By Steven T. Dennis

Bloomberg News

WASHINGTON, D.C. — The Mexican border wall proposed by President Donald Trump wall must be at least 18 feet tall, resistant to sledgehammers and “aesthetically pleasing,” at least on the north-facing side, according to a request for contractors to bid to produce prototypes.

Trump’s plans to spend billions of dollars on the wall to run the 1,933-mile border with Mexico has already sparked a warning from Senate Democrats about a potential government shutdown, but the administration isn’t waiting around for Congress to act.

While the government’s initial concept is for a 30-foot wall, the request for proposals posted by Customs and Border Protection late Friday said “designs with heights of at least 18 feet may be acceptable.”

The designs — one for a reinforced concrete structure and a second that could use other materials — must be able to prevent people from climbing the wall unassisted, include features to prevent scaling via grappling hooks and other climbing aides, and prevent tunneling within 6 feet of the surface.

The wall must also be able to repel someone equipped with a “sledgehammer, car jack, pickax, chisel, battery operated impact tools, battery operated cutting tools, …. torch or other similar hand-held tools,” the government said in its request for bids. The concrete wall should be strong enough to delay a breach 12 inches in diameter by at least an hour, the alternative wall for at least 30 minutes.

And the north-facing side must be “aesthetically pleasing in color, anti-climb texture, etc., to be consistent with general surrounding environment.”

Initial concepts are due to the government by March 29, and contractors will first be asked to build 10 foot by 10 foot mockups of their prototypes in San Diego within two weeks of getting a notice to proceed. That’s a sign of just how quickly the Trump administration wants to act on one of the president’s signature campaign promises. A 30-foot prototype is expected within a month.

Trump has insisted that Mexico will pay for the structure. His budget proposal released this past week seeks $1.5 billion in 2017 and $2.6 billion for 2018, to cover border infrastructure and technology, including partial construction of the wall.

“Does that build the whole wall? No. It doesn’t. But it gives us a start on the program, and you see some of the wall being built this year,” Office of Management and Budget Director Mick Mulvaney said March 16. “Obviously we increase funding in 2018. But the wall will take longer than two years to build.”

Construction estimates have ranged from $12 billion to $21.6 billion. Mulvaney said March 6 that per-mile estimates for the wall run from $8 million to $25 million. “It just depends on, when you’re talking about across 2,000 miles or so, what you decide to build in what areas,” he said on the syndicated “Hugh Hewitt Show.”