The height of the driveway over the backyard varies from 4-6 feet in height, is 35ft wide at the end and is held up with railroad ties which have all nearly rotted and collapsed. I want to build a 2 terrace, boulder wall sloped out minimum 12feet from wall base, partially bury the largest boulders and backfill with limestone gravel. How high and how much gravel should be used? How large and how many layers of boulders are needed? Can the gravel be capped with soil for planting purpose? if so, how much soil can be put down?
How do i determine exactly how to design this wall? I plan to replace badly weathered and cracked driveway with concrete which i don't want to crack b/c due to a poorly designed retaining wall. Thank you.

This answer cannot be done with the information you provided. You need to secure the services of a professional landscape designer to get the answers to your question. Lots of issues to consider. What type of soil is behind the old ties? What about the soil that would support the new stones, These and a host of other questions come to mind. Sorry we cannot help you.