Shale for rubble trench?

I am planning a cob house and have a lot of shale on my property. Does anyone know if shale is suitable fill for a rubble trench?

How deep is your trench going to be to get below the frost line?  I am going to guess you are looking at 36 to 48 inches?  What is your base soil type?

My concern with shale would be shifting.  It could be contained with a wire gabbion.  A more practical way would be to add gravel as you filled.  Shale would take up a lot of volume and reduce the amount of material that would need to be brought in; but a deep layer of shale will shift and settle.  With frost heaves and expansive soil, I think you will not be happy with shale alone.  It would not take a lot of gravel but needs something to stabilize the lateral forces.  

I don't have shale in my area, but have hiked in areas that was heavy shale.  Uncontained and/or unstablized, I think you would have problems.  I envy you though.  I think it would be a good resource for natural building.  

I think shale would be okay.

It would be contained within a dug trench, and really it is just a static load that you are putting on top of it. Once the shale shifted and settled in, it would be able to support the static weight of a home I would think. Really the "rubble" is to ensure water is drained away, and that frost does not play havoc under the structure.

I would think the hardest part would digging a trench down to depth if you have so much of it nearby. That kind of says bedrock is pretty close to the surface of the ground, but sometimes you can shatter the shale enough a you are digging to make the trench.

I only semi-have shale here. We have what is called shalely-ledge.

Thanks for those opinions. Our frost line is 12”. I had planned to dig around 24” deep x 24” wide, line with feed bags and hoping to avoid putting a pipe in. It is a very well draining area. I could easily put part shale/ part rocks. That may just ease my mind enough.