Sand/Gravel Mix for Earthbag: how much gravel is acceptable?

Hi all,
First of all, I want to thank you all who have helped me so much with my past earthbag questions. I keep pretty busy and have difficulty finding time to post a proper thank-you. You guys, and this site, have been a great resource.

Anyways, my next question is, how much gravel is ideal for earthbag construction mixes? Most sources that I see say that approximately 20% clay to 80% sand and gravel mix is ideal. However, what I have not seen yet is what the ideal ratio of sand to gravel actually is. So I wonder, what is the absolute maximum and the absolute minimum amount of crushed gravel that should be present in an earthbag?

Personally, I'm stuck in a position where I need to purchase the sand and gravel mix separate of the clay mix from the local gravel pit. Attached is a picture I took of their sample of sand and gravel mix, which contains three-quarter inch crushed gravel pieces. After looking at the picture, I would like to know if this looks like too much gravel. To me it looks like a lot, but after adding the clay mix, do you all think this looks like a good choice? They have other mixes available, like plain sand, but my sources say that something with a bit of small gravel bits is more ideal. Can strong earthbags be made with just sand and clay?

Anyways, would love your thoughts. Hopefully this hasn't already been discussed elsewhere and can help others.

This is a matter of compaction, how well you can compress the mixture.
The gravels are like the gravel in concrete, they are there to form a matrix that will not fall apart under high pressure, so from that POV, what you show, would be good, especially once the clay is added to it.
Such a mix will lock together well when you tamp it down and that means a good, sturdy wall.

Hey thanks Bryant, glad to hear! So, would you say that up to half or so gravel in the sand mix is the ideal? Have you worked with a wide variety of soils in earthbag before?

I wonder if it's even common to find a mix like this in nature, since I know many people can dig the fill right out of their land. Most of the rock around here, if you find any, is smooth round river rock.

Hey, here's a somewhat related question, does anyone have experience using soft clay shale to add to their sand/gravel? This may suddenly be my only option. I found a nice patch with shale already naturally pulverized a bit into a fluffy, heavy clay mix (see pic). Shale is still very visible, but most pieces are tiny, and it can form a good clay ball that is sticky and pliable. Some visible bits of shale still float in the matrix though.

Anyways, this stuff is apparently ground up and used in ceramics and pottery, does anyone have experience with it?

Jared, what you may have there may not really be shale but mudstone. Same raw materials, just not "cooked" and so they can be broken down mechanically. If it turns back into clay when you grind it and mix with water, then it's all good.

Where I live is sedimentary country and nearly everything around us is on its third or fourth trip to the sea and back, so we've got it all. Our bedrock is greywacke, a super hard and slightly marbled sandstone, but on top of it are layers of crumbly limestone loaded with shells, and lots of soft papa rock, which is basically varying degrees of silt and mud that would have turned into shale or sandstone but got lifted up too soon and is now eroding and headed back out to sea again. Makes for very interesting bluffs along the rivers (great diving potential) but they fall down at the drop of a hat, so we're always warning the kids about the danger.

Sure hope it works that way!  Thanks Phil! The layers nearby are a bit harder shale, and has the many horizontal layers characteristic of it. The stuff is so soft though that it gets a layer of clay slip when mixed with water. The stuff I'm actually keeping is the shale/mudstone that is already broken up like a soil. I'm guessing that millennia nearer to the surface and exposure to moisture and frost heave may have broken it all down for me.  Not entirely sure why I've only been able to find that trait in just one small area, but I'll take it. When squeezed into a ball with a higher moisture content, it dried quick in the sun as hard as cement and without a single crack. Sounds like a good sign I think!