Unveiling the deterioration formation process of the rammed earth city wall site of the Ancient City of Pingyao, a World Heritage Site: occurrence, characterizations, and historic environmental implications

Large, immovable rammed earth buildings in UNESCO World Cultural Heritage sites are at serious risk of disaster due to environmental changes. In this study, the rammed earth city walls (REWs) located in the Ancient City of Ping Yao, a World Cultural Heritage site, affected by heavy precipitation in early October 2021, were used as the research object. The study aimed to specify the details of deterioration through multiple indicator data collection, semi-quantitatively evaluate the degree of deterioration of REWs, and investigate the causes of typical deterioration in REWs with different spatial distributions under the influence of heavy precipitation. Based on the current research and experimental data, and considering the mode of action and form of deterioration, a classification system for the landslide-like collapses of REWs under the influence of precipitation was constructed. Factors such as the influence of heavy precipitation and the REWs' intrinsic features resulted in significant variations in structural properties and deterioration development of REWs with different spatial distribution characteristics. Finally, based on the research in this paper and the conservation ideology of cultural relics that respects the original and minimizes intervention, we propose protection recommendations for the daily management and conservation of the REWs to provide guidance for future research.