It won't even keep out the people trying to get in illegally. 45% of the illegal population had legal Visas and chose not to leave when they were supposed to. I agree that the border should be monitored but a physical wall is probably not the best option except perhaps for some specific locations. Enforcing existing laws against those who would hire illegal immigrants would be much more effective. Then you just have to watch for the drug dealers and human traffickers. Ending the war on drugs would probably solve much of cross border drug issues as well.
Tunnels have been dug under the existing walls/fencing many times in the past. There are many ways to hide what you are doing including starting your tunnel from within a structure.
I repeat, those who are identified and can get visas are the least of our worries. And those tunnels were from walls across town borders, not the trackless desert that is proposed now. Minimal but smart acoustic sensors would reveal all tunneling anyway.
But I agree that the war on drugs (as well as on prostitution) is a motivating issue. It causes there to be a huge profit motive. I go further, and suggest that the State governments grow, manufacture, and sell at a cut-rate price, all drugs to any adult. They could sell it for pennies on the current dollar, and still make profits enough to fund drug rehab for those who want it. After the panic, revolution, starvation, and cannibalism settled down in those countries currently supplying our drugs, the States could decriminalize small home production and sell yearly licenses by lottery for reputable local suppliers. This prevents smuggling, bootlegging, and pushing. Eventually we could remove all drug laws with no disruption.
But that won't happen because democrats and republicans won't vote for a libertarian (no matter how bad their candidate is, as shown in 2016), and libertarians won't vote for anything but a "perfect" libertarian. Incrementalism is working marvelously against us, so I can't figure out why we don't get it to work for us.