Disentangling the erosion protection characteristics of the rammed earth Great Wall in Shanhaiguan and understanding the role of biological crusts with salts

Abstract Rammed earth is a green material that is widely used in all kinds of buildings around the world. Rammed earth buildings are often subject to erosion and collapse hazards due to prolonged heavy rainfall and exposure. The Great Wall of Shanhaiguan is a typical world cultural site built of rammed earth. However, an investigation found that there were no erosion marks on the partial walls of the Xiaowan and Nantuzhuang gaps of the Great Wall of Shanhaiguan. To study the reasons why rammed the earth Great Wall have strong erosion resistance, we carried out different experimental tests. The results show that the erosion resistance performance of the rammed earth Great Wall is due to the formation of a gray‒green biological crust layer on the rammed earth surface. Scanning electron microscopy indicated that the polysaccharide organic matter secreted by biological crust caused soil particle agglomeration. Moreover, the algal filaments and the hyphae overlapped, wound and wrapped the soil particles, forming a complex spatial network structure. Together, they improved the soil erosion stability. on the other hand，the high-valent cations in the rainwater and the low-valent cations in the clay exchange ions, forming crystal growth and blocking the soil, also enhance the erosion resistance performance of the wall.