New revelations come almost by the minute in the Trump-Russia affair. The White House moves into full-defense mode. The Trump agenda stalls on Capitol Hill.

A reasonable observer might conclude that is all that is happening in the Trump administration. But even as those troubles fill news sites and cable TV, administration officials are quietly moving ahead on one of the president's top campaign promises: the construction of a wall on the U.S.-Mexico border. Although it hasn't received much attention relative to the president's many problems, extensive planning for the wall is under way, officials are evaluating specific proposals, sites are being studied, and yes, there is money available to get going.

The work is being done under President Trump's executive order of Jan. 25, which declared the administration's policy to "secure the southern border of the United States through the immediate construction of a physical wall …" The order went on to set a high standard of effectiveness: "the prevention of all unlawful entries into the United States" along the border. Finally, the order cited an existing law, the Secure Fence Act, which in 2006 called for the construction of "at least two layers of reinforced fencing" and "additional physical barriers" on up to 700 miles of the 1,954-mile border.

"The executive order calls on the authority in the Secure Fence Act for us to begin immediately," said a senior administration official who recently provided an extensive update on the state of the wall project. In March, U.S. Customs and Border Protection sent out a request for proposals for companies to bid on the construction of prototypes — not little models to sit on someone's desk, but full-scale sections of proposed wall designs that will be put in place on the border. So far, Border Protection has received more than 100 proposals.

"We are evaluating what started out as a solicitation to industry and request for proposals — 18 to 30 feet high, concrete, impenetrable, hard-to-scale, the correct aesthetics," the official said. "We've tried to capture the intent [of the executive order] in the requests for proposals, and those proposals are being evaluated now."

There are some important points to remember before going any further. First, there is no intention to build a wall to stretch the entire border, from San Diego, Calif., to Brownsville, Texas. In his campaign, the president made clear that the wall need not cover every mile of the border. Certainly, no expert who supports more barriers at the border believes it should, either.

And the wall does not always mean a wall. The Jan. 25 executive order defined "wall" as "a contiguous, physical wall or other similarly secure, contiguous and impassable physical barrier." Planners say that in practice, that will certainly mean extensive areas with an actual wall. But other areas might have the type of fencing outlined in the Secure Fence Act, or some other barrier yet to be designed.

And that leads to a third point: The border barrier will not look the same at all points along the border. The terrain of the border is different — some parts are so imposing they don't need a barrier at all — and officials plan to design walls and barriers that fit each area, rather than one long, unchanging structure.

Right now, officials are studying how many "buildable miles" will need a barrier. Whatever the precise number, it will be big. In 2015, the Department of Homeland Security told Congress that, of the 1,954 miles of border, 1,300 miles, or 66.5 percent, have no fencing or barriers at all; 299.8 miles, or 15.3 percent, have vehicle fence; and 316.6 miles, or 16.2 percent, have pedestrian fence. Only 36.3 miles, or 2 percent, have the kind of double-layer fencing required by the Secure Fence Act. (The law was passed by Congress and signed by President George W. Bush, but neither Bush nor Congress really wanted to build the fence. So they didn't.)

"We've asked the nine sectors on the Southwest border, if you have to meet the standards in the executive order and the Secure Fence Act, where is it that barriers are required to complete the task?" said the senior administration official. "We've then evaluated those areas where the traffic [of illegal border-crossers] is highest." Planners are considering those factors in light of the executive order's "prevent all entries" standard — administration officials are taking that edict very seriously — to come up with areas in which a wall would be the best solution, or where some other type of physical barrier would do the job better.

At the moment, planners believe that about 700 "buildable miles" of the border will require a wall or other barrier. That just happens to be about the same amount called for in the Secure Fence Act.

Does the government have that much land available? The answer is mostly yes. Remember, from the numbers cited above, that there are more than 650 miles along the border with something on them — vehicle fence, simple pedestrian fence, whatever. That means the government has already gone through the land acquisition and approval process required to erect a barrier. "It's federal property now because we've either condemned it or purchased it," said the official.

There's no doubt that hundreds of miles of truly impenetrable barriers would have a huge effect on illegal border crossings. Talk to some experts who favor tougher border enforcement, and they will say that even as few as 100 well-chosen miles of barrier would make a difference.

In any event, there is a significant amount of border land that is already in government hands. "West of El Paso, a lot of the land is public," the official noted, while "as you go further east from El Paso towards Brownsville, a lot of that land is private." Going through the process of condemning or buying land — with all the legal and financial issues involved — will depend "on how we choose the priorities."

Once planners decide where to build, there will then be the question of what to build. If the decision is to build a wall, then the question is: a wall of what? Planners have decided that concrete will definitely be involved, even though it hasn't played much of a role in earlier barriers. Why concrete? "It's an interpretation of the vision," the senior administration official explained. By "vision," he meant it is a way to make Trump's oft-repeated promise of a "big, beautiful wall" a reality. Trump didn't mean a fence.

On the other hand, using concrete presents one obvious problem. Whatever barrier is built, Border Protection agents on the U.S. side need to be able to see through it. That's always been a requirement with earlier barriers. So now, officials are looking for creative ideas for a wall that will still allow them full sight of the Mexican side.

That touches on the most important consideration for planners. A wall isn't just a wall. It is a system — a "smart wall," as they call it. It involves building a barrier with the monitoring technology to allow U.S. officials to be aware of people approaching; to be able to track them at all times; to have roads to move people around; the facilities to deal with the people who are apprehended; and more. "It's not just a barrier," noted the official. (Last year, with the Obama administration still in office, a number of Border Protection officials traveled to Israel to study that country's highly effective barriers; they came home big believers in a smart wall.)

At this point, it's impossible to say what building a smart wall will cost, because officials haven't yet decided on a plan. But how much money does the administration have to get started now? Begin with money that was already to available to the Department of Homeland Security.

"Congress gave us a re-programming for money we were planning to do other things with — mostly technology — to get us through this request for proposals and to get the prototypes underway immediately," the senior administration official said. "That has happened already. We found $20 million to get that effort underway."

"Then, the 2017 budget resolution gives us substantial money to continue doing real estate and environmental planning and design, and then replace some fencing," he continued. "That's in the neighborhood of $900 million."

"You won't get a lot of new fence for that," the official conceded. "You'll get some upgrades. But you'll get some behind-the-scenes work underway — engineering, design, real estate acquisition, title searches, the kinds of things that have to happen to make it work."

That is a start. Republicans on the Hill argue that they got as much money in the recent spending bill as they could for the project, given that they had to work with Democrats to avoid a government shutdown and fund the government through Sept. 30. "We weren't going to get anything passed that said, quote-unquote, 'wall,'" noted one GOP staffer.

The next funding hurdle will come when Congress considers spending for 2018. Most House and Senate Democrats appear determined to stop a border barrier. They say it will be expensive and ineffective, while some Republicans believe Democrats oppose the wall mainly because they fear it will work.

After the recent spending bill passed, some opponents of the wall declared the project dead. (Sample headline: Vanity Fair's "How Trump's Wall Failure Will Forever Doom His Presidency.") But any victory dance right now is premature. Yes, it's certainly possible the wall won't be built. But it's also possible it will be built, or that significant parts of it will be built. The work is already under way.