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