Question
Sean,
The railroad tie retaining wall holding up the asphalt driveway have rotted and collapsed. Height from base to top of the driveway ranges from 4-6 feet. I have removed the ties and want to construct a 2 terrace sloped support wall with large boulders at the base about 10-12 feet from the base of the wall, backfill with limestone gravel up to 2-4 feet and then soil on top with another layer of smaller boulders 1-2 ft from the top.
The width of the driveway at the end is 35 feet. The terraces would have perennials.  
Does this design seem plausible?  How do i calculate the required weight and size of boulders and gravel to support the driveway? Do i need water drainage with gravel and boulders? The driveway desparately needs replacement and i'm concerned with installing a concrete drive and the propensity for cracking if this retaining wall design is inferior.  The boulder and terrace design is important to us, the artificial interlocking block look will not provide the natural appearance that we want to maintain in our back yard.  By the way this wall will extend an additional 50 feet to support a red brick patio that ranges from 1-4 ft in height. Thank you very much in advance for any comments and help you can provide. Tom

Answer
Tom: yes it seems plausible and possibly quite attractive. It would be difficult to estimate the weight because most boulders over the size of a softball are not sold with any average coverage information. Estimating the needs of large boulders is something that comes from years of experience. Gravel coverage depend on size. You can contact local supplier for this info. They can tell you 1 ton covers x sf at 4" depth, etc. The you can alter the math to figure out what you need.
Note building a wall to support your driveway with boulders is very tricky. If not properly set, fortified and back filled it could easily fail.
I suggest back filling with lots of gravel, installing a french drain and possibly using geotextile fabric every 3 feet like you would with a "keystone" type wall.
Finally, I am very much a  do-it-yourself kind of guy but this is definitely NOT a do it yourself project unless you are very familiar with setting boulders and proper wall design. You'll need large boulders that will take either a backhoe or 3 or 4 guys to move with steel rods to coax the boulders.
I wouldn't hire anyone who doesn't have insurance and wont guarantee the work for 36 months or longer. There is nothing to break or die so they should be willing to do this.

Best of Luck