geophysical surveys, considered as innovative diagnostic techniques, allow for the identification, working from outside the surfaces, “hidden” (thus, non-visible) anomalies, voids, structures, chains, pipes, ... They are based on the registration of physical phenomena that occur naturally, without external stimulation, or that are artificially caused by ex- citations of various kinds (sonic, electric, electromagnetic, radar, magnetometric ....).

They result, being a fundamental addition to the direct measures (excavation, drilling, chemical and physical analysis), can be considered useful and convenient only whereby programmed, performed and interpreted correctly. The principal geophysical techniques applied to Cultural Heritage assets can be outlined in the following groups:
> ELECTRICAL TOMOGRAPHY: the study of spatial variations of electrical resistivity “of a medium”; allowing for the simple location of buried structures from the resistivity contrast between these and the soil in which they are contained;
> SEISMIC TOMOGRAPHY: the study of the spatial distribution of sonic wave velocity in a medium, for which one returns to the elastic characteristics of the same; high-velocity characteristics corresponding to healthy, homogeneous and compact walls. The presence of voids or discontinuities reduces the velocity of wave propagation;
> ULTRASONIC INVESTIGATIONS: timely seismic surveys at higher frequencies for the determination of velocity characteristics and thus the detection of smaller abnormalities;
> MAGNETOMETRIC SURVEYS: measure variations in the Earth’s magnetic field or its gradient; in particular, they are used in archaeological and environmental fields for the detection of buried structures;
> RADAR SURVEYS (GPR): the georadar operates in accordance with principles analogous to those of conventional radars. This studies the electromagnetic echo captured by a receiving antenna and produced by the interaction of a pulse, emitted by a transmitting antenna, with a target. In contrast with the second point, the georadar transmits in means characterised by elevated values of signal attenuation, being much more conductive than air. The georadar is also a broadband device that can operate at a frequency range of between 10 and 2,500 MHz. The depth of the reflective “object” is determined from the interval of time between the moment of transmission and reception, whereby the signal propagation speed is known. In regards to masonry, in addition to the identification of voids or metallic/plastic structures, the study of the degradation and constructive characteristics is of much interest.

An integrated approach aimed at the awareness of Cultural Heritage should follow a logical progression based on the use of non-invasive techniques, without alteration of the structure or terrain under study, accompanied by a limited number of direct inspections, with the advantage of being operations that are targeted and optimised on the basis of the interpretation of the results obtained by non-invasive investigations, often performed in an extensive manner, achieved by overlapping with any other diagnostic steps through- out the restoration.
The advantage of the realisation of such an articulated and organised integrated diagnostic campaign is being able to maintain the utmost integrity and therefore the maximum protection of the Cultural Heritage, taking advantage of the interdisciplinary nature of the professionals involved, not only throughout the data acquisition phase but particularly in that of diagnosis, and thus the global interpretation deriving from the total overlay of the data obtained, effective and necessary for undertaking a restoration project.

The Integrated Cognitive Approach is coherent with the DPCM 09-02-2011 “Assessment and reduction of seismic risk of cultural heritage with reference to the Technical Standards for construction pursuant to Ministerial Decree 14/01/2008”.
The study items proposed in this approach are, in fact, consistent with Article 4 “Knowledge of the Artefact”, of which the fundamental points are:

4.1.6 The construction material survey and the state of conservation:
… The construction material survey must allow the complete identification of the resistant organism of the construction, keeping in mind the quality and state of conservation of the construction components and materials.
Such recognition requires collating information that is often hidden (concealed beneath plastering, under false ceilings...) that can be executed through non-destructive investigation techniques of an indirect nature (thermography, ground-penetrating radar, sonic tomography...) or slightly destructive direct inspections (endoscopies, stripping plaster, probes, small breakages...) .... to be utilised only whereby well-motivated, or if useful in the evaluation and intervention project.

4.1.8 Geotechnical aspects
…The determination of the type and consistency of the foundation system, together with the geotechnical characterisation of the terrain included in a significant volume of subsoil, constitute necessary components for the evaluation of seismic action and its effects on the construction.
Among the possible survey measures, the non-destructive testing techniques are preferred, such as geophysical and tomographic tests, to be effectuated even after the execution of exploratory wells, to bring to light the foundations and highlight the imposing levels. If necessary, continuous core drilling can be carried out.
The surveying holes can be effectively utilised for video inspection and geophysical tests.
The geotechnical surveys should enable the physical and chemical characterisation of the foundation soils, through in situ and laboratory tests, aimed at determining the geotechnical models suitable to the local seismic response analysis and soil-structure dynamic interaction.
The tests should be aimed at the study of the stability of the site in which the building under review is located, particularly with respect to slope instability and soil liquefaction.

A particular case for the application of such techniques has been the integrated monitoring approach performed as a training session on structures (walls, beams, slabs, …) of the “Palazzina Reale and the Mazzoni Power Plant of the Santa Maria Novella Railway Station, to which the images refer.

[by Annalisa Morelli]