No matter where you stand on Trump's wall, this is an interesting read. It helps understand the real world issues such a project would face. Trump doesn't have a magic wand that will just make a border wall appear out of nowhere. It also helps quantify the current $5bn price tag is most likely a minor down payment when given the overall scope and cost of such a project. Again, try to read this without a partisan, political slant. It'll help you come to grips with the challenges such a gigantic project entails, and perhaps provide a sobering analysis of what the realities are on the ground. It should also shed light on why so many people have wet feet cutting Trump a blank check on a massive project like this. Particularly since there are no specific plans or feasibility studies, it's just an idea in Trump's head with some hastility assembled prototypes.
This was originally posted on twitter, so I'll line-up the individual tweets for ease of reading.
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From Amy Patrick, an engineer, Wall Expert.   
I’m a licensed structural and civil engineer with a MS in structural  engineering from the top program in the nation and over a decade of  experience on high-performance projects...
...and particularly of cleaning up design disasters where the factors  weren’t properly accounted for, and I’m an adjunct professor of  structural analysis and design at UH-Downtown.
I have previously been deposed as an expert witness in matters regarding  proper construction of walls and the various factors associated  therein, and my testimony has passed Daubert....
Am I a wall expert? I am. I am literally a court-accepted expert on walls....
Structurally and civil engineering-wise, the border wall is not a  feasible project. Trump did not hire engineers to design the thing. He  solicited bids from contractors, not engineers....
This means it’s not been designed by professionals. It’s a disaster of numerous types waiting to happen.  What disasters?...
Off the top of my head... 1) It will mess with our ability to drain land in flash flooding.  Anything impeding the ability of water to get where it needs to go  (doesn’t matter if there are holes in the wall or whatever) is going to  dramatically increase the risk of flooding...
2) Messes with all kind of stuff ecologically. For all other projects,  we have to do an Environmental Site Assessment, which is arduous.  They’re either planning to circumvent all this,...
...or they haven’t accounted for it yet, because that’s part of the design process, and this thing hasn’t been designed.
3) The prototypes they came up with are nearly impossible to build or don’t actually do the job. This article explains more:  
https://www.google.com/amp/s/mobile.engineering.com/amp/17599.html …  And so on....
   The estimates provided for the cost are arrived at unreasonably. You can look for yourself at the two-year-old estimate that you see everyone citing.  
http://fronterasdesk.org/sites/default/files/field/docs/2016/07/Bernstein-%20The%20Trump%20Wall.pdf …
It does not account for rework, complexities beyond the prototype  design, factors to prevent flood and environmental hazard creation,  engineering redesign...
It’s going to be higher than $50bn. The contractors will hit the  government with near CONSTANT change orders. “Cost overrun” will be the  name of the game. It will not be completed in Trump’s lifetime....
I’m a structural forensicist, which means I’m called in when things go wrong. This is a project that WILL go wrong....
When projects go wrong, the original estimates are just *obliterated*....
And when that happens, good luck getting it fixed, because there aren’t  that many forensicists out there to right the ship, particularly not  that are willing to work on a border wall project—
.. a large quotient of us are immigrants, and besides, we can’t afford  to bid on jobs that are this political. We’re small firms, and we’re  already busy, and we don’t gamble our reputations on political  footballs....
So you’d end up with a revolving door of contractors making a giant,  uncoordinated muddle of things, and it’d generally be a mess. Good money  after bad. The GAO agrees with me....
And it won’t be effective. I could, right now, purchase a 32 foot  extension ladder and weld a cheap custom saddle for the top of the  proposed wall so that I can get over it...
   I don’t know who they talked to about the wall design and its efficacy, but it sure as heck wasn’t anybody with any engineering imagination.
Another thing: we are not far from the day where inexpensive drones will  be able to pick up and carry someone. This will happen in the next ten  years, and it’s folly to think that the coyotes who ferry people over  the border won’t purchase or create them.
They’re low enough, quiet enough, and small enough to quickly zip people  over any wall we could build undetected with our current monitoring  setup.
Let’s have border security, by all means, but let’s be smart about it.  It’s not effective. It’s NOT cheap. The returns will be diminishing as  technology advances, too. This is a ridiculous idea that will never be  successfully executed and, would be a monumental waste of money.  <>