Foundation Idea

Now don't be too hard on me....this may well be a terrible idea.

So my understanding of the frost line depth for foundation, gravel trench, etc. is that drainage or protection down to frost line is needed to prevent moisture from getting under the foundation of your structure, then freezing and thus causing frost heave that can move/damage the entire structure. I had a thought: could you combine a shallow stone stemwall with a thin, reinforced concrete shell on the outside of it that goes all the way down to the frost line to prevent moisture from getting beneath the shallow stone portion? Attached is an image of what I'm trying to explain. Again, could be a horrendous idea, so please be nice (but honest). I used cob for the sake of having an example.

Tom, good question. Simply put there is more to it than frost heaving and frost lines. Some non-frost soils(6% granular's or less by weight) or non-granular cohesive soils do not need depth and raised foundations can be built on them if the bearing/shear capacity is there. Plastic Index(PI) is an indicator of swell/shrink of different soil types. The higher can retain, expand, swell, shrink, alot. Put those under a slab/floor, and a low PI outside the slab/footing, cupping or drying from high to low causes the floor to sag down and footings wall to bend/crack from difference in bearing capacity of soils. Just the opposite or "doming" occurs in the presence of water pressure expanding soils under a floor/slab sucking water in from outside the footing's. You want positive drainage away from the building as well. So you can see even in normal seasonal weather issues can occur.

We dig down deep or below what is thought to be frost depth where there is less variations in PI, ground water impact, or where there is high PI that allows frost (freezing) to penetrate deeper below grade and keep all these actions away from the slab/floor/walls. The best thing to do at a minimum is obtain PIs at different locations and depths to design a foundations to. There are other test too.

So to answer your questions that design won't solve the issues of having the wrong soils types in the wrong locations and improper drainage, or frost or pressure heaving the floor/stone/wall, and in some soil types the concrete is not needed.

Hope that helps

Hi Tom,

Look up shallow frost protected foundations... it may help you in your journey.

One of the principals of this is to artificially raise the frost line around the building... no water no frost heaving.

Thanks to both of you for the responses Chad - I actually have done some research on shallow frost protected foundations, and that will likely be the solution I pursue....to be honest this was just one of those things that popped up when I was sketching plans and thinking too much.

Tom, you are welcome thanks for the thanks.....This thread should help get you started on FPSF: https://permies.com/t/52387/natural-building/Frost-Protected-Shallow-Foundation-Piers

well, since I already put one strange/possibly terrible idea out there, I figured why not ask about some other strange, possibly terrible idea. I've also thought in the past about doing a partially earth bermed/sheltered structure, but didn't want to use too much concrete and have heard that stone foundation walls tend to leak.....so what about something like this? Terrible idea? Better/easier way to earth berm with stone? Again, please be kind. These are random thoughts/ideas that, once again, may admittedly be terrible.

Tom... Unless you are thinking of having a basement to the interior (can't quite tell), your new plan is great, and in fact it's even simpler than what you've drawn. It's called a rubble trench, and you don't need either the concrete skin or the mortar in the stone! Rubble trenches are very common in the natural building world since they are cheap, effective, require no concrete (or very little), and are DIY friendly. Basically you dig a trench directly under your walls, as wide as the wall or wider depending on your soil, and down to the frost depth, and fill it with crushed rock. That way, any water that is present around the foundation will flow into the trench. The trench then must run down slope to "daylight," so the water can freely run away from the house. Before filling the trench with rock, line it with landscape fabric, which lets water seep through but prevents silt from building up and clogging the spaces between the rocks. For the same reason, use washed crushed rock. Usually, people put a 4" perforated pipe in the bottom of the trench as well (inside the fabric and surrounded by rock), which facilitates the flow of water. Once the pipe is out from under the building and on its way to daylight, you can switch to solid pipe. The pipe in the trench itself isn't strictly necessary since water can flow in and around the rock. But I don't see any reason not to include it. The most important thing is that the bottom of the trench is below frost depth, and the trench slopes consistently from the high point all the way to daylight, 1/4" per foot minimum.

Once the trench is filled to grade, you just build up from there. Frequently, builders will pour a concrete grade beam which both raises the wall off ground level and helps distribute the wall loads evenly. But you can also do a stacked stone or urbanite stem wall.

Tom - your thin concrete wall will not take the loads. International Residential Code "Foundation" chapters has different types of professional engineered design paths that are proven in detail. I already explained in detail why all foundations need not go below frost line & is dependent on soils. Code will also explain that. Setting some walls on rubble trench would be a mistake. I have a thread on Permies named "Breathable Walls" with a ton of field and test data denoting why. But yes, there are many shallow rubble trench foundations that perform well. If you want to learn more about that read permies thread "Raised Earth Foundations" many proven foundations that never get below ground level more less frost depths.

You seem like a nice guy with more questions than incorrect answers and I for one appreciate that. Thank you!