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Thursday, December 8, 2016

Update below.

There are very few occasions in American political discourse that require the input of a structural engineer, but when Donald Trump took a question from Univision’s Jorge Ramos regarding his proposed United States-Mexico border wall at a press conference on August 25, I heard the clarion call:

RAMOS: How are you going to build a 1,900-mile wall?
TRUMP: Very easy. I’m a builder. That’s easy. I build buildings that are — can I tell you what’s more complicated? What’s more complicated is building a building that’s 95 stories tall. Okay?

No. Donald Trump is not a builder. Donald Trump could not build a doghouse. Donald Trump is a developer who pays what he would call “very, very smart people” to build things on his behalf. His response to Ramos’ question was meant both to exaggerate his understanding of construction and to downplay the challenges posed by his border wall project.

Though I would never classify the construction of a 95-story building as simple, it is a feat that has been achieved many times before. There are at least 30 buildings that have reached a height of 95 stories or more, according to the obsessively detailed database at SkyscraperPage.com, and there are even more in the design phase or under construction.

On the other hand, human beings have built a 2,000-mile-long frontier wall exactly one time. Once. And it was accomplished only through a centuries-long building campaign that necessitated the forced labor of millions of Chinese peasants.

The challenge of Trump’s border wall is not technical, but logistical. The leap in complexity between “building a wall” and “building a 2,000-mile-long continuous border wall in the desert” is about equal to the gap between “killing a guy” and “waging a protracted land war.” Trump’s border wall, if built as he has described it, would be one of the largest civil works projects in the history of the country and would face an array of challenges not found when constructing 95-story skyscrapers.

In order to adequately answer Mr. Ramos’ question, let’s first make some assumptions on the project’s scope: A successful border wall must be effective, cheap, and easily maintained. It should be built from readily available materials and should take advantage of the capabilities of the existing labor force. The wall should reach about five feet underground to deter tunneling, and should terminate about 20 feet above grade to deter climbing.

wall_1
Elevation view of proposed border wall. (Click to enlarge)

To be classified as a “wall” rather than a “fence,” the barrier must also be a continuous, non-porous construction. This distinction might seem purely semantic, but Trump has made himself very clear on the matter, saying, “A wall is better than fencing, and it’s much more powerful. It’s more secure. It’s taller.” So we’ll take him at his word: He wants to build a wall.

One of the biggest choices that a builder has to make is what material to use for his or her project. For Trump’s wall, I would first dismiss concrete masonry unit (commonly called cinderblock) construction because each block would have to be put in place and set in mortar by hand. The finished product would probably be acceptable, but construction would be outrageously labor intensive and therefore costly.

Next, I would dismiss steel wire mesh. While it is cheap and readily available, it can be easily penetrated by a pair of wire cutters, an angle grinder, an oxy-acetylene torch, or just a Chevy going really fast. Even though extant barrier sections along the border make use of wire mesh, the United States Border Patrol is constantly battling to repair breaches and, as stated above, this kind of barrier really falls into the category of “fence.”

That leaves concrete. A concrete wall would meet all of the basic project requirements, and as a bonus would also embody the gray-faced antipathy of America’s immigration policy. There are two major types of concrete construction:

  • cast-in-place, where wet, plastic concrete is brought in trucks to a job site, cast into formwork, and then cured; and
  • pre-cast concrete, where the concrete is cast in a controlled indoor environment, cured, and then shipped to the construction site for assembly.

The hot, dry climate in the border regions would complicate cast-in-place construction because high heat tends to screw up the chemical reactions that cause concrete to harden.

I drew up a quick design option for a pre-cast concrete wall, not dissimilar to many proprietary systems currently on the market. This design consists of I-shaped concrete columns spaced at 10 feet on center, with eight-inch-thick wall panels spanning in between them. In such a design, the only concrete that would need to be cast on site would be for the foundations. The columns would anchored to the foundations, and the wall panels are slipped in place from above.

Click to enlarge
Section cuts of proposed border wall. (Click to enlarge)

If we assume a border wall length of 1,954 miles (there are 600 or so miles of existing border barrier, but much of this would not qualify for Trump’s wall), then we can make some estimates as to the volume of concrete needed for the project:

  • Foundation: 6 feet deep, 18 inch radius = 42.4 cubic feet
  • Column: 4 square feet area by 30 feet tall = 120 cubic feet
  • Wall panels: 25 feet tall by 10 feet long by 8 inches thick = 166.7 cubic feet
  • Total concrete per 10-foot segment = 329.1 cubic feet
  • 1,954 miles = 10,300,00 feet = 1,030,000 segments (10-feet long each)
  • 1,030,000 segments * 329.1 cubic feet per segment = 339,000,000 cubic feet = 12,555,000 cubic yards. (The cubic yard is the standard unit of measure of concrete volume in the United States.)

Twelve million, six hundred thousand cubic yards. In other words, this wall would contain over three times the amount of concrete used to build the Hoover Dam — a project that, unlike Trump’s wall, has qualitative, verifiable economic benefits.

Such a wall would be greater in volume than all six pyramids of the Giza Necropolis — and it is unlikely that a concrete slab in the town of Dead Dog Valley, Texas would inspire the same timeless sense of wonder.

That quantity of concrete could pave a one-lane road from New York to Los Angeles, going the long way around the Earth, which would probably be just as useful.

Concrete, of course, requires reinforcing steel (or rebar). A reasonable estimate for the amount of rebar would be about 3 percent of the total wall size, resulting in a steel volume of 10,190,000 cubic feet, or about 5 billion pounds. We could melt down 4 of our Nimitz-class aircraft carriers and would probably be a few cruisers short of having enough steel.

But the challenge is far greater than simply collecting the necessary raw materials. All of these hundreds of miles of wall would need to be cast in concrete facilities, probably project-specific ones that have been custom built near the border. Then, the pre-cast wall pieces would need to be shipped by truck through the inhospitable, often roadless desert.

The men and women doing the work of actually installing the wall would have to be provided with food, water, shelter, lavatory facilities, safety equipment, transportation, and medical care, and would sometimes be miles away from a population center of any size. Sure, some people would be willing to to do the work, but at what price? Would Trump hire Mexicans?

This analysis also ignores the less sexy aspects of large-scale engineering projects: surveying, land acquisition, environmental review, geological studies, maintenance, excavating for foundations, and so on. Theoretical President Trump may be able to executive-order his way through the laser grid of lawsuits that normally impede this kind of work, but he can’t ignore the physical realities of construction.

Trump’s border wall is not impossible, but it would certainly be a more challenging endeavor than he would ever lead you to believe. Maybe he should stick to 95-story buildings.

Update: In the Oct. 28 debate, Trump amended his wall proposal to a mere 1,000 miles. The author responds here.

Ali F. Rhuzkan is the pen name of a professional engineer and unprofessional writer living and working in New York City. The author can be reached at [email protected].

This post has been updated.

Photo: Gage Skidmore via Flickr

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