Forbidden City Constructed Using Road of Ice

By combining an ancient document and modern experiments, a team of US and Chinese researchers have developed the most detailed description yet of the process by which the builders of Beijing's Forbidden City transported rock slabs weighing hundreds of tons.

According to a 500-year old manuscript the team translated, the slabs were transported over a road of ice from a quarry 45 miles away on a sledge, despite the invention of wheeled vehicles 3,000 years prior.

"In the history of mechanical engineering, it is commonly believed that transporting huge stones with sliding sledges hauled by men did not occur in ancient China, because there were well-developed wheeled vehicles in China around 1500 B.C.," the researchers wrote in the Proceedings of the National Academy of Sciences. "However, some books contain brief remarks that the Large Stone Carving in the Forbidden City in Beijing was pulled to the site with a sledge sliding on an artificial ice path."

Ice roads posed several advantages, Princeton University engineer Howard Stone notes.

"The roads were pretty bumpy and rough," Stone told NPR, "and one thing ice does is give you a pretty smooth surface."

Though details are sparse, the document reports that a slab weighing about 120 tons was transported the almost 50 miles in 28 days, and suggests that wells were dug along the way in order to wet the road.

"If you didn't lubricate it with additional water then ... the object would have just frozen to the ground," Stone said, according to NBC.

However, by dousing it in water, those transporting the slabs could have greatly reduced the number of men needed to haul the heavy rocks.

"If you look at the frictional characteristics of ice for the rocks of this size, we estimate that 300 people were needed for this kind of dragging," he told the BBC, rather than thousands, which would have been tremendously unwieldy.

"The technique took full advantage of the natural properties of ice," the study states, "such as sufficient hardness, flatness, and low friction with a water film."