Any advice on retaining walls

[AsianEngineer]

i have a residential client that has a one story rambler with a daylight basement(sort of). The problem is he has a work area underneath the garage. The work area is only 7ft tall, has retaining walls on three sides with a structural slab above. What he wants to do is dig down and install another retaining wall. he wants the room to be at least 8 or 9 ft tall. Is this possible? The problem I'm running into is how to support the existing retaining walls so that we don't affect the structural integrity of the home.

 

[mcutchi]

Scenario 1 : The floor is independant of the walls ; In this case you can just rip the old floor out, dig down another 18 inches (upto top of wall footing), and cast a new floor.

Scenario 2 : The floor is either integral with or acts as a strut for the base of the walls ; in this case, you may be able to do it in sections, digging out and casting U-sections of slab + wall extension as you go along. I wouldn't leave more than 1/4 unsupported at a time, i.e. do in 4 alternate sections. (Dont park the car in the garage during this time !)