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

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Abstract
Lalibela is located in the northern–central part of Ethiopia, c. 600 km north of Addis Ababa in Northern Wollo (Ahmara Region). The town, which has about 12 000 inhabitants, is situated at an altitude of 2500 m. In its centre a unique complex of 11 rock-hewn Christian Orthodox churches is located, cut out of the rock some 800 years ago. Their construction is attributed to King Lalibela (1167–1207). All the churches exhibit widespread evidence of structural damage and weathering processes, which are posing a serious problem for the preservation of the monuments. Preliminary geological field surveys, in situ geotechnical analyses and laboratory tests have been carried out in the area to understand the cause of weathering and the likely consequences on the structural stability of the churches. Laboratory tests on volcanic materials detected a deep alteration of rocks as a result of the widespread presence of montmorillonite. This is the primary cause of the progressive deterioration of the physical and mechanical characteristics of the slope-forming rocks in Lalibela. Major consequences are the alveolar weathering of churches' façade, the degradation of the roofs and a reduction of rock strength, which are causing a progressive structural instability in some rock-hewn monuments. Also, sliding of façades along bedding joints was detected. Understanding the weathering processes affecting the Lalibela churches allows the detection of problems and the timely implementation of medium- to long-term protection strategies for conservation of the monuments.