The Great Wall of the Ming Dynasty starts from the Hushan in eastern China's Liaoning Province, to the Jiayuguan in western China's Gansu Province, totalling 8851.8 km in length. The Great Wall is a precious historical and cultural heritage in China. It was listed as a world heritage site by UNESCO in 1987. The Chinese government has always attached great importance to the protection of the Great Wall and has taken a lot of effective measures. However, because of the ages and economic and social development, the Great Wall is still facing quite serious human threats and natural destructions. Therefore, it has become the consensus of the Chinese government and all the researchers to further strengthen the protection of this precious cultural heritage of mankind, which is of outstanding and universal value throughout the world. Based on field investigation and sampling in Yongning of the Great Wall of the Ming Dynasty, basic physical and mechanical properties and SEM experiment of the Great Wall soil were carried out, and the main mineral composition, chemical composition, and microstructure characteristics of rammed soil of the Great Wall were obtained. According to the natural environment conditions of the Great Wall, a one-way freeze-thaw cycle device was developed to study the migration law and microscopic mechanism of water and salt of rammed soil. The results provide scientific data for the long-term preservation, display and utilization of the Great Wall site.

Salt damage caused by the complex interaction between water and salt in the heritages is the main factor that deteriorates the materials and destroys the historical information of the relics. The influence of environmental conditions, especially humidity, on salt damage of heritages has been emphasized by many researches. In this study, the water-salt migration characteristics in soil columns under different humidity were studied by laboratory tests. First, water vapor adsorption test was carried out to investigate the soil samples adsorption capability in 6 relative humidity conditions (RH11%, RH23%, RH43%, RH60%, RH75%). There is a linear relation between relative humidity and water vapor absorbed by soil, and the water vapor adsorption curves of samples can be well described by first-order exponential attenuation equation. Second, the water content and conductivity distribution within samples (hygroscopic and non-hygroscopic samples respectively) were investigated after capillary migration tests using 4 types of saline solutions (0.2 mol/L NaCl, 0.2 mol/L Na2SO4, 0.2 mol/L NaCl Na2SO4 mixed solution and distilled water). Results show that high conductivity appears on the top of most samples, and the values have a correlation with type of capillary migration fluid: NaCl > Na2SO4 > NaCl-Na2SO4 > H2O. In addition, the distribution of water content and conductivity becomes complicated under different relative humidity conditions.