Integrated geological-architectural pilot study of the Biet Gabriel-Rufael rock hewn church in Lalibela, northern Ethiopia

The paper present a geological and architectural integrated pilot study, aiming at the preservation of the Biet Gabriel-Rufael church, located in Lalibela, the worldwide known Ethiopian rock hewn monumental site protected by UNESCO since 1978. The town developed since the Neolithic up to the medieval age, as inferred from the traces of three distinct architectural phases. Lalibela was built on a geological substratum made of rocks belonging to the Ethiopian Plateau suite, which is mainly composed of basalts of fissural origin or derived from shield volcanoes. The geological units are composed of alternating massive and scoriaceous basalts. The main scoriaceous basalt level, embedded within the massive basalts, is 30-40 m thick and corresponds to the horizon within which the Biet Gabriel-Rufael church all the other monuments of Lalibela have been carved. Therefore, the evolution of the town was strongly conditioned by the occurrence and extent of the softer scoriaceous basalt level. Many fracture systems of both natural (i.e. geological) and anthropic origin (these latter connected to the carving of the church), were recognized. The fracture pattern determined the subdivision of the church into different blocks that can behave independently, thus compromising the stability of the monument. A net of deformometers and fracture gauges was installed for the monitoring of the fracture system and a preliminary Finite Element analysis, following the approach used for underground excavations, was performed, with the aim of elucidate the mechanical behaviour of the rock. The integration between geomechanical approach to the rock mass and the architectural study of the critical situation due to the carving and connected to buildings, resulted in the precise individuation of future interventions devoted to the conservation of these monuments.