Even the more modest $5.7 billion in wall funding Trump directly requested during a prime time Oval Office address Tuesday to address what he called “a growing humanitarian and security crisis” would take an army of 10,000 workers more than two years to build and yield only 230 miles of barrier, according to estimates.

And even at 1,000 miles long, the steel-slatted border wall would still be too small to be a boon for U.S. steelmakers.

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The full version of Trump’s envisioned border wall — featuring rarely tested heights cast over almost unimaginable distances — would cost at least $25 billion, said Ed Zarenski, who teaches construction estimation at Worcester Polytechnic Institute in Massachusetts. Zarenski spent 30 years figuring out project price tags for Gilbane, one of the nation’s largest construction firms.

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“I wouldn’t say it’s impossible. But you’ve got to factor in engineering considerations,” Zarenski said. “And then I would say, is the project realistic? Probably not.”

After almost two years of Trump demanding Congress fund his desire for an expanded southern border wall, little time has been spent determining how the project might actually come together. A project of this scale has rarely been attempted — not even by the developer-president himself when he was erecting New York skyscrapers.

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The border’s landscape is uniquely remote and difficult. The project site is narrow and runs for miles. And there are unknowns, such as the maximum wind load for a fence reaching about three stories high.

The wall design currently favored by Trump appears to make heavy use of steel — which the president said would be good for the U.S. steel industry. About 3 million tons of steel would be needed for 1,000 miles of steel-slat wall and concrete base, according to Zarenski’s calculations, which factored in 8-inch hollow steel tubes standing 30-feet high and spaced every 14 inches.

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But the demand for that steel would not land all at once. It would be stretched over the project’s life. If it took an optimistic 10 years to build, the wall would require less than half a percent of the annual U.S. appetite for finished steel.

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The wall “would have a very limited impact for U.S. steelmakers,” said Josh Spoores, a steel analyst with the research firm CRU.

Trump has made supporting the U.S. steel industry one of his administration’s focal points. Last year, his administration slapped tariffs on steel imported into the United States, ostensibly to help domestic steelmakers. But the steel tariff also has hurt U.S. manufacturers who use steel and must contend with increased prices, said Jim Doyle, of the industry trade group Business Forward.

“Trump is disrupting the auto industry and other manufacturers in order to help steel, a much smaller, less important industry,” Doyle said.

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The same dynamic would be seen in the steel used for a border wall. The steel tariffs add about $1 billion to the estimated $25 billion border-wall project price tag, according to Zarenski.

The design of Trump’s border wall could still change — and already has fluctuated with the political winds. During the 2016 campaign, Trump talked of a solid concrete border wall. Then it was steel slats. Sometimes he called it a wall, other times it is a fence. He has described it stretching for 2,000 miles and 1,000 miles and even just 700 miles.

Gary Winek, a professor and construction program director at Texas State University, recalled how excited the concrete industry had once been — back when the wall was going to be all concrete. Concrete is a fickle material that does not travel well. It has to be mixed close to a project site at either a permanent or temporary plant.

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“The logistics are not going to kill the project,” Winek said, “but it will make it challenging.”

If Trump’s border wall gets funding, construction would not begin for at least six months — and likely longer, Zarenski said.

Land along the border still needs to be acquired.

Soil and environmental studies need to be done.

The first thing a construction firm would build is not a wall but a road. A roadway running parallel to the border would be needed to allow the backhoes, dumptrucks and cement trucks to reach the remote construction sites.

Finding enough skilled workers in the current tight labor market also would be difficult.

And the project’s massive price tag comes with its own constraints. The construction industry’s rule of thumb, Zarenski said, is it takes 5,000 to 6,000 workers a year to build $1 billion worth of construction. But you cannot fit them all on one job site. For a project like the border wall, you would want to have dozens of different sites going at once.

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Zarenski calculated how fast the work could go — assuming 10,000 workers spread equally over 50 sites. Then, they could build 37 feet of border wall each workday at each site — about 1,850 feet each workday across all the sites.

Even if these huge crews broke ground today, they would finish just 86 miles of border wall by year’s end. By Election Day 2020, 161 miles of border wall would be done. It would take 11 years to reach 1,000 miles. And that is assuming 10,000 workers going all at once, five days a week.

Cut that to 5,000 workers and a 1,000-mile long wall would take more than 20 years, Zarenski said.

About 700 miles of the 2,000-mile-long U.S.-Mexico border already has some form of fence or wall.

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In October, the Trump administration touted the completion of about two miles of steel-slat and concrete border wall near Calexico, Calif., as the first section of the president’s hoped-for border wall. The project had been planned since 2009 and replaced an existing barrier. And it still took eight months from contract award to completion.

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It is also increasingly clear Mexico is not going to foot the bill for the border wall, despite Trump’s insistence Tuesday night that it would pay “indirectly” through a renegotiated trade pact.

But it might make more sense for Mexico, Spoores with CRU said, because if Mexico built the wall, “the steel would likely come from their mills.”