This generally falls under GM purview, because there are technically no rules for digging your own tunnels. (Should be, especially if mining and other such similar concepts are meant to occur in these games, otherwise the rules fall flat upon themselves and create impossible paradoxii.)
Per RAW, only trained individuals could properly construct and dig a tunnel (which somewhat makes sense), meaning your fellow party members couldn't realistically help you.
Per RAI and/or a permissive GM, I would allow your party members to aid you in a Craft check (even while untrained) to dig a tunnel, with results based on your check.
What those results would entail is up to GM FIAT, which I guess I can give you my personal estimate on what that entails, but your GM would ultimately decide how that works.
In my opinion, I would probably use a more realistic approach as to how far you would progress each day. That is, you need the appropriate tools (such as mining picks and shovels) for you to get any sort of reasonable result. I guess you could try digging with axes or your bare hands, but that will get very little results, and also serve as a potential hazard. Specialized tools (such as Adamantine Picks and Shovels) will certainly yield more results (at least double the original progress). As to how far a single person could get in 8 hours worth of basic tunnel work, I don't know for sure, and largely depends on how stable and structured you want the tunnels to be, as well as how good your check is, and whatever anecdotal knowledge I have. Assuming no support beams or anything like that (like in typical mineshafts to prevent cave-ins), I could see it being in the neighborhood of 40 feet. If you want to add in support beams, that would likewise improve the difficulty of the check (if not make it require a higher tier proficiency to even try), as well as reduce the progress of your check on average (cut it down to 20 feet for simplicity purposes, though it might even be 10 feet with how much added work that is). I probably wouldn't require a support beam for every check, but for maybe every other check, since having too much distance between support beams can be problematic, and honestly, every 40 feet seems like it's a little too much distance for support beams.
The support beams, assuming they are made to make a steady and safe tunnel, also requires its own set of materials (such as wood for the beams themselves, along with basic metal and steel combined with a forge to help shape the metal to the proper shapes you need them to be). This is also important if you would like to create doorways and other similar structures.
Well I think the biggest X factor you (or your GM in this case) need to decide on is the level of the task. That sets the DC, and then you would roll to see if you succeed at it. A tunnel like this might take more than 4 days though, I dunno.
I'd maybe run it on a fail forward model, where you still make some progress on a failure but it is very slow going. A critical failure would trigger a cave in and there'd be some consequences to that.
Well fundamentally, to use the Practicing a Trade, or Crafting Progress parts of second edition rules, the GM would need to decide on how expensive the necessary tunnel would be. You'd technically also then need to pay half that in raw materials, for a pure crafting progress.
Although technically the Practice a trade doesn't have an untrained category, it isn't a horrible stretch to potentially consider the Level 0 job something that one might be able to do untrained, under the supervision of someone who is. That means that you could potentially justify a 5cp per day increase in progress per laborer helping with the work, up to as many as you decide the team lead can supervise successfully getting their own work done. Potentially, instead of doing their own progress, you might allow one or more of them to do Aid action. That aid action could be used to help insure you get a good roll on your craft roll or practice a trade roll. If you base it on Practice a Trade, knowing you have a good chance of getting a boost from the aid, might allow them to try to make a higher level Job roll, to try and make faster progress. [and practice a trade does't require the up-front materials requirement that crafting does]
If you have them make untrained practice a trade rolls to make 5cp a day for an arbitrary time such as a week. If they fail, they only contribute 1cp progress, and might have caused a problem. Have the supervisor have to make an extra crafting check successfully, or the worker creates an issue, a cave in, or other problem that would set back the progress, or create an scene that might have to be played out. [chipping into a pocket that has something in it that is dangerous, or have the cave in do a certain amount of damage to everyone working with some save for half damage, a couple examples]
Something to keep in mind is that technically, to craft something other than a purely common item listed in the equipment chapter, you are supposed to have a Formula to follow at a certain designated level. [Level x: tunnel; for instance]
But the object is to make the game fun, so if it makes sense for them to be able to start tunneling, then let them. However, it isn't unreasonable to consider that things like support beams and such might be materials they might need to be able to make in the progress, so not all of the work may be 'digging'.