Mexico border wall: What is Donald Trump planning, how much will it cost and who will pay for it?

Donald Trump is setting out to fulfil one of his key campaign promises, with an executive order to build his proposed wall along the US-Mexico border.

To restrict illegal immigration, Mr Trump has promised to build a wall on the southern US border and to deport illegal migrants living inside the United States.

On his personal Twitter account on Tuesday, Mr Trump tweeted: "Big day planned on NATIONAL SECURITY tomorrow. Among many other things, we will build the wall!"

Here's everything you need to know about the planned wall.

When did he propose building the wall?

Mr Trump's presidential campaign got off to an explosive start in June 2015 when he used incendiary language in announcing plans to build a wall along the border with Mexico.

"When Mexico sends its people, they're not sending their best," he said. "They're sending people that have lots of problems. ... They're bringing drugs. They're bringing crime. They're rapists."

In quotes | Donald Trump on Mexico
In quotes | Donald Trump on Mexico

Mr Trump pledged to build a huge wall along the border to keep migrants out. "I will build a great, great wall on our southern border, and I will have Mexico pay for that wall. Mark my words," he said.

The remarks drew a swift reaction from America's neighbour. "The remarks by Donald Trump seem prejudicial and absurd," said Miguel Angel Osorio Chong, Mexico's interior minister.

Despite the outcry over the remarks, Mr Trump's proposal resonated with a large block of voters and it became a pillar of his presidential campaign. 

How long will it be?

The border with Mexico is roughly 1,900 miles long and spans fours state: California, Arizona, New Mexico and Texas. In comparison, the Berlin Wall was 96 miles while the Great Wall of China is 13,000 miles long. 

There is already roughly 700 miles of fence along the border - the remainder is either open, nearly impossible to actually build on or impassable. Mr Trump said in 2015:

"You know, the Great Wall of China, built a long time ago, is 13,000 miles. I mean, you're talking about big stuff. We're talking about peanuts, by comparison, to that."

Mr Trump has said his wall will cover 1,000 miles, with natural obstacles protecting the rest of the border.

How much will it cost?

Estimates vary hugely. In February last year, Mr Trump told MSNBC he could finish the wall for $8 billion. “Of the 2,000, we don’t need 2,000, we need 1,000 because we have natural barriers … and I’m taking it price per square foot and a price per square, you know, per mile,” he said .  

But most other estimates are far higher.

The existing border fence cost about $2.4 billion. Building the rest of it would cost between $15 billion and $25 billion, with an annual maintenance cost of $700 million, according to an estimate by Marc Rosenblum, the deputy director of the US Immigration Policy Program at the nonpartisan Migration Policy Institute, who is cited in a report in January last year.

"It's a lot more expensive than we expected when we started, and it was much more difficult," said Ronald Vitiello, deputy chief of border patrol for the US Customs and Border Protection, at a Senate Committee hearing in May 2015.

Who will pay for it?

The Mexicans, according to Mr Trump. No chance, says Mexico.

During his campaign, Mr Trump boasted he would “make Mexico pay” for the wall by seizing remittances from undocumented immigrants from the country and increasing fees on entry visas to the US for Mexican citizens.

“It’s realistic if you know something about the art of negotiating. If you have a bunch of clowns negotiating, it’s not realistic,” he said.

After the Republican candidate met President Enrique Pena Nieto in September, he said the pair has not discussed the financial issue - a claim swiftly contradicted by the Mexican leader. "At the start of the conversation with Donald Trump I made it clear that Mexico will not pay for the wall," Mr Pena said at the time.

Mr Trump angered the Mexican leader hours later when, delivering an immigration speech at a rally, he declared: "We will build a great wall along the Southern border and Mexico will pay for the wall. One hundred percent. They don't know it yet but they're going to pay for it."

Earlier this month, Mr Trump said the building project would initially be paid for with a congressionally approved spending bill - as early as April - and Mexico will eventually reimburse the US, though he has not specified how he would guarantee payments.

Mr Trump will meet with Mexican President Enrique Pena Nieto at the White House next week. 

Does he need approval from Congress?

No. 

In claiming authority to build a wall, Mr Trump is expected to rely on a 2006 law that authorised several hundred miles of fencing along the frontier. That bill led to the construction of about 700 miles of various kinds of fencing designed to block both vehicles and pedestrians.

The Secure Fence Act was signed by then-President George W. Bush and the majority of the fencing in was built before he left office. The last remnants were completed after President Barack Obama took office in 2009.

However, the Trump administration must adhere to a decades-old border treaty with Mexico that limits where and how structures can be built along the border.

The 1970 treaty requires that structures cannot disrupt the flow of the rivers, which define the US-Mexican border along Texas and 24 miles in Arizona, according to The International Boundary and Water Commission, a joint US-Mexican agency that administers the treaty.

Is it actually a wall?

"A wall  is better than fencing and it's much more powerful. It’s more secure. It’s taller," Mr Trump said, as he described the barrier he had in mind.

The plans have evolved slightly since then. He admitted after his election in November that it may in fact be part wall and part fence. “I’m very good at this, it’s called construction,” he said at the time. 

In an article for National Memo, Ali F. Rhuzkan, a New York-based structural engineer, said concrete would be the obvious choice for construction. He said the slabs would need to be made nearby - likely in dedicated plants - and transported to building sites.

A section of border fence - Credit: Getty
A section of border fence Credit: Getty

Mr Trump has said it building the "wall is easy, and it can be done inexpensively".

Others are more sceptical. “It’s extremely challenging to put a brick-and-mortar wall along the Southwest border for any number of reasons,” Richard Stana, who wrote multiple reports on border security for the nonpartisan Government Accountability Office before retiring in 2011, told the Washington Post. “It seems very simplistic.”

How effective would it be?

Most experts say it would be nearly impossible to make the barrier truly impenetrable. “Every wall can be circumvented. People can go under it, they can go over it. . . . No one should go into this with the idea that if you just build the right kind of wall, no one will get through," Thad Bingel, a former senior US Customs and Border Protection official, told the Washington Post.

The Department of Homeland Security is already spending millions of dollars a year to maintain existing fences, while drug smugglers are increasingly using tunnels to pass underneath.

Mr Trump, however, has said a wall “would be very effective” in deterring illegal migrants and seismic and other equipment could prevent any tunnels. 

The idea of such a wall has been tried elsewhere, with more than 45 such walls and border fences worldwide, perhaps most prominently Israel’s West Bank barrier.