This is our West retaining wall/root cellar now fully encased in concrete.  As you may have read in a previous post, this is an old septic tank. (fully cleaned and sanitized:-)  We had two inches of structural foam sprayed on the sides and top. This was then tied into the retaining wall with lots of fiberglass rebar, basalt roving (for reinforcement), and of course even more concrete.  The bottom, which now has a concrete floor, was not insulated so as to allow the coolness of the earth to transfer into the cellar. Over half of the tank will be buried within the berm.  We will eventually extend a hip roof off of the North side to create covered firewood storage.  This will also help shade the exposed concrete on the tank from absorbing solar heat.