From Reddit, so take it for what it's worth, but the dude sounds like he knows what he's talking about.
Do you want to take a realistic look at the proposed wall? I can help. I am a construction estimator that specialized in reinforced concrete for 12 years. The wall promised on the campaign trail was to be 30’-0” tall above grade, and made of concrete and rebar. To prevent tunneling, the wall should extend at a minimum 10’-0” below grade. Now, this wall is not a retaining wall and will, for the most part, have even backfill on both sides. So I would recommend a thickness of 18” to prevent easily breaking through it. Let’s say the footing is 15’ wide and 24” thick. We won’t even worry about the rebar right now. The border with Mexico is over 1900 miles long. Lets say that due to existing natural deterrent elements like canyon walls, that we only need to do 75% of the border. 1900 miles x .75= 1,425 miles Typically when we talk about concrete wall construction, we speak in linear feet. Let’s convert it. 1,425 miles x 5,280 feet in a mile= 7,524,000 linear feet Say that the following things have already happened: -Court battles over eminent domain when we take people’s land to build the wall -Site engineering, architectural design, and structural design -Soil testing for a 7+ million foot long job site -Newly constructed roads that can handle fully loaded concrete trucks and rigs carrying excavators -Concrete batch plants that can produce the types of volumes we will be discussing in a paragraph or two So, it’s day one and we are ready to start our wall. Say that we do such a good job today, that we get an incredibly ambitious 500 (!) linear feet of the wall built in the first day. We excavated 10’ into the ground, we formed our footings, we set our reinforcing bars, we poured the footing, it fully set, we formed our walls with 40’ high aluminum forms backset to dead men at every 10’ or so to prevent blowouts, we set our reinforcing in the forms, we got the pump trucks to the site on our new roads and they pumped the concrete 30’ into the air to go in the top of the forms. The walls set too, and we stripped our forms and backfilled all the grade back to normal. Now, imagine we work every single day until the job is done. Every weekend, every holiday. We celebrate Christmas by pouring another 500 linear feet. We have perfect weather every day and we do not have a single problem that sets us back a day.Let’s see how long it takes us to finish this thing! 7,524,000 LF/500 LF per day= 15,048 days/365 days per year= 41.23 years. In absolute perfect, some would say inconceivable conditions, we would finish the wall in 2060.