An international research team has discovered large areas of… Great Wall of China They are held together thanks to “biocrust,” thin layers of organic material that have helped protect the architectural marvel from corrosion.
According to a study recently published in the journal Science Advances, the Great Wall of China extends an astonishing 8,851.8 kilometers across mostly dryland environments, and is recognized as a World Heritage Site due to its unparalleled construction duration and geographic extent.
The Great Wall of China has existed for more than five centuries, serving as an indispensable manifestation of the Chinese nation and a priceless treasure of human civilization. It was built in many locations and time periods using rammed earth, which was one of the most common materials used to build large structures in the ancient world. It includes natural raw materials such as soil and gravel and is used to build walls and foundations.
As an emblematic structure of rammed earth, the Great Wall is highly vulnerable to wind erosion, rainfall, salinization, and freeze-thaw cycles, leading to serious problems such as cracking, disintegration, and eventual collapse.
Given the effects of global climate change, the Great Wall is at risk of severe deterioration, which could jeopardize the long-term durability of its earthen structure. To date, only 5.8% of its total length is still well preserved, while 52.4% has either disappeared or been severely deteriorated. Therefore, conservation strategies must be implemented as a matter of urgency.
Biocrusts… How did they protect the wall?
Biocrusts – photosynthetic autotrophic communities consisting mainly of cyanobacteria, algae, lichens, other microorganisms and tightly bound soil particles – are known to cover large parts of the Great Wall of China.
Although biocrusts only colonize a few centimeters on the soil surface, they can act as ecosystem engineers supporting and regulating many key processes of soil and terrestrial ecosystems.
But biocrusts may also affect the mechanical stability and physical and chemical properties of the rammed earth used to build the Great Wall, serving as a natural living cover to protect terrestrial heritage objects in dry climates. However, how biocrusts contribute to the preservation of the Great Wall is almost unknown.
“The ancient builders knew which materials could make the structure more stable,” says study co-author Bo Xiao, a professor of soil science at the College of Land Science and Technology at China Agricultural University in Beijing.
“To enhance mechanical strength, the wall’s rammed earth was built with clay, sand, and other adhesives such as lime by the original builders,” Xiao told Live Science. “These ingredients provide fertile ground for the organisms that build the biological crusts.”
To test the strength and integrity of the Great Wall of China, researchers collected samples from eight different sections built between 1368 BC and 1644 BC during the Ming Dynasty, and found that 67% of the samples contained “biocrusts” that Xiao called “engineers.” environmental system”. Using portable mechanical tools, both on site and in the laboratory, the researchers measured the samples’ mechanical strength and soil stability and compared that data to sections of the wall containing only bare rammed earth, according to a statement.
They found that the “biocrust” samples were sometimes three times stronger than solid Earth samples. According to the study, samples containing algae were particularly fatty. This is because cyanobacteria and other life forms within the biocrust secrete materials such as polymers, which would “tightly bind” to the rammed earth particles, helping to “enhance their structural stability” by forming what is essentially cement, Xiao said.
He added that these cement materials, biological threads, and soil aggregates within the biocrust layer finally form a cohesive network with strong mechanical strength and stability against external erosion.
