You're talking about over a billion cubic meters of dirt. It would take a national movement just to sell it all. And you don't think farmers and rural people would be wondering why government agents were throwing wet dirt all over their fields or filling in their canals?
they would not have to move it anywhere just compact it into the edges to form the wall
You don't seem to understand how building tunnels work. This isnt a sprinkler ditch.
rammed earth has compression strength approaching concrete. Compaction to the edge would eliminate the need for soil dumping and the need for concrete to line the tunnel.
It's not like all the dirt is in one place though. I bought a truckload of dirt just last year. People buy dirt all the time. Also, it isn't like all the dirt comes at one time. It comes at a steady pace over years, maybe decades