Photo of the Forbidden City via Shutterstock

A new study by a team of engineers from two Beijing universities and Princeton University reveals how ancient Chinese workers hauled 100 ton plus stones over a distance of 43 miles to build the Forbidden City. The study published in Proceedings of the National Academy of Sciences states that workers dug wells every 500 meters in the deep winter to reach water that they could pour over the ground to create ice pathways that helped them slide the stones. They were able to determine this by studying the friction that different methods of hauling produce.

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The engineering team discovered that the recorded winter temperature in Beijing in 1557 was cold enough to maintain ice roads sturdy enough for hauling very large and heavy stones. In fact, it was so cold that the ice alone could not have provided a good enough lubricating layer between the bottom of the wood sledges and the surface of the ice roads. But it was just warm enough that any water poured over the road would stay liquid long enough to provide the needed lubrication to slide the heavy stones. This extra step of adding water made all the difference, according to the study. Without water, 338 people would have been needed to pull a stone that weighed 123 tons over a distance of 43 miles. With water, however, it only took 46 men.

The team calculated that the coefficient of friction for logs rolling along the ground, which was a common ancient method of hauling, is 0.2 to 0.4. With rails, that drops to 0.02 to 0.04. Water and ice sliding has a coefficient of friction of about 0.02. The engineers concluded that it would be easier to slide sledges over ice than over logs and planks as ice is smoother and easier for negotiating turns. Moreover, wheeled vehicles at the time could only have supported up to 97 tons and could not have handled the largest stones that sometimes weighed over 100 tons. Ice roads and a team of men were considered more reliable than carts pulled by mules, and their safety (the stones’, not the men’s) was of the utmost concern to the architects of the Forbidden City.

Via Daily Mail