I have built two soldier pile retaining walls (where you slot the boards between I-Beams you sink into the ground) with good success - but that success was due to:
sinking the piles at least their above ground height (ie. 5 feet above and 5 feet below)
very generous "deadmen" going backwards under the retained material
But now ... we have a much bigger wall to build - 80 feet long and running from 4 feet tall out to a corner that is 12-13 feet tall. Soil samples show 8 feet of "creeping" soils and our wall engineers tell us they want us 9 feet PAST the creeping, so a 12 foot corner requires a 12+17 (let's call it 30) foot H beam. And that's just one of them ...
So that's getting really expensive and complicated and the engineering firm said to just forget the soldier pile wall and switch to a segmental retaining wall - just the plain old versa-lock pre-fab blocks that everyone uses (although we'll probably use "big blocks" that are 200-500 pounds each ...
My question is this:
Apparently a segmental retaining wall is placed right on grade. As simple as that. You just level your grade and start stacking.
That sounds great, but I don't understand how that can be that simple ... if we have creeping soils and we need to go 9 feet past them and everyone (including myself) is scared to death of not drilling pier holes deep enough ... how can we go from that to just tossing them right on grade and calling it a day ?
Why won't the grade move just like the creeping soils move ? Why won't the combined weight of all that wall (which will probably be canted backward 10-15 degrees ?) push the soil underneath down and out ?
What am I missing here ? When we switch to building with concrete legos, basically, why do we suddenly not care at all about the dirt underneath the wall ?