Task impossible? Excavation for the new footings right next to the edge of existing slab 2

[enriko12]

Working under severe space constraints, and need to put a new footing and grade beam as close as possible to the edge of existing slab, which bears a 15' tall 2x6 wood frame/drywall partition. Nothing on the left hand side of the partition to be disturbed.
This looks like an impossible task to me, so I am calling for 48" space instead of 2" to prevent a cave-in and to have some room for the concrete forms and backfill. I thought will ask, just in case some of the gurus on here have their magical way?

 

[SlideRuleEra]

Do a small scale exactly the same thing that could be done on a large-scale project - drive light-weight sheeting with a post driver to create a cantilever retaining wall. Clearance could be more that 2" but much less than 48".
Probably not worth the cost just to gain an extra 2 or 3 feet.

http://www.SlideRuleEra.net

 

[r13]

I suggest use double angles as solider beam with wood lagging in between. For better result, fasten the angles to the existing slab that remains, and place the lagging as the excavation proceeds.

 

[enriko12]

My concern here is the sand. Unless, something is driven into the ground before the excavation, it will all come out and leave the partition wall on the cantilevered slab. wwf was not pulled up to the middle during original construction and simply sits under the slab, so it does not provide any flexural strength in that direction.
And while 2x6 wood frame with 5/8 drywall is relatively light, I am still a little hesitant to rely on flexural strength of unreinforced concrete, even it checks out with a safety factor of 10.
I guess a 1/8" thick steel plate could be driven through the sand and a little bit into clay first, and then angles driven a ft or so below excavation depth and tapconed to the edge of the slab.
Gaining extra space would simplify some things, but the additional cost will probably be unacceptably high to the owner.

 

[PEinc]

Underpin the slab along the wall.

http://www.PeirceEngineering.com