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Can someone tell me about rubble trench foundations?

 
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As near as I can tell, to make a rubble trench foundation, you dig a trench in the shape of your future building deeper than the frost line, and then fill it with whatever rock you have available.  The frost line here can be as deep as 4 feet, so let's say I dig a trench 5 feet deep and fill it with rock.  Once I get to surface level, I pour a 6 inch slab of concrete on top.  That's it? Surely there must be more to it than that?  What am I missing?
 
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Trace Oswald wrote:As near as I can tell, to make a rubble trench foundation, you dig a trench in the shape of your future building deeper than the frost line, and then fill it with whatever rock you have available.  The frost line here can be as deep as 4 feet, so let's say I dig a trench 5 feet deep and fill it with rock.  Once I get to surface level, I pour a 6 inch slab of concrete on top.  That's it? Surely there must be more to it than that?  What am I missing?




Yes, but not much more.

You would want the rubble filled trench to drain to daylight (downhill). You can do this in one of two ways.

Continue digging the trench until, at 1/4 of an inch per foot in slope, you reach daylight. Or do the same but with drainage tile, which is just cheap, black plastic pipe.

It works pretty simply. Because the rubble filled trench allows water to easily drain (also called a French Drain), the water drains away from your foundation. It has to be able to drain to daylight because otherwise that water would just pool up. But without water, when the ground freezes, there is no water to expand and heave the building, so your building stays firm.
 
Trace Oswald
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Thank you Travis, I appreciate that.
 
Travis Johnson
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I should have said that they work exceptionally well.

They are just so simple. Gravity drains the water. Because there is no water, when the ground freezes, there is nothing to expand, so the building does not move. In warm, but wet weather, or when the snow melts in the spring, again the water drains so there is no mud. In either situation, the building sits on dry rock.

But they do have to drain to daylight.
 
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Trace, all sounds good but I would look into FPSF (Frost Protected Shallow Foundation) combined with rubble trench. At five feet deep you are looking at a whole lot of gravel and excavation. Shallow foundation allows you to dig much shallower and still protect from frost upheaval. At 4b you would need at least 14" down and wouldn't need the horizontal insulation skirt employed in places like Alaska. Either way, rubble trench is the way to go over traditional foundations.
freeze.PNG
[Thumbnail for freeze.PNG]
 
Trace Oswald
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Thanks Daniel, I'll take a look at some material about it.
 
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Trace,   To add a few more tidbits to what Travis and Daniel said according to Ianto Evans/Michael Smith in their book Hand Sculpted House (chapter 10) they recommend after digging the trench to test it with a few buckets of water to ensure the water does flow and not pool and to adjust as needed. Also to tamp the base really well as this is what will hold the weight of the house. An excellent book that I would highly recommend reading.
 
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To add/clarify...  Do you just toss the rubble and stones into the foundation trench or do you place them as tightly together as possible?  
 
Travis Johnson
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Kind of both.

You need the rock to have plenty of spaces so water will drain, but it also needs to be compacted enough to support your building. Granted your concrete grade beam will "float" some anyway, but I would just back fill with potato rock, and then drive over the filled trench to help get things to settle a bit. Then I would add more rock so that the tops are level, so that I could build my forms so my grade beams could be made.
 
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