The boxes were constructed by a friend of the client. He grew up laying forms for concrete which is not so different from retaining soil.
Our boxes are of 2″x10″ treated lumber. There is a plastic membrane separating the soil and wood.
The box must be strong in two ways. It must stay in place horizontally and vertically.
The lumber must retain the soil. The strength for this is provided by using strong lumber that will not bend, by driving rebar stakes along the outer edge of the lumber box, and by reinforcing the box’s corner joints with 2″x2″ wood. Our boxes are nailed together with box or decking nails (twisted) and they do not pull out. Additional retaining strength will be provided when the areas between the boxes are backfilled with gravel, and eventually, pavers.
It is very important to affix the lumber to the ground. The soil will want to creep out underneath the box, lifting the box up and causing it to fail. These boxes are held down with 2″x2″ stakes into the ground. When possible, the stake was driven in inside corners. Then the box edge was nailed to it, providing not only hold-down, but corner reinforcement.
The 2″x10″ lumber was purchased, as were the rebar stakes, however, the 2″x2″ stakes were salvaged, as was all the gravel, from nearby construction.