A holistic approach to diagnose deterioration of rock-cut structures in Lalibela, Ethiopia
Keywords: Moisture
Rock-cut
Lalibela
Clay swelling
Semi-natural
Deterioration

Rock-cut architecture is made through a subtractive process of removing material from rock outcrops or cliff faces to create spaces. Rock-cut sites are at the intersection of the natural and built environments, producing a unique semi-natural setting in which both naturally existing geological features and human activities contribute to their deterioration. This thesis develops a holistic approach to diagnosing deterioration of rock-cut structures by focusing on the rock-cut church complex at Lalibela, Ethiopia. The diversity of rock-cut church types in the UNESCO world heritage site of Lalibela makes it a suitable case study to investigate the factors that influence the deterioration of rock-cut structures.
In semi-natural settings, key drivers of weathering can be broadly classified into climate, lithology, architectural form and history of conservation. These drivers were investigated in Lalibela using field studies, laboratory-based experiments and archival resources to develop a comprehensive understanding of weathering of rock-cut sites.
Rock-cut sites share lithological characteristics; many of them are built in low-strength rock outcrops. This inherent characteristic is correlated with the presence of secondary minerals such as swelling clays. The churches at Lalibela are carved from basaltic scoria that contains > 30 \% swelling clays. The climate at Lalibela is characterised by intense seasonal rainfall (main rains season) and high rock-surface temperature fluctuations (dry season). High, but fluctuating relative humidity (RH) and intense rainfall during the main rains season are favourable conditions for surface and deep-seated wetness and for frequent wetting-drying cycles to occur. To determine the impact of wetting-drying cycles on clay swelling/shrinkage and salt crystallisation/dissolution cycles, dilatation was measured in salt-free and salt- contaminated basaltic scoria samples in simulated wetting-drying cycles. Results showed that the introduction of salt (NaCl) to clay-rich basaltic scoria can substantially accelerate decay and that the range of RH fluctuations at Lalibela can cause significant dilatation.
The architectural form influences the degree of exposure of rock-cut structures to climatic variables such as rainfall and insolation. Moisture dynamics assessments done in the main rains and dry seasons demonstrated that the saturation state of the walls is influenced by exposure of the roofs to rainfall. Salt distribution assessments showed that the thickness of the walls (which lack foundations to retard the flow of moisture) influences salt loading. Conservation at Lalibela has mainly focused on protecting the exposed roofs from water ingress and has often been destructive or altered the cultural landscape. Revisiting traditional conservation techniques that mimic soft capping may present a new opportunity to preserve the sites in a manner that is in keeping with the intangible heritage values of the site.
A holistic methodology to understand the processes that lead to the deterioration of rock-cut architecture should be based on a semi-natural site framework, as has been attempted in this thesis. Two scales of approach are recommended to assess the state of and threats to the preservation of rock-cut structures: architectural form and landform. The architectural form will influence micro-climate, moisture dynamics and salt distribution, and overall, the post-exposure history of transformation and conservation of rock-cut sites. At the landform scale, inherited properties (pre-exposure history) such as topography and geological discontinuities can influence weathering.
By identifying and assessing the key drivers of weathering of rock-cut structures, this thesis lays the foundation for a scientific approach to understanding the deterioration of rock-cut architecture, which can be applied elsewhere beyond the seasonal tropical environments of northern Ethiopia.