Questions about black locust roundhouse in frosty environment

I'm contemplating a project where I get some wonky black locust logs and make a roundhouse.  The logs would be stuck down in the ground with more logs as beams between them at the top (making a circle or decagon).  Big rafters from there to the peak with a healthy overhang.

My initial questions are:
1. I have a very sandy soil.  Black locust reportedly lasts a very long time in the ground.  Should the logs be debarked before setting them in the ground?  

2. The building code frost depth is 4'.  I suppose I should sink the posts at least that deep and probably set them on a bigger flat stone or paver.  Any reason to think I could get away with only putting them 2' in the ground?

3.  I'd hope to do slip straw for the walls.  If I used more black locust as a sill plate (half buried in the ground), I could build right from there on up with the straw slip.  But would the sill plate rise and fall with frost?  If I make a rock stem wall instead, what would keep it from rising and falling just the same?  I don't want to dig down 4' for a foundation wall since I don't have many rocks and would have to haul them in.

4.  I realize with having slip straw (and an earthen plaster overcoat) so close to the ground is a risk for both wicking and splash back from rain.  Would a 3+ foot overhang (natural roof plus maybe gutters) resolve the splashing concern?  Is wicking a risk?  I could use a layer of birch bark between the sill log and the wall.

Thanks!

Interesting, I'm trying to picture what you mean by a roundhouse. Are you intending to use it as an abode?

Can't offer much advice on how deep to set the posts or much else but I can say that black locust does last a long time. Parts of my gardens are slightly terraced with black locust logs on the downslope edges and all of my fence posts are black locust. They have been there for thirty years and still as solid as ever. There are old fence lines in my area where black locust posts are still intact after at least 100 years. The pole structure of my garden sheds and workshop are also locust. I imagine in a colder climate they last even longer. Come to think of it they are a full three feet deep which in my climate is fine.

I've heard both ways about leaving or removing the bark. I don't and have never seen any do so, except when used inside as roof rafters and I think that was more about shaping and flattening the surfaces that anything else.

[edit] Thinking a little more I suspect you can get by with just two feet deep, depending on what exactly what the structure is to be used for and more specifically how important is it to maintain perfect square in joint, for windows, doors etcetera. Sounds from the description exact angles and joints aren't part of the plan anyway so what difference does it make if it moves around a fraction of an inch with every freeze and thaw? Wood has that much flexibility anyway and as long as your joints / fasteners can do the same, it would just all go through the ride together without anything breaking losing integrity.

I guess though, that if you need a more rigid and unforgiving structure, then deeper for your posts might be in order.

Thanks Mark, it's good to hear the bark doesn't have to be removed.  I'm used to peeling pine but I'm guessing peeling locust is harder work.


Here's what I'm talking about.  A circle of 8-12 posts with a matching number of beams tying their tops together.  Then a matching number of rafter beams to hold the roof.

Cool, how big is your structure going to be? Looks like large carriage bolts holding the top beams to the posts? I think they call them carriage bolts because they are made for staying together under constant flexing. That's what's I would use. Drill the holes about a 1/16 or maybe an 1/8 inch bigger around than the bolt and those joints aren't going anywhere.

From what I can tell locust and pine have very little in common no matter what they are used for, other than shade. In that application, peeled or not, pine would last about six months in my climate.

What a coincidence, just today I had a young fellow to cut down nine, big and I do mean big black locust trees. Firewood for two or three years in just the tops and side branches and at least 18 BIG posts and he's coming back tomorrow to cut three more. I was actually planning to cut them to firewood too, but your idea sounds pretty good now.

I'd probably aim for maybe a 16' diameter.  I'm a long way from needing to know that :)  

That pic above uses carriage bolts and the beams bypass the sides of the posts.  Most roundhouse pictures I've seen have the beams meeting on top of the posts.  Then a bit pin/spike could tie them together and attach them to the posts.

An earthfast post structure, unless there is significant diagonal bracing above grade, would be more stable if dug in three feet. Two feet is iffy. Frost heave could make the structure heave unevenly, but insufficient depth will let it tilt and eventually fall over. If the structure is heated, and there is any kind of insulation in the ground outside the walls, you will have much less danger of frost damage (as well as a more comfortable floor.)

The outer surface of locust will rot if not debarked, but the bark will fall off in a few years in any case if exposed to weather. Inside, the bark will harbor insects and slowly crumble and make a mess, and hinder a good connection between posts and wall infill. I would debark everything. When freshly cut, locust is pretty easy to debark, not so much when it has seasoned a bit.