He was lucky.
Magnificent!
hh
Hey, Surbey, are you in jail by any chance ? 
It is especially depressing given that this type of accident is completely preventable in the first place. But workers often get overconfident based on statements like “I’ve been digging in this town for 20 years and I know when a trench is safe.” The OSHA regs cited above are quite clear, safe, and often very difficult to adhere to in real life situations, so despite the clarity they are often ignored.
But the easiest regulation to follow is the requirement to have a “competent person” on site who knows treching regulations. Following this easy requirement would prevent decapitation type problems.
But to the OP…
IF you are tunnelling directly under concrete in reasonable soil (sticks in a loose ball if you squeeze a handful) and you just want to squeeze body through and you will not have vehicle traffic on the concrete, then the concrete will make a nice roof and your 2’ x 2’ tunnel will not collapse too much from the sides. But you might still suffocate in the tunnel.