support existing foundation/single pour/or sections


I am considering building a workshop under my house. Have to dig out the dirt. I will be digging down anywhere from 3.5' to 5.5' under existing foundation.
Plan to pour new wall under existing foundation. Will have house resting on crib and beams.
Do not want to take out existing foundation. Walls are from 3' high and go down to 18". Would prefer to make new wall under old foundation.
PROBLEM: I will have to undermine the existing foundation. Remove dirt. While this is in process, is there any way that I can support the existing foundation? It seems that the weight of the foundation will be to heavy for the crib and beams support system.
1) pour columns and footing with rebar in them. Every 8'. Columns will then be hidden/buried in the TOTAL foundation pour. COLD JOINTS? Leakage, Unstable foundation due to two pours?
2) Steel bar with plate on top and bottom? How thick would the bar have to be? Steel bar placed on footing. Bolted under old foundation?
3) I have looked up permajacks. To expensive to leave in the concrete pour.
Was trying to dig out and have one continuous pour for the new foundation.
The other thought was to pour the new foundation in sections. Jam rebar 2' into dirt adjacent to new wall form. When wall is poured, rebar will be exposed when digging out the next section. Attach next set of rebar to new wall. Was advised that this may allow water leakage in cold joints. Planning to us a meridrain system against the dirt (outside of the new foundation wall).
Any suggestions?
Mark
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You would have to be nuts to try and do this job yourself.
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