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WASHINGTON — It should be tall, climb-resistant, difficult to penetrate and blend into the landscape.

Those are among the requirements for the US-Mexico border wall released by the federal government late Friday, kicking off a bidding process that will lead to one of the largest government construction projects in US history.

“The wall design shall be physically imposing in height,” Customs and Border Protection outlined in the notice for contractors. That means 30 feet tall, although the officials wrote that “designs with heights of at least 18 feet may be acceptable.”

The wall must also be impossible to climb without a ladder, and should make it difficult to use “common and more sophisticated climbing aids,” like grappling hooks.

In addition, the wall must resist attempts to penetrate through or under it. The request specifically mentions it must successfully endure for at least 30 minutes — but ideally more than four hours — attempts to bore through it with a “sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools.”

Among the requirements is that the wall be “aesthetically pleasing in color” and blend into or match the surrounding landscape on the US side. It does not address the aesthetics of the Mexican side.

After initially saying the wall should be made of concrete, the government widened its call and said it would consider “other designs.” On Friday, it suggested non-concrete designs “may not be entirely solid” and could feature “a see-through component” so the border patrol would have “situational awareness” of the Mexican side.

The specifications leave nearly all of the design work to interested bidders, who now have about two weeks to develop and submit their plans, known as proposals. A federal contracting expert told CNN this approach is common when the government knows generally what it wants, but is interested in seeing different ideas to accomplish the goals.

It will select from those proposals several 30-foot-long prototypes to be built. Each prototype is expected to cost between $200,000 and $500,000, the government notice said. Companies will also build smaller 10-foot models for penetration tests.

The prototypes will be used to decide upon a design for the wall that ultimately is constructed along the border.

The wall — and having Mexico pay for it — was a central promise President Donald Trump made on the campaign trail. The government’s proposal reflects many of his campaign promises, including that the wall be tall and “beautiful.” It also includes a provision that contractors use American materials whenever possible.

Friday’s release did not address the overall cost of the wall. Various estimates have put the project at between $10 billion and $25 billion. Trump’s recent budget proposal requested $1 billion to begin the process.

In recent weeks, hundreds of companies indicated interest in the project. One construction contracting expert told CNN, however, that there are only a handful of construction firms with the size, capability and experience to handle such a large-scale contract. The expert pointed to federal requirements that contractors set aside substantial amounts of money as a guarantee that the project is completed satisfactorily.

As is standard with government contracts, a secretive selection committee will evaluate the proposals based on the merits of the design and the contractors’ ability to complete the job. The committee will select up to 20 plans to move to the next phase, where contractors detail how much their plans will cost and answer other questions.

Multiple contracts to build wall prototypes will then be issued, the government has said. It hinted that bidding may be reopened in the future, saying the current process “is not intended as the vehicle for the procurement of the total wall solution for the border with Mexico.”

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