The secret of the Great Wall of China remaining strong for centuries has been revealed by scientists. Photo/Live Science/Sputnik Globe
BEIJING – The secret of the Great Wall of China remaining strong for centuries has been revealed by scientists. Apart from having a strong architectural structure, the wall, which stretches for 21,000 km, uses a mixture of organic materials of moss and crust to protect against erosion.
During the construction process, ancient workers often used rammed earth, including a mixture of organic materials such as compacted soil and gravel, to build large walls. These materials may be less susceptible to erosion than other materials, such as solid rock, but help encourage the growth of “biocrusts.”
Biocrusts are thin layers of organic material that help protect wall architecture from erosion. This living plaster consists of cyanobacteria (microorganisms capable of photosynthesis), moss and lichens that help strengthen construction, especially in arid and semi-arid regions.
“The ancient builders knew which materials could make structures more stable,” said Bo Xiao, professor of soil science at the Faculty of Land Science and Technology, China Agricultural University in Beijing, to Live Science, Sunday (10/12/2023).
To test the strength of the Great Wall of China, researchers collected samples in eight different sections. They found that 67% of samples of the Great Wall of China built during the Ming Dynasty between 1368 BC and 1644 BC contained biocrusts.
Using portable mechanical instruments, both on site and in the laboratory, the researchers measured the mechanical strength of the samples and the stability of the soil. Then compare this data with a wall segment that only contains bare ground.
They found that “biocrust” samples were sometimes three times stronger than flat soil samples. Samples containing moss are particularly sturdy because cyanobacteria and other life forms in the biocrust secrete a polymer-like substance.
“This polymer tightly binds soil particles to strengthen their structural stability by creating a material that is basically similar to cement,” said Xiao in a statement published in the journal Science Advances, Friday 8 December 2023.
The cement, biological filaments and soil aggregates in this biocrust layer form a cohesive network with mechanical strength. This combination makes the Great Wall of China very sturdy and stable against external erosion.
“To increase mechanical strength, compacted walls are always made with clay, sand and other adhesives such as lime by the original builder,” Xiao added. These materials provide fertile ground for organisms that build “biocrusts”.