A recent international research team has discovered that large sections of the Great Wall of China are held together by “bio-crusts,” thin layers of organic material that have helped protect this architectural wonder from corrosion.
According to a study published in the journal “Science Advances,” the Great Wall of China spans an astonishing 8851.8 kilometers across predominantly arid landscapes. Recognized as a UNESCO World Heritage site due to its unparalleled construction duration and geographical expanse, the Great Wall has stood for over five centuries, symbolizing an indispensable aspect of Chinese identity and a priceless treasure for human civilization. Constructed at various sites and time periods using rammed earth, a common material for large structures in the ancient world, including raw natural materials such as soil, gravel, and more, it was employed for building walls and foundations.
As a symbolic structure made of rammed earth, the Great Wall is highly vulnerable to wind erosion, rain, salt, freeze-thaw cycles, leading to severe issues like cracking, disintegration, and potential collapse over time.
Considering the impacts of global climate change, the Great Wall faces the imminent danger of significant deterioration, jeopardizing the long-term stability of its earthen structure. Currently, only 5.8% of its total length is well-preserved, while 52.4% has either disappeared or severely deteriorated. Urgent conservation strategies must be implemented.
Bio-crusts: Safeguarding the Wall
Bio-crusts, self-nourishing communities primarily consisting of blue-green bacteria, algae, lichens, and other microorganisms intricately linked with soil particles, cover substantial portions of the Great Wall. While these bio-crusts colonize only a few centimeters on the soil surface, they serve as ecological engineers, supporting and regulating essential soil processes and terrestrial ecosystems.
However, bio-crusts may also influence the mechanical stability and physical and chemical properties of the rammed earth used in the construction of the Great Wall, acting as a natural cover to protect the earthen heritage pieces in dry climates. Yet, the exact contribution of bio-crusts to preserving the Great Wall remains largely unknown.
The study’s co-author, Professor Bo Xiao from the College of Soil and Water Conservation at China Agricultural University in Beijing, stated, “Ancient builders knew about materials that could make the structure more stable.”
To enhance mechanical strength, rammed earth for the wall was built using clay, sand, and other adhesives such as lime by the original builders. These components provide fertile ground for living organisms that build bio-crusts.
To test the strength and integrity of the Great Wall, researchers collected samples from eight different sections built during the Ming Dynasty between 1368 BC and 1644 BC. They found that 67% of the samples contained “bio-crusts,” which Xiao referred to as “ecosystem engineers.” Using portable mechanical tools, both on-site and in the lab, researchers measured the mechanical strength of the samples, soil stability, and compared the data with sections of the wall containing only bare rammed earth.
They discovered that samples with “bio-crusts” were sometimes three times stronger than samples of bare soil. The study noted that samples containing algae were particularly robust, as blue-green bacteria and other life forms within the bio-crusts secrete substances like polymers that tightly bind with rammed earth particles, aiding in “enhancing its structural stability” by essentially forming cement, according to Xiao.
He added that these cementitious materials, biological threads, and soil groups within the bio-crust layer ultimately form a cohesive network with strong mechanical strength and stability against external erosion.
