Holistic approach to stone heritage preservation on rock masses: experience at Gozo Citadel, Malta

Conservation of outdoor stone heritage built on unstable rock masses can benefit from a holistic approach which leads to an analysis of instability mechanisms of the geologic substratum, deterioration processes of the architectural surfaces and their mutual relationships over time. An analytical procedure combining on site and laboratory diagnostic investigations is here proposed, to deal with the conservation issues due to weathering in historic hilltop towns where the construction and restoration materials share petrographic properties and similar deterioration phenomena with the rocks of the substratum. Conventional and recently developed minero-petrographic, chemical and geotechnical tests were employed to analyze rock and stone samples from the calcareous rock mass, historical masonries and underwalls of the Citadel fortifications in Gozo, Malta. Appearance and spatial distribution of erosion patterns visible on the cliff surfaces were correlated to the rock textural properties, while the morphological study and mapping of the ledges and recesses clarified the past instability events and current rockfall hazard for the enceintes. Diagnosis of stone deterioration highlighted the intrinsic weaknesses of the historically used restoration and mitigation strategies. The identification of the most critical sectors led to the installation of on site monitoring system, to early warn in case of (re)activation of block detachment and/or tilting of the walls.