…how worried do I have to be about a cave in?
You don’t have to worry about it at all; you could, for example, be excited about the idea.
What sort of tunnel in what sort of material and what method of digging?
I thought site like the SDMB were blocked in most prisons. 
lol. Ok, lets say that I was tunneling to safety. From one point to the other. I would be about 5-10 feet below the ground and I’m not sure what kind of weight I’d be worrying about above me. It wouldn’t be too large. Just enough for me to get through. And I’m not in prison (or atleast not yet :D)
oh and it would mostly be in dirt, once I got past the level on cement. ( but it doesn’t necessariy have to be in ground below cement, just a for instance) And I would mostly be doing the work with a shovel and alot of elbow grease.
It depends greatly on the soil conditions but generally speaking I’d be pretty damn nervous about crawling into it.
About once a year I see on the news somebody that dies from the walls of a hole collapsing on them (let alone a tunnel) and even if 1/4 or so of their body is above ground they sometimes die from the injuries.
I don’t know what type of shoring they use in small hand dug tunnels but I’d be an expert on it before I crawled into one.
The others are quite often killed (usually decapitated) by their would-be rescuers.
Back to the OP; yes - if you’re digging through any kind of soil, you’d need to line the tunnel with boards and props to prevent a very probably fatal cave-in. Solid clay might have less tendency than, say, sand to cave in (the Great Escape tunnels were dug through soil that was almost pure loose sand), but even so, there’s no way you’d get me into an unshored soil tunnel.
If you’re digging through solid chalk, then you might get away without any shoring; if it’s limestone or harder, you’d probably be OK as long as it’s solid material, not loose blocks.
They recently put in sewer lines down my street. One of the trenches they dug from the main line over to one of the houses collapsed, burying one of the workers up to his chest. And this was just a trench, not a tunnel.
So yeah, be worried.
The OP should note that in the US, in some States and jurisdictions, it may not be legal to dig a tunnel below a certain depth and past a certain length without the plans for said project approved and stamped by a licensed Professional Engineer who can legally practice in said State.
duely noted
And if you can get a licensed engineer to sign off on a homemade tunnel of any length you’re a wonder.
In our town, when they do sewer work and dig those large trenches, they lower into place a sort of temporary wall system that is essentially two large, thick walls, connected and braced by four strong horizontal columns. We’re near Lake Michigan and the ground is undoubtedly sandy, but they don’t take any chances of a cave in, even if they’re only digging a stretch of 15 or 20 feet in length. I’ve never seen guys go down into those trenches to do sewer work without that protection. xo, C.
Care to elaborate?
Let’s just say that if you’re buried under a bunch of dirt, it is expediant, but risky, to be dug out with a backhoe.
Yes, and another unrecognized problem with being buried in loose or sandy soil is that it acts sort of like a constrictor. I.e. when you breath out it settles in around your chest making it very difficult to breathe in.
You’d have to be nuts to dig a tunnel without shoring. Even if the soil feels firm, ther’es tons of ways for it to fail. In engineering geotech design, we use a factor of safety of three or greater because there are so many unknowns with soil behavior.
For fun, here’s the OSAH info on trench shoring with an illustration of methods of failure. I know in Texas they take trench shoring so seriously, it’s required to be it’s own seperate item on the scope so the contractor can’t cheap out. Can’t remember if that’s a national requirement or a state one tho’.
Don’t forget to also build an air-duct system out of old “Klim” tins.
http://www.virtualmuseum.ca/Exhibitions/Militaris/eng/air/people/pplpopup/d.1.3/l_rafm2d.jpg
And, of course, you already have your forged papers ready, right?
This account is despressingly typical of trench cave-in rescues. If you’re in a trench and it collapses, it’s most likely you’ll suffocate, but if your rescuers have the wherewithal to dig fast enough to reach you before you suffocate, it’s quite likely that they will not be employing care and finesse and will accidentally kill you in their haste; if you were in anything like an upright position when the cave-in occurred, this death will most likely take the form of decapitation.