Diagnostic Process of a Megalithic Monument by Ultrasonic Measurements

Silvana FAIS^{1, 2} / Paola LIGAS₁ / Francesco CUCCURU¹ / Paola BASOLI³ / Graziella DETTORI³ / Paola DUI⁴
(¹Dipartimento di Geoingegneria e Tecnologie Ambientali, University of Cagliari, Italy / ²Consorzio Interuniversitario Nazionale per l’Ingegneria delle Georisorse (CI.NI.Geo) Roma, Italy / ³Soprintendenza per i Beni Archeologici di Sassari, Italy / ⁴VI Comunità Montana di Monte Acuto, Sassari, Italy)

Abstract: Ultrasonic measurements have been implemented in past years in the Cultural Heritage Field with the aim to improve the diagnostic process of a monument prior to any restoration. This paper presents the results of a study regarding the application of non-destructive ultrasonic techniques in evaluating the conservation state of the Dolmen Sa Coveccada in northern Sardinia (Italy). The study was carried out within the framework of a relevant project of restoration of megalithic monuments promoted by the Soprintendenza per i Beni Archeologici di Sassari (Italy) (National Archaeological Authority of Sassari – Italy), Comunità Montana di Monte Acuto (Mountain Community Administration of Monte Acuto) and Comune di Mores (Local Town Authority of Mores) (Italy). The study Dolmen is a megalithic structure of great historic and scientific importance and is the only example of its kind in Sardinian archaeology (Italy). It is made of the dacites of the Oligo-Miocene calcalkaline volcanic cycle of northern Sardinia. At the petrographic microscope, the volcanic rocks present a porphyric vitrophyric structure and an isotropic texture, with phenocrystals of plagioclase (often united in glomerocrystals), quartz, biotite, amphibole, clinopyroxene and sanidine, included in a vitreous fluidal mass with laminar flow, glass shards and spherulitic devitrification. In the femic minerals, which are more unstable at environmental temperature and pressure, intense oxidation has caused an increase in microporosity and microcracks which at the macroscale have triggered some important forms of degradation such as exfoliation and fissuration that have drastically changed the stability and safety of the monument.

In this context the use of the ultrasonic techniques was very effective both to define the elasto-mechanical properties of the building materials such as longitudinal ultrasonic velocity, quality index and fracture coefficient, and to detect the presence of discontinuities caused by degradation phenomena and cracks.

In addition, the ultrasonic techniques were used in laboratory on samples of the same volcanic building materials of the Dolmen to investigate the repairing efficiency of chemical consolidating materials such as epoxidic resins and to test their compatibility with the investigated materials. The selected epoxidic resin (EPO 150) was applied on fresh-cut dacite volcanic samples so that changes on elasto-mechanical properties were also evaluated in time.

Keywords: megalithic monument, non-destructive ultrasonic techniques, elasto-mechanical properties, repairing efficiency