Geological and geotechnical properties of the medieval rock hewn churches of Lalibela, Northern Ethiopia

Lalibela is a medieval settlement in Northern Ethiopia famous for its 11 beautifully carved rock hewn churches, registered as World Heritage Site in 1978. The rock hewn churches are grouped into three based on their proximity: the Bete Medhane Alem (Church of the Holy Saviour), Bete Gabriel–Rufael (Church of St. Gabriel–Rafael) and Bete Giorgis (Church of St. George) groups. The churches are carved out of a single, massive scoriaceous basalt hill which was deposited along an East–West extending palaeovalley in the Oligo-Miocene Trap basalt of the northwestern Ethiopian plateau.The Rock Mass Rating (RMR) classification scheme was used to classify the rock mass (assuming each church as a separate rock mass) based on their uniaxial compressive strength and the spacing and conditions of discontinuities. Though most of the churches are hewn from medium to high strength rock mass, discontinuities make them vulnerable to other deteriorating agents mainly weathering, and water infiltration. Most of the rock hewn churches are affected by pre-carving cooling joints and bedding plane discontinuities, and by mostly but not necessarily post-carving tectonic and seismic induced cracks and fractures. Material loss due to deep weathering triggered by rain water infiltration and uncontrolled groundwater seepage affects most of the churches, particularly the Bete Merqorios (Church of St. Mark) and Bete Aba Libanos (Church of Father Libanos) churches. The scoriaceous basalt which is porous and permeable allows easy passage of water while the underlying basalt is impermeable, increasing the residence time of water in the porous material, causing deep weathering and subsequent loss of material in some of the churches and adjoining courtyards.

Research highlights► This research applied the Rock Mass Rating method to classify the rock hewn churches of Lalibela in Northern Ethiopia based on in situ uniaxial compressive strength measurements and detailed discontinuity characterization. ► The rock hewn churches are generally carved out of medium strength to strong intact scoriaceous basalt rocks which are however strongly affected by both natural, horizontal to sub-horizontal bedding plane discontinuities, and by vertical to sub-vertical structural discontinuities formed due to unloading or geostatic pressure release during and after carving. These might led to the formation of a thrust system and sliding of separate blocks of the churches. ► Weathering accompanied by material loss as well as slow but persistent water infiltration to the permeable scoriaceous basalt are also leading to a continued deterioration of the rock hewn churches.