Reinforcing existing retaining wall

[milkshakelake]

I have a 12' deep 24" thick concrete retaining wall (no tiebacks) that needs to go about 6' lower. I plan to add tiebacks and underpin with epoxied rebar so the whole thing acts together continuously. I'm a structural engineer, not geotechnical, but I want to do this safely. I've been reading about retaining walls for 2 days and couldn't find a definitive answer. How to I check the existing wall, global stability, and if this method actually works? I could just buy software to do it for me, but I don't function that way. I'm a hand-calcs kind of person.

Plus, there are a series of stepped wood retaining walls beyond this thing. Should I reinforce the wood retaining walls with helical tiebacks and angle anchors? I don't know how to analyze this; I'm at the point of just saying that I can't do this. Not to mention seismic concerns, which I can't even fathom. But I really want to learn this and do it the right way.

I'm at my wits end; I just need a starting point to do a code-justified design, and I want to be able to sleep at night so that nobody gets hurt from my design. A starting point, please.

 

[milkshakelake]

Update:
I talked some sense into the contractor. He finally agreed to take out the whole wall. Now we're doing a steel soldier pile wall with helical tiebacks. He's used to doing this, and wants to stay away from bored pile walls. Tangentally, there are some places near the top where the soil slope is no good in my opinion (and calculations), and I'm going to present the contractor with some kind of slope stabilization with plates or small piles.

01523339552609:
Thanks for the reference!