"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 that Congress fund his desire for an expanded southern border wall, little time has been spent determining how the project might actually come together. The US-Mexico border, photographed from Tijuana, Mexico. Credit:AP A project of this scale has rarely been attempted – not even by the developer-president himself when he was erecting New York skyscrapers. The border's landscape is uniquely remote and difficult. The project site is narrow and runs for hundreds and hundreds of kilometres. And there are unknowns, such as the maximum wind load for a fence.

Loading The wall design currently favoured by Trump appears to make heavy use of steel, which the President said would be good for the US steel industry. About 3 million tons of steel would be needed for a steel-slat wall and concrete base, according to Zarenski's calculations, which factored in 20-centimetre hollow steel tubes standing nine metres high and spaced every 35 centimetres. But the demand for that steel would not land all at once but be stretched over the project's life. If it took an optimistic 10 years to build, the wall would require less than half a per cent of the annual US appetite for finished steel. The wall "would have a very limited impact for US steelmakers," said Josh Spoores, a steel analyst with the research firm CRU.

Trump has made supporting the US steel industry one of his administration's focal points. Last year, his administration slapped tariffs on steel imported into the United States, obstensibly to help domestic steelmakers. But the steel tariff also has hurt US manufacturers who use steel and must contend with increased prices, said Jim Doyle, of the industry trade group Business Forward. Workers replace sections of the existing border wall in Tijuana, Mexico. Credit:AP "Trump is disrupting the auto industry and other manufacturers in order to help steel, a much smaller, less important industry," he said. The same dynamic would be seen in the steel used for a border wall. The steel tariffs add about $US1 billion to the estimated $US25 billion border wall project pricetag, according to Zarenski. The design of Trump's border wall could still change – and has already 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's a fence. He has described it stretching for 2000 miles (3210 kilometres) and 1000 miles (1609 kilometres) and even just 700 miles (1126 kilometres).

Loading 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 doesn't travel well. It has to be mixed close to a project site at either a permanent or temporary plant. "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 wouldn't 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.

Loading 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 labour market would also be difficult. And the project's massive price tag comes with its own constraints. The construction industry's rule of thumb, Zarenski said, is that it takes 5000 to 6000 workers a year to build $US1 billion worth of construction. But you can't 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. Zarenski calculated how fast the work could go - assuming 10,000 workers spread equally over 50 sites. Then, they could build 11 metres of border wall each work day at each site.