Deliberately making a road icy sounds like a recipe for disaster, but in 16th-century China it helped things run smoothly. Repairs to the Forbidden City in Beijing in 1557 called for huge stones from a quarry 70km away. The biggest was the 300-ton “Large Stone Carving.” Such stones were too heavy for any cart, and too fragile for rollers. The builders adopted an ingenious alternative approach.
They dug a series of wells, spaced a few hundred metres apart, along the route to the quarry. Then, in the depth of winter, when temperatures reached around -4C, buckets of water were poured on to the dirt track, transforming it into an ice road.
The stone blocks were hauled along the road on wooden sledges. Modern engineers have calculated that it would take 1,500 workers to drag a sledge on the dirt road, but only 300 on ice. Ancient texts suggested the ice was made slippery with more water; this reduced the friction further and just 50 men could pull a sledge. This technique only works when the temperature is close to zero, otherwise the film of water freezes too quickly.
The researchers at Princeton University estimated that the blocks could be hauled at six metres a minute, and the journey could be completed in 28 days. This would be well before the spring thaw.
It was once suggested that similar ice-sledges transported Stonehenge stones, but the porous ground and comparatively mild conditions probably rule this out.
