I'm about to attempt my first Oehler style earth integrated structure, any suggestions would be appreciated.
The poles, girders & beams are Larch timbers.
My main concern as of now is on the shoring boards. I have access to rough cut 1x6 boards from a local hardwood sawmill.
Species like: Beech, Maple, Oak etc.
In Mike Oehler's underground book he mentions the use of softwoods ei. pine. for shoring.
Am I correct in assuming that hardwood would be even better a choice? Perhaps hardwood would let me get away with thinner boards?
Anyways, here's some 3d model views of the structure.
I haven't read up on Oehler structures in particular, but 8' spans for planking 10' below grade are not viable for the long term, even 2" solid oak. Without calculations, I would want at least 4" solid hardwood planking or larger round logs. I think doubling the perimeter posts would be more viable. Again, I don't know the exact capacity of larch, but as a softwood I wouldn't trust it buried in the ground for decades, depending on the climate. What is the diameter range of the posts on hand?
Mike Oehler mentioned that 6' would probably be better than 8', perhaps in a Youtube video of his place. I don't recall him saying it in the video courses. If the off cuts you can get are 16 feet long for the sides and are 2" thick, then that will probably work if the wood is protected. There should be a layer of poly between the shoring and the earth, and it also comes up under the bottom board if following Mike's floor setup. If you dig a trench around the building, with gravel and drain pipe going down below the frost line, then the chance of moisture seeping up to reach a board seems highly unlikely.
The biggest point of concern I have been considering is protecting the posts. Especially if green when installed, a lot of moisture will come out of the wood and if the logs are wrapped in water-tight plastic then that moisture will cause faster rotting. Some suggest charring the logs to create a more rot-resistant coating, others have thought a layer of borax underneath would reduce the organisms trying to feed on the wood. My personal thought is to have landscape fabric inserted in the hole, then 2-3" of gravel in the bottom, tamped down, so water doesn't remain in contact with the wood. Then some borax might be added but I think it would settle into the gravel... but then the log on top of that. I would want the fabric to come up the sides of the log to limit soil contact there either.
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