Forbidden City is a historical symbol of China, located in the center of Beijing and was constructed between 1406 and 1420 . It served as an imperial palace for almost 500 years and consists of 980 buildings on 180 acres area. Large number of massive buildings required mining and transportation of gigantic stones to the Forbidden City, including Large Stone Carving, which was 300 tones in weight.
By the moment of the construction, large wheeled vehicles had been well developed in China, and researchers Li, Chen and Stone in their research paper published in PNAS, try to answer the question, why sliding sledge was used for the transportation? They argue, that the latter approach was safer and more reliable for transporting very expensive huge stones and that wheeled vehicles load capacity was not enough. The lubrication was necessary to make the sliding transportation even possible. Moreover, water lubricated wood-on-ice transportation was more efficient than other lubrication technologies available at that time. Taking as an example a transportation of 123 tone stone, the researchers estimated the number of men necessary to tow the sledge with a stone on it under various lubrication conditions, as shown in the figure below.
Before sliding, the ice path was created by artificial supply of water to the ground. Although the ice friction is typically small, it may reach value of 0.36 if rubbed with wood at low speed without lubricating water.At the same time, if lubricated with water, it may be as low as 0.03. The speed of transportation of the massive stone to the Forbidden City was estimated to be 8 cm/s and is not enough for generation of the water film due to frictional heating. Therefore, authors conclude, that water was supplied directly by pouring water on the surface of ice prior to moving the sledge. Therefore, water was used not only to create the ice path for the sledge, but also for the further reduction of the friction.
As authors mention, in modern engineering, the approach for transportation of large objects using artificial ice path was successfully applied as recently as in 2013, to move a 1200 tone building in China. Therefore, although old, the combination of ice and water can be useful.
Credit for the title image: By user:kallgan – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=978574
More details can be found in the original article: Ice lubrication for moving heavy stones to the Forbidden City in 15th- and 16th-century China Jiang Lia, Haosheng Chen, and Howard A. Stonec.