Digging a tunnel? (CEB and other eng questions)

Hi,
Ok so first off, this started off after I found out how much a few truck loads of dirt would cost me, which I wanted to use to level parts of my yard out a bit. Then it turned into a well, it'd be nice to have a cellar and now is somewhere between that and "reliving childhood project" where we dug a hole down then tunneled about 6 ft (insanely dangerous in retrospect as we didnt use supports or anything). This is a fun-when-I-have-time project that doesnt have a "must-be-done-in-X-mos" time frame, could be years - just when i have time and feel like it. So in short, not a I am making a house to live in forever project, half fun half see-what-happens.

That said, it would be nice if it worked out and wasnt a death trap so I'd like to do it right. While I am not looking to spend much money on this, if it looks like its going somewhere then I might put a bit of money into it (like for cement?). First a bit of background i guess:

I am in the NC pediment so we have red clay up top and some sort of yellow-orange clay once you go down 7ft or so. It has some interesting properties, not the easiest to dig through (by hand) but while it mixes together when well like clay it dries into something more brittle, and drains pretty fast for clay - its like clay mixed with sandstone? (I am sure thats not it but it does have those properties). I will try to post some videos later if that would help.
My yard slopes, not super steep but probably about 6 feet drop over a distance of maybe 100ft?
I am no engineer but it would be nice if it could be to code(ish?)
The brittle clay i was refering to *seems* like it would make good CEB, from what I have read you shouldnt use just clay and sand should be mixed in but what if the clay was already kinda sandy? From what I understand plain CEB should not touch the ground but if it was mixed with a bit of Portland cement then maybe?
I have access to quite a bit of untreated wood (2x4s mostly) thanks to some of the housing divisions around (ugh, but hey, they are there so have had huge success dumpster diving for wood) so was thinking about using those as *temporary* supports?
I have actually started the hole at the end of my yard, bottom of the hill, about 9 feet down and started (like 1ft) digging horizontally, and have stopped until i get some clarity on temp supports then more permanent options.

For the tunnel, what I was thinking was building supports so that i, in effect, am never working with unsupported ground over my head so would dig a foot or two then support it and so on. Obviously untreated, salvaged 2x4s wont last long which is where I was wondering how to go. Engineering question. If i make supports, do they literally have to contact with the ground or if i have a tunnel in a tunnel, like a 5ft wide 7ft tall tunnel dug, with a 4.5x6.5 wooden tunnel in that would that protect me if there was some sort of collapse? Pretty much every mine shaft i see has supports that are up against the ground/sides of the tunnel. The reason i ask is if I built a tunnel in a tunnel, then put cement and rebar (not sure how I'd get the rebar in there) would 6 inches of wall/ceiling be safe? For getting the cement in there (keep in mind, I am fine with years and literally going a foot or two a week - fun project, no rush) I was working out how to either "blow" cement in as i go, every few feet or literally put some pipes from the surface down to the space between the earth and the wood tunnel then pouring cement in (seems to have kinda worked for Colin Furze, minus the hundreds of thousands of dollars he likely had).

Another cooler thought was to make a CEB tunnel (in the tunnel) then fill in with cement... or is portland cement with clay an option?

Anyway, I have started reading some other posts and about wofati which are interesting but the vast majority are about excavating the entire area, then building a structure, then covering it in dirt which is just not what I am out to do.

Thanks in advance.

I am a Civil Engineer in Australia.
And I have worked with tunnelling and observed many things.
- There are 2 types of tunnellers, those that come out and those that dont.
- collapses occur only in good ground
- shafts need to be timbered all the way down
- tunnels always need support against the soil the way it is supported depends on the tunnel size
- tunnel boring machines move and place support about every 3 -4 ft depending on the ground.
- soil loads are immense.

There is a topic about tunnelling on Permies.
You mention you dont want to spend a lot of money, well that idea will need to change.
You will need reinforced concrete to make it safe.
I would recommend a cut and fill process it will be safer.

Thanks!!!
I was hoping an engineer would respond
A few questions:
- you mention "timbering all the way down", that was the plan, would 2x4s be sufficient? (you say tunneling machines do every 3-4 feet, I was thinking I'd need to literally have the entire length supported - that is, not every 3-4 feet but side by side? overkill?) Do they have to abut up against the ground or can I leave space for "filling in" with cement and wire?
- "tunnels always need support against the soil the way it is supported depends on the tunnel size" and "soil loads are immense" so if I had something that was 7ft tall x 5ft wide and then had 6in of reinforced concrete should that be more or less about right?
- cut and fill, is that the method where one effectively digs a pit, builds the structure, then covers it back up?

As for the money, that was the preference, as i see it (admittedly not a well formed plan yet) the main expense would be cement and reinforcement (wire?). If I look at a driveway it would be a few $k and thats doable. The temp supporting wood i get is free and labor is, well mine so am not counting that. Are there other costs that you would forsee?

Thanks so much for the response!

btw, can you point me to the other tunneling topic on permies?

I had seen the Colin Furze videos, asides from almost gettnig a cramp i laughed so hard it seemed to have some good points, for me one take away was never digging  with unsupported dirt overhead which seemed like solid advice, though he had unlimited funding and was quite handy with a welder, I have neither. The CEB part of my question was essentially about would CEB be acceptable for building an arch "box" as I go? (likely cemented together, and some cement poured or blown into the area in between the arch and the ground?

In looking around some key bits I took away from the reading were:

"All you really need to dig a safe(ish) tunnel is a soil with enough stand-up time to get a liner in place (and a basic grasp of how to design an adequate liner). Then you dig, and line, dig and line. It takes forever, but it is also a lot of fun."

"If you still want to go under I would use standard tunneling techniques by excavating a small distance under, propping the slab and boxing the sides to prevent collapse.
eventually you may need to  pour a concrete support structure under there..
Then moving forward again."

Hi Reid;
Sounds like a fun project for your spare time.
Who knows in 10 years you may have a whole complex of tunnels and rooms secreted away!

Until then, while your getting started I believe this is the thread that John was referring to.

https://permies.com/t/171780/Building-cave-hillside

#1 Thank you for the "sounds like fun" part, I dont necessarily expect everyone to agree on what constitutes fun but I love this community - the idea that a bit of "work" can actually be fun seems to be totally normal here

#2 Thank you for the link!