Rising Damp: The Rising of Moist Earth-Air in the Mogao Grottoes, China

Usually, the term 'rising damp' refers to capillary water rising from the ground which may damage architectural heritage. In this work, the essence of rising damp in extremely arid regions and its driving force are revealed based on experiments monitoring the relative humidity (RH) and atmospheric pressure (AP) in the Dunhuang Mogao Grottoes. The air in the vadose zone, the unsaturated region between ground level and the top of the water table, is here referred to as 'earth-air'. When the AP rises, the earth-air is compressed, and atmospheric air enters into the soil. Then, when the AP drops, moist earth-air expands and rises into the structure. The RH in the soil is thus negatively correlated with the AP, yielding a correlation coefficient of up to -0.94. Under the action of this long-term dry-wet alternation, the salt present in the building near ground undergoes repeated cycles of crystallization and dissolution, resulting in efflorescence and a deterioration zone. Therefore, the deterioration due to rising damp in extremely arid regions is caused by the rising of moist earth-air rather than capillarity. The height to which it rises is directly proportional to the amplitude of the daily AP fluctuation and thickness of the vadose zone, exhibiting a bimodal fluctuation pattern on a daily scale. The discovery of this mechanism of rising damp provides a scientific basis for preventive conservation interventions.