Should I build a segmental retaining wall on top of a grade beam with piles?
When you build a grade beam with piers every 6’ apart, you are essentially building a “post and beam” foundation.
The grade beam will need to be designed to span from pier to pier. The size, amount of rebar, etc. will be based on the load, (i.e.: 4’ to 12’ tall blocks stacked on the grade beam).
The reason the foundation system changes from the traditional “footings on grade” to the “post and beam” style is that the piers will act as posts pushing against the deeper (and more resistive) soil. (Soil 8’ down in the ground is more compacted than soil near the surface.)
Question #1: Is it something people do? No.
The one thing that foundation engineers fear is differential settlement. When a traditional foundation system rests on the ground, the ground can support a fairly uniform load. When you install piers they extend down to more “resistive” soil (soil that’s been compacted over eons of time from the soil higher up.) This will settle less than the soil near the surface, causing differential settlement when loaded (when the blocks are stacked on top).
Question #2: Is it unbelievably overbuilt and guaranteed never to move? Could be.
If the grade beam and piers were adequately designed, then yes, they would not move. In order to design this type of foundation system you’ll need geotechnical engineers to take samples of the soil and test them for compaction, etc. Plus, the size and complexity of the system is far more expensive.
Side Note: This type of engineering requires the highest type (most expensive) of insurance. In the medical field, anesthesiologists and OB/GYN pay the most, (except for special surgical doctors.) These engineers pay higher rates than them.