Dense clay soil

I live in the western edge of east Texas near the Trinity River.  Our soil is nearly pure clay (gray shale).  I am purchasing select fill (a combination of sand, small rock and clay) to fill my earth bags since my clay is way too dense.  I will be lining it with 6 mil stringed poly.  I am afraid the clay around the construct might expand enough to push the walls inward.  As far as rain here, it is either feast or famine.  Can be extremely wet and then go extremely dry.  Don't want walls caving in on me!  Anyone have any experience or advice with this?  I have researched until my eyeballs nearly fell out and have found zero about it! 😁

Some questions I have lots of experience with soil  / earth construction
- have you done any soil tests with your soil?
- bringing in soil must alter the economics of your build.
- your soil needs to be similar to earth block requirements, it sticks together over time.
- loose sand or screenings are no good for earthbag building.
- Did you research , 'soil types for earthbag building?'
I do not understand your concerns, particularly the underlined parts

Our soil is nearly pure clay (gray shale).
I am purchasing select fill (a combination of sand, small rock and clay) to fill my earth bags since my clay is way too dense.
I will be lining it with 6 mil stringed poly.
I am afraid the clay around the construct might expand enough to push the walls inward.
As far as rain here, it is either feast or famine.
Can be extremely wet and then go extremely dry.  Don't want walls caving in on me!  

Create a temporary roof to keep the rain away and keep the first layer about the ground.

Sabrina, welcome to the forum.

Carl said, "Heavy clay should be advantageous to what you are planning. I dug a tunnel into clay in the woods behind my parents place when I was a kid, and it is still standing 20 years later.

https://permies.com/t/166806/plan-root-cellar#1309734

Some other threads regarding clay that might help you or others:

https://permies.com/t/15911/Root-cellar-bare-clay-walls
https://permies.com/t/16377/stabilizing-excavated-clay-walls-root
https://permies.com/t/186749/determine-proper-structures-tunnel-cellar

I haven't tested the soil per se, but as a former water well driller I've seen plenty of it.  There is a layer of sand and silt about the first 18" and then there is pure dense clay, or shale.  My daughter's property is a short distance from me and lacks this layer and is just the shale.  Cracks form in it during the dry season big enough to lose small animals in, literally!  I know I can't use it to put in the bags, thus the reason I am purchasing 50 tons of the other material.  Wondering if i use the select fill or a different aggregate as back fill if that would help to absorb the expanding of the clay so as to not affect my bag walls?  I am going to make a temporary roof after I dig the hole to protect from rain.  Tried it without one a few years ago and now I have a pretty deep pond instead! 😁  I can get free pallets, so going to fill the bags and store them covered to protect from direct sunlight until I have them all ready.  Then going to dig the hole, rent a gas powered tamper, and move quickly to get the walls up.  Lining it to keep the water out as the shale stays wet for a long, long time.  Do you think if I waited until the ground was soaked to put walls up would make a difference?  

Carl, normally I would agree with you as my grand parents had one just dug out of their red clay soil and it lasted their lifetime.  However, I live where the water table is extremely high, so I think I would just be digging another pond.  But it might be worth digging a small "sample" hole and roofing it for test purposes.

What are you actually building?
It sounds like a cistern in the ground?
Normally its easier to fill and stack the earth bags at the same time, it reduces handing and allows the soil to 'go off'.

I am building a storm shelter.  This is high tornado risk area.  If the conditions, such as temp and humidity are ok I will use as canning storage also.  I am building it the way I am, by first filling bags, because my hubby and I are old and I am pretty much disabled, so not much help.  We are building a form with 6" to 8" casing to measure same amount of aggregate in each bag using a tractor to fill 6 to 8 bags at a time.  If all is ready to go, then I can borrow a couple of stout grandsons for a couple of days to lay the bags and barbed wire.  Gas powered tamper will speed the bag laying.  Thinking I might hammer rebar through bags kinda like with concrete blocks, bringing it up through roof, coating the last bit with rubberized coating to prevent it rusting away, then bending it over to create stability.  The outside clay soil might still push the walls in some, but might prevent total collaspe.  🤔