

Donald Trump has issued an executive order calling for a 1,000-mile-long wall on the US-Mexican border. The order allows for six months to survey all 1,000 miles before the groundbreaking.

Smithsonian magazine consulted a panel of expert geologists on the logistics of such a wall and came away with the impression that Donald Trump has no idea what this involves, and will end up with an expensive, useless disaster.

Take surveying: field geophysicist Mika McKinnon has been working on a three-mile stretch of pipeline, which is now in its fifth year of surveying. The terrain that Trump wants to cover with his wall includes hydrophilic clay soil (which swells and moves, shattering foundations), sand, and regions where the bedrock is thousands of feet down.

A "megastructure" like a 1,000-mile wall is substantially different from a single dwelling or even a skyscraper: each section depends on the integrity of the adjacent one. Heterogenous soil conditions (including acidic soil that dissolves rebar and other materials) mean that each section has to be built differently, but must still adapt to soil and weather and seismic conditions in lockstep with its neighbor.

These are just the first-order difficulties with the wall. China's Great Wall took 2,000 years to build (and didn't keep the barbarians out).

Dirt can also eat up the wall's support system. Soils that are naturally acidic or have high chloride levels can rapidly degrade iron-rich metals, says McKinnon. These soils could "corrode any, say, nice big metal rebar that you're putting in there to stabilize your foundation," she says. Other soils have a high amount of sulfates, a compound found in the common mineral gypsum that breaks down both metals and concrete. Sulfate-rich soils are common in what's known as the Trans-Pecos soils along the border in the southwestern arm of Texas.

Upkeep of such a lengthy structure is challenging. And even if such a wall can be erected, the size of budget necessary to keep it standing remains unclear.

"You're going to encounter hundreds, if not thousands, of different types of soils along [such a lengthy] linear pathway," says Clendenin. (In fact, there are over 1,300 kinds of soil in Texas alone.) And many of those soils aren't going to be the right type to build on top of. At that point, would-be wall-builders have two options: Spend more time and money excavating the existing soils and replacing them with better dirt—or avoid the region altogether.

One thing they can't always avoid, though, are regions at risk of earthquakes and floods. Rivers run along a sizeable portion of the U.S.-Mexico border, which can create a very real danger of flood. Building adjacent to rivers can also present unexpected legal issues: A 1970 treaty necessitates that the fence be set back from the Rio Grande river, which delineates the Texas-Mexico border. Because of this, the current fence crosscuts Texas landowner's property and has gaps to allow landowners to pass.

[Maya Wei-Haas/Smithsonian]

(via Skepchick)