fence post root cellar

I'm building an above ground root cellar in my yard. I've dug down through a giant mound of soil from other projects to create a flat area a foot below the original grade, which I have now partially filled with tamped gravel. I am thinking about building the structure out of timbers. Specifically, I have a huge stack of cedar fence posts from an old fence. The posts rotted at the ground level, but the main sections are fairly sound. I was planning to build with them retaining wall fashion, pinning them together with rebar. For backfill, I plan to use concrete rubble against the wall with the earth further back to help retard rot, and the base layer of timbers will be set on cinder blocks to keep them dryer (the root cellar will be moist, of course.) The roof will be above ground, a standard frame structure.

My questions; how long would something like this last? Would coating each timber with limewash help? I'm imagining something more the consistency of lime plaster, actually, spread between the layers like chinking. And, the timbers have some very faded varnish or stain on them, would this need to be plained or sanded off to get the limewash to stick?

Please forgive my side note here but you may want to consider if cold air can get down into the concrete rubble and cool the root cellar more than you intend.  Capping the rubble with something to block frigid air from entering could be worthwhile.  Or controlling it so that you can circulate cold air at night in the fall and spring to help cool the cellar and then block it for the winter to avoid over cooling it...

Sorry for the sidebar.

I don't know how the cedar will last.  I'm guessing that if you protect the inside and leave the outside exposed to the rubble (to dry) it could last a decent amount of time.  Will the posts be strong enough to hold back the weight of the surrounding dirt?

Cedar will usually last about 10 years when in contact with soil (it is the dampness that gets the rot started in fence posts).
If these are 6 inch diameter cedar posts, then they should last the ten year life expectancy, if they are only 4 inch diameter they will last about 6 years.

To get the longest life from them you will want to put a moisture barrier over the post prior to backfilling.
The strongest way to build would be to stack them like a log cabin, that gives the most strength against backfill pressure.

Don't forget the roof, it will need to be strong enough to hold out/up the soil that you put over it, and don't forget the vent pipes, you really need those or you will have mold issues big time.

Thanks Bryant and Mike!

The rubble will be capped with earth. Also, the whole structure will have an EPDM liner over the surrounding earth, covered with more earth for protection and then wood chips as insulation. The roof will be exposed and heavily insulated; it will probably be a living roof. Rain falling on the structure should run off the roof membrane and then off the membrane covering the surrounding earth. (After soaking through the mulch.)

The backfill will only come up to about four feet up, at which point the walls will also be insulated. So I hope they will be strong enough.

Bryant, will the lime chinking/ plaster help to preserve the posts? Would tar be better? Of course, I probably wouldn't want to use tar anywhere around food in any case. Is there anything I can to to extend their life? They are only 4x4s.

hau Gilbert, I would use a mix of borax and lime and make it stiff enough to be used as chinking first then you could use a second layer just a bit thinner to coat the entire wood structure on the exterior. For even more rot prevention you could thin this coating mix enough to use it like a paint on the interior and let it dry completely (inside and out) before applying the barrier material. A coating system like this should allow cedar wood to last nearly indefinitely (the cedar main beams are still present in one Solomon's gates that's well over 2000 years).

Never use tar for a root cellar, it just creates too many problems down the road and that isn't even getting into the obvious issues of odor, sticky mess, etc.