We used to create a 4 sided "box" using SRW
materials, usually from UNILOCK. The center fill was CA-6, compacted in 2" lifts. Then on the outer 1' around the infill directly behind the retaining wall, we used 3/4" hard sided washed stone. To keep it separate, we took a sheet of 1/4" steel, and bolted handles on the top upper edge. A piece of fabric was placed at the bottom of the washed stone course. As the two types of gravel filled the inner parts of the patio, we took the skidsteer and pulled the steel. On raised patios where elevation went over 24", geo grid was used every other course and we wrapped the wash stone in it's own cells to allow water to drain.
We did not use any piping, but probably should have .
In this neck of the woods, the few raised patios we have done have had the toe of the outer wall on the center of a slope. Engineering has had to be used to make these come together, which, includes an up to date soils report, geotechnical data, and, a stamped drawing from a licensed engineer. We have 5 different soil types here, and, in rare occasions, there can be all 5 on one property.
90% of the time, the footing of the walls are set on concrete rather than gravel, and that footing is 18" below grade and 3' into the earth. If you pour footings for SRW
's you need to be very careful that you finish the concrete as though you were walking on it daily. If there are bumps and dips in the concrete, the wall will just follow the curves.
Costs on raised patios have really gotten stoopid because of all the steps people have to take to get them approved.