What is the soil like? Sand? Clay? Loam?
I am not that familiar with Calexico, but I know Otay Mesa in San Diego where many tunnels have been found. The dirt is hard decomposed granite and limestone. Ideal for tunneling. The walls of the tunnel look like concrete and does not cave in. Calexico is probably the same.
BTW, El Chapo Guzman reportedly visited his daughter in Los Angeles County while he was on the lam. No hay problem