Growth is usually considered harmful to architectural monuments. But in the case of the parts of the Great Wall of China made of rammed earth, the opposite is true, according to a study: thin biocrusts made of mosses, lichens and cyanobacteria protect these structures from erosion by wind and weather. In poorly vegetated areas, targeted vaccination with these creatures could even be worthwhile in order to slow down the decline, say the scientists.
Stretching thousands of kilometers from the desert in northwest China to the sea in the east, the Great Wall of China is one of humanity's most famous monuments. It is actually a series of ramparts that were built piece by piece over centuries to protect the Middle Kingdom against invaders from the north. The emperors of the Ming Dynasty expanded the wall particularly intensively from 1368 to 1644. The most famous section north of Beijing, which is characterized by stone and brick structures, also dates from this period. In large parts of the wall of the Ming era, however, it was built using a different construction method: they were walls made of rammed earth - structures that were made from clay-rich earth and other materials through mechanical compaction.
Is the crust good or bad?
Since rammed earth is a relatively erosion-prone material, the centuries have taken a toll on these parts of the wall. Chinese researchers hope to at least be able to counteract this process. To do this, they are researching the factors that play a role in the erosion of rammed earth structures. Yousong Cao from the University of the Chinese Academy of Sciences in Beijing now had living creatures in his sights: In some areas, the remains of the Great Wall of China are covered by a layer of mosses, lichens and cyanobacteria.
Until now, it seemed unclear whether this so-called biocrust promotes or limits the erosion of wall parts made of rammed earth. It is known from other buildings that vegetation can lead to the softening of materials. However, given the special characteristics of rammed earth, an opposite effect is also conceivable - the crusts could form more of a protective layer. In order to clarify the effect of the biocrust, Cao and his colleagues took a closer look at the growth on various parts of the wall from the Ming period.
Living protective layer
First of all, the study showed: Around 67 percent of the surfaces of the rammed earth structures of the wall are covered with biocrusts. In dry regions they were dominated by cyanobacteria, while in wetter climates mosses tended to make up the majority of the biocrusts. From the on-site inspections and the analyzes of the materials in the laboratory, a clearly positive effect of the layers emerged: The thin vegetation improves the resistance of the rammed earth structures to erosion processes by up to three times compared to the unvegetated areas.
The laboratory studies also clarified the causes of the positive effect: The crust protects against wind erosion, leads to less water absorption due to increased beading of rain and buffers destructive temperature fluctuations. As the scientists explain, the special properties of the biocrust are due to certain substances that mosses, lichens and cyanobacteria release in order to attach themselves to the substrate. These are polymer-like substances that act like glue between the particles of the rammed earth and thereby stabilize it.
The study results have shown that the biocrusts significantly benefit the preservation of the Great Wall of China, the researchers conclude. According to them, this probably also applies to other clay monuments. “The results suggest that we should preserve naturally occurring biocrusts on these heritage structures rather than removing them. “It could also be worthwhile to investigate whether colonization by biocrusts can be promoted through artificial vaccination with the organisms,” the scientists conclude.