The Game of Thrones (the HBO series based on George R. R. Martin’s books, A Song of Fire and Ice) features a giant wall made of ice. Seven hundred feet high. It’s an imposing structure, but it has to be, in order to keep out the terrifying dead people who inhabit the north.
According to Martin, “You could see it from miles off, a pale blue line across the northern horizon, stretching away to the east and west and vanishing in the far distance, immense and unbroken. This is the end of the world, it seemed to say.”
Such an extraordinary structure couldn’t help but draw attention from our geotechnical engineers and staff, who responded to some of the quotes from the books.
“The wall is a hundred leagues long.”
A league was supposed to be the distance that a person could walk in one hour. An English league, once upon a time was about three miles long, which would make the wall three hundred miles long.
“The wall is 700 feet high.”
This is almost as tall as the 1201 Third Avenue Building in Seattle, a 55-story building, which coincidentally has beautiful blue coloring as well. Certainly it takes a lot of work to design and build a high-rise—imagine building so many adjacent high-rises that they would stretch for 300 miles. That’s never been done.
At a height of 700 feet and a unit weight of 57.4 pounds per cubic foot (pcf) for fresh water ice, the base contact pressure on the underlying soil/rock would be on the order of 40,000 pounds per square foot (psf). (Compare that to a high-rise on glacial till at 14,000 psf).
“The wall has stood for, what, eight thousand years?”
Assuming a coefficient of secondary compression, C-alpha, of 0.02 and assuming that the base upon which the wall is built is comprised of some reasonable thickness of compressible organic muskeg (say ten feet), and assuming the wall was built over a period of one hundered years, the wall will likely have settled about three to five feet under its own weight.
“The top wide enough for a dozen armored knights to ride abreast.”
How wide is an armored knight? Say five feet? 5 x 12 = 60 feet wide? To safely travel, there would need to be at least three feet between riders so the total width (including four feet on either side for shoulders and jersey barriers) would be 101 feet.
“The gaunt outlines of huge catapults and monstrous wooden cranes stood sentry up there, like the skeletons of great birds, and among them walked men as small as ants.”
To anchor the catapults and cranes, it is likely that the overturning and uplift forces on the catapults and cranes would control the design. The overturning forces associated with the action of the catapults and the wind loads on the structures (resulting from the unobstructed exposure to the predominant winds due to the height of the wall) could be resisted by using high-capacity drilled micropiles.
“It was older than the Seven Kingdoms and when he stood beneath it and looked up, it made Jon dizzy. He could feel the great weight of all that ice pressing down on him, as if it were about to topple, and somehow Jon knew that if it fell, the world fell with it.”
One of the Seattle-Tacoma International Airport Third Runway walls (135 feet tall) was built stepped in, in order to avoid this feeling when you stand at the base of it. However, typically, a tall wall looks shorter when looking up than when it does when looking down. Jon is a weenie.
“Eight hundred feet above the forest floor, a good third of that was earth and stone rather than ice.”
It seems that people got creative over the years, sometimes making use of on-site materials, a good practice to save cost, time, and the environment. It also makes sense, when you are building a structure with a contact bearing pressure equal to 40,000 psf, to do overexcavation and replacement with densely compacted (i.e., 95 percent of the maximum dry density, within two percent plus or minus of optimum moisture content, as determined by ASTM D1557 Test Procedure) well-graded sand and gravel with less than five percent passing the U.S. No. 200 sieve based on the minus three-quarter-inch fraction.
Have questions about the geotechnical design of other giant structures? Need a dragon or two? Contact Garry “the Hound” Horvitz.