You see how the west porch is narrower than the main part of the house? ...that's where I intend to pour a 15 x 15 foot slab of concrete, plus the narrow area to the right between the shop & house will be concrete too to carry rainfall away.
The camera is pointing south. The tunnel will be northward of west porch, starting in the middle of porch. Tunnel depth will be 7' under concrete and then elevate at 12 degree incline to ground level.
I intend to drill 6" diameter holes 2' from main house foundation, two rows in parallel, spaced 4' apart north of porch. Then I'll insert 4" x 4" x 10' PWS posts in the holes to help support the cement & give me something to screw PWS sheet to to crib the vertical walls. I'll place 6" square 1/2" plate steel on top of posts, plus lots of rebar in the concrete, plus drill holes into house foundation to insert rebar... that should be one solid mass, with 80% of concrete resting on undisturbed ground.
I think the reach of the JD850 is about 30" vertical, so the tunnel will be that height till I get all the way across. Then cut the floor down another +4'.
Before I pour cement slab, I'll lay three 1" plastic pipe the length of tunnel & drill 1/4" holes every 12" for the length of each pipe, so I can add water here to soften up the ground to dig it out... for every hole I reach while digging inward, just tape it off so I can keep adding water to keep the ground moist for easier digging.
Once the concrete work is completed & cured, then the lean to structure will be put up. Then get the spray insulation guy to do his thing and lastly any electrical necessary... with a little luck, it should all be done before next winter.
Where I'm standing with the camera (north east corner of shop), I intend to put a hot tub in this corner! It'll be out of the way & out of the elements... and I hope it's the cure to my lower back pain that has been getting worse & worse the last 2yrs.