Moisture plays a key role in rock decay in the built and natural environments. Rock-cut sites are particularly vulnerable to moisture-related weathering as they are carved into rock outcrops and do not have impermeable foundations or roofs to retard the flow of moisture. To characterise the moisture dynamics and its influence on weathering of rock-cut sites, we undertook a moisture monitoring campaign using a non-destructive Microwave Moisture Measurement System (MMMS) at two monolithic rock-cut churches in Lalibela, Ethiopia. The results showed that the walls were more saturated at depth than on the surface during the wet season. This suggests that low surface temperature and higher moisture content at depth will lead to constant-rate drying and accumulation of salts on the surface of the walls during the wet season. In the dry season, there was higher saturation near the surface than at depth (falling-rate drying). High rock surface temperature during the dry season contributes to subsurface drying and accumulation of salts below the surface. This seasonally shifting moisture dynamics will lead to a complex and dynamic damage profile. This study highlights the significant wetting facilitated by a lack of impermeable roofs and foundations at rock-cut structures during rainy periods.
<p>Weathering of rock-cut structures exposed to the environment is strongly influenced by fluctuations in climatic variables. Both macro and microclimate data are needed to identify key weathering types and rates likely to affect rock-cut structures in a specific region. The aim of this paper is to study the macro and micro climatic conditions affecting the rock-cut churches in Lalibela, Ethiopia to determine how the climate influences weathering at this site. Macro climate data collected over a 26-year period and microclimate data monitored on the north, east, south and west walls at one of the churches in the Lalibela church complex (Bete Mariam) are used to make these assessments. Microclimate data was monitored during the long rains (Kiremt), short rains (Belg) and dry (Bega) seasons in 2018 and 2019. The results showed a high seasonal variation in macro climatic conditions like rainfall and ambient relative humidity. The micro climatic (rock surface) conditions also tended to vary seasonally. The diurnal range of rock surface temperature during Bega varied significantly depending on which cardinal directions the walls were facing, with south and west facing walls having high diurnal thermal ranges. The influence of aspect was less pronounced in Belg and Kiremt, but cloud cover played an important role in varying the range of diurnal thermal and humidity cycles from day to day during these seasons. These climate trends are likely to cause seasonal variations in wetting and drying cycles, deep wetting, increased time of wetness and thermal cycling. These wetting/drying and heating/cooling characteristics affect weathering processes. During Kiremt, biological weathering, salt weathering and clay swelling are more likely to occur than in Belg and Bega. High diurnal thermal ranges in Bega are likely to cause thermal fatigue in this season. This is the first paper to address the macro and micro climatic trends that influence rock weathering at the rock-cut churches in Lalibela. The results of this study also have implications for rock-cut structures in northern Ethiopia having similar environmental conditions as Lalibela.</p>
This study concerns the eleven monolithic churches in Lalibela, in northeastern Ethiopia, a UNESCO World Heritage Site and currently the main pilgrimage site in Ethiopia. In 2019, on the initiative of Prime Minister, the French authorities proposed their support in the management on the site of the churches. To do so, the French Development Agency (AFD), in collaboration with the Ministry of Europe and Foreign Affairs and with the support of the Ministry of Culture, granted a feasibility study to examine ways of restoring, conserving, and developing the rock-hewn churches. The objective of the feasibility study conducted was to produce the preliminary technical diagnostics required for the preparation of the comprehensive project to restore, conserve, and develop the site. In order to propose a protection and conservation solution, diagnostics and analysis of the pathology of the rocks were made during two campaigns in November 2019 and November 2020. The rock pathology teams implemented non-destructive and minimally invasive analysis. The complementary methods acquire data from the rock surface and the different forms of differential alteration of the scoriaceous basalt. The objective is to characterize, through comparative analyses, the impact of a protective shelter on the alteration kinetics of the rock. The analysis, coupled with on-site observations, suggests that deterioration linked to liquid water and the persistence of a state of high water content is more damaging than the deterioration risk linked to salt crystallization. As water is the key factor in the very harmful alteration for the conservation of scoriaceous basalt as a heritage material in humid natural environments, it seems useful to fully cover the churches. Doi: 10.28991/HEF-2022-03-02-01 Full Text: PDF
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