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In essence, your knee walls are retaining walls, and I follow the the recommended method of installation for retaining walls in this instance. Directly behind the blocks, I put 1' of clear stone, covered by filter fabric and separated from the base material in the same way. This gets compacted the same as the base material and my sand course gets laid on the filter fabric on top of the stone.
If possible, a perforated PVC pipe should be installed at the base of the stone and exit at some point through the wall. This will relieve any hydraulic pressure that could develop due to poor drainage, and will minimize any tendency for a frost push to the wall.
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