I think you need to talk to a structural engineer. Your building department will probably require it anyway.

Thanks. I should mention they're big boulders and not the rip rap. I wound up with 7 tons and think i might be light a couple of tons.

Like HERE says; an engineer would know, and you'll need one anyway.

Actually, I do not. Permits have been issued already.

Wow. Where is this? Really surprising. Are there drawings showing the volume of boulders and a spec on their size and you just need the weight? If so, then I gather you just need a weight/volume ratio, which the quarry or re-seller can give you.

If not? I'd still call an engineer. Going by a formula someone with unknown qualifications gives you over the internet seems like a dicey way to design a retaining wall.

I'm in Denver. I do have a structural engineer but oddly we never really discussed the garden level retaining wall. I did consult with two friends that own landscaping companies and feel like I've got a pretty good grasp on the design. The plan is to start with really big half ton boulders on the bottom and gradually reduce them as elevation increases.im going to do a french drain with perf pipe wrapped in a weedblock type sock. Also going to do some native grasses and flag steps.
I do appreciate the comments and suggestions and love the feedback I get on this site. Totally valid points. I just couldn't find any equation anywhere for the volume of boulders I'd need anywhere. I'll post update and photos in a few days when I'm done. Thanks again.

If your engineer was hired to design the building, they may have regarded the garden wall as outside of the scope - or may just not have noticed it. Same with the building department. Personally, I would never design a 6' retaining wall myself. It is one thing to have a hill fall to your waist or chest, but 6' is above my head. There are all kinds of regulations about trenching deeper than 5' for that very reason. I'm not certain whether they pertain here, but, I would not be surprised if they did. Be very careful - especially after all that rain! :)

Yeah. Thanks. I think you might be right.
The crazy thing is I pulled a dead sapling out of the ground after 5" of rain and the dirt was bone dry. I think it was raining so hard it just ran off in my area. Appreciate your comments.