Weathering processes and slope-structure stability of rock-hewn churches of Lalibela (Ethiopia)

Lalibela is located in the northern-central part of Ethiopia, approx. 600 km north of Addis Ababa. The town, which has about 12000 inhabitants, is situated at an altitude of 2500 m. In its center a unique complex of 11 rock-hewn Christian Orthodox churches is located, cut out of the living rock (mainly tuff and basalt) some 800 years ago whose construction is attributed to King Lalibela (1167-1207). Over the years, the churches have been exposed to wind, rainfall, thermal changes and human activities. This has resulted in a severe structural damage of the monuments most of which are now considered to be in critical condition.
Because of the outstanding value and the nature of the monuments, the conservation of the Lalibela churches attracted long ago the interest of the international community. A first restoration attempt of the Lalibela rock-hewn churches was carried out in 1920. Presently, almost all churches are protected by shelters in order to avoid rain drops to produce surface erosion and direct circulation over the roofs and temporary scaffolding to prevent from collapse of vulnerable structures.
Geological field surveys, in-situ geotechnical analyses and recognition of various weathering processes have been preliminary carried out in the area providing a classification of the churches according to structural damage. Laboratory tests on volcanic materials have detected a deep alteration of rocks due to chemical and physical weathering as well as the diffuse presence of montmorillonite and related weathering, in the poor silica volcanic rock.
This may be recognised as the main cause of the progressive deterioration of the physical and mechanical characteristics of the slope-forming rocks in Lalibela. One of these churches, Biet Aba Libanos, is a monolithic church anchored to the rock out of which it was carved. Two major damaging phenomena affect the church: weathering of volcanic tuff in the lower part of the edifice and sliding of the façade and lateral walls, as consequence of a prone discontinuity. A first destruction of the façade has already occurred in the past, as consequence of an old planar sliding, still in coincidence of the same joint; presently, the walls prone to slide are the structures constructed to replace the original rock collapsed for the rock slide as well as some of the original rock hewn lateral walls. Kinematic analysis and numerical modeling clear highlight the hazard for the rock hewn church of Biet Aba Libanos and the need of a prompt and proper intervention. Following the new scientific findings on the weathering processes affecting the Lalibela Churches it will be possible to detect and implement urgent and medium/long-term protection strategies for the conservation of the monuments.