RF;
For a tiered wall, say 3 short walls (<6 ft stem Ht.), the surcharge of one wall to the other is only one concern. We also do static and seismic slope stability of upper wall onto the middle wall and then jump the middle wall and do a static/dynamic slope stability of the upper wall into the lowest wall,and then do a global static/dynamic of wall 1, 2, and 3 combined.
Again, if the walls are short and the slope stabilty calculations work out, generally, there is no concern. For walls, say talller than 6 ft, you can do Boussinesq lateral stress analysis.
Then there is individual wall stability, like overturning and sliding factors of safety. For tiered walls, it is best to suggest a shear key below the footing, possibly placed directly below the stem. This will avoid large heel footing and still will give you adequate sliding resistance. Also try to discount toe soil cover, unles you are in frost condition. Watch for daylight distance (descending slope height/3, min. of 2 m)requirement.
As an option, you can select gravel backfill above the heel to reduce your active thrust. If the lateral setbacks are tight, then you have no option but to use drilled pier support.
I have Bowles, 1st, 4th and 5th editions. His first edition is more hand calculation style and is my favorite one, except in lateral load analysis. I alos like the 4th ed., because most units are in English and the computer code emphasis is balanced.
Good point about passive/active wedge intersection. If you get a chance draw by hand a 3 tier system and do the geotechnical analysis. Then design the stem, footing, key, and see how all elements influence each other- it is revealing excercise!
