Nowadays, we have the advantage of combustion engines and hydraulic cranes to help us build enormous structures like the Hoover Dam or the new World Trade Center tower. But people living hundreds or even thousands of years ago managed to construct equally impressive structures without the benefit of modern technology. Some of their methods are well-documented; others, half-buried by history.

The Forbidden City, a sprawling palace complex in the center of Beijing, was built over almost 20 years in the 15th and 16th centuries. Some of the giant stones used to create the palace were as large as 30 feet across and weighed more than 100 tons. So how did 15th-century Chinese workers haul them into place? A newly published paper highlights one of the key construction elements for this symbol of Ming and Qing dynasty power: ice roads.

Princeton University engineer Howard Stone and two Chinese colleagues – Jiang Li of the University of Science and Technology Beijing and Haosheng Chen of Tsinghua University – assessed the construction of the Forbidden City with an eye for both practical physics and known history. Their report was published on Monday in the Proceedings of the National Academy of Sciences.

While scholars know that the Chinese have had the wheel since around 1500 B.C., they also know that sledges were still used to transport large stones as late as the fifteenth century. There are also some brief mentions – including on a sign on the palace grounds itself -- of the stones of the Forbidden City being transported on roads of ice.

“That got us a little interested in why you would choose an ice path when there are other methods possible,” Stone said in a phone interview.

Stone and his colleagues calculated that an ice road provides just the right amount of ease of sliding for somewhere between 50 and 100 workers to drag 100-ton stones from one area to the other. Ice roads, they found, were ideal for transporting these heavy loads because they were naturally hard, flat and slippery.

“The wheel would have failed you, because you couldn’t make a wagon strong enough to support such a large rock,” Stone explains. “Even if you were making a wagon that was strong enough, the roads are pretty bumpy, and that would’ve broken the wagon. It was only hundreds of years after the Forbidden City was built that they were able to make stronger wagons with more wheels.”

The ideal ice road was one that had a thin film of lubricating water on top, the researchers found. And weather conditions in a Beijing winter would have been cold enough to create an ice path – using one of the many wells dug around the construction site – while still allowing a water film to sit atop the path.

And, Stone points out, this method is still used today. Last winter, in fact, workers in the northeastern Chinese province of Heilongjiang shifted the Anda Railway Station by about 200 meters on an ice road.

Ice roads, of course, aren’t the whole story to the construction of the Forbidden City. Once the giant stones arrived at their initial destination, they had to be moved and lifted into place, perhaps using some combination of raw manpower, ramps or lifting contraptions. 

We may never know exactly how they laid every stone of the Forbidden City, or the Great Pyramids, but the structures remain a testament to the feat.

“You know, someone said, you get enough people together, you can get a lot of things built,” Stone says.

SOURCE: Li et al. "Ice lubrication for moving heavy stones to the Forbidden City in 15th- and 16th-century China." Proceedings of the National Academy of Sciences, published online 4 November 2013.