"Red Clay" determining concentration to build on

[AlanB]

Folks I missed on this and must not have explained myself.

I Have a hole.

I dug the hole down to the stable soil (red clay)

I am filling that hole back up to build on.

Red Clay is what I WANT.

The ground here is too "squishy" to build on (when wet the backhoe sinks) which is why I have the big hole now.

The normal operation in this area is to dig too red clay, then fill with compacted red clay fill to grade or near grade then top with gravel and pour concrete etc.

I am building a shop, concrete slab floor.

It will probably be a pole barn (as Ken said, sinking the posts down further to get to stable ground)

The question I have.

How do you determine the % or volume or suitability of the "red Clay" that you are looking at. I am thinking there should be some form of basic field test like a perc test or separation test to "grade" the soil.

rScotty, I will see if I can find that book.

http://www.clarksville.tn.us/downloads/Teeter Preliminary Geotechnical Study.pdf

If interested, this is out there, that site is about 3 to 4 miles from the house, similar ground. Page 5 pretty much sums up what I am doing.

The question though (I see the jargon, want to know what it translates to in reality) is how do they actually judge or grade that fill?

 

[rScotty]

Folks I missed on this and must not have explained myself.The question I have.

How do you determine the % or volume or suitability of the "red Clay" that you are looking at. I am thinking there should be some form of basic field test like a perc test or separation test to "grade" the soil.

The question though (I see the jargon, want to know what it translates to in reality) is how do they actually judge or grade that fill?

OK. Yes, that isn't at all what I thought you were asking. I read the Clarkesville report, and think I understand what you need to know. Getting there without some specialized equipment will be a challenge though. At least I can walk you through the basics and maybe that will help to zero in on your question.
Yes, As you suspect, a typical first step is to grade your soil to see what you have. This is done by taking some samples and running them through about a dozen screens - they look like flour sifting screens - and weighing what percent passes and also looking to judge what each percent is composed of. Experience or a library book will tell you how to identify the material. Percent of clay is the final screen and is the most important one. All that screening is just grunt work and you can do it as well as anyone. Most soils have some percent of clay and most soils engineers will have a set of screens and an accurate weight scale. I've seen those sets of screens for rent at contractor's yards, so you can probably find them if you look. Once you get a profile of what the composition of the soil is, then you will need two more tests - actually four tests as you will to do them both on the overburden soil and on that red clay you are getting from somewhere and potentially using as a part of the fill. The first of these next tests is a percent moisture (done by drying and weighing) of all the soils involved and you can certainly also do that yourself in the kitchen oven and using the weighing scale from the screening test. Next is to get a feel for the compressive strength of the overburden soil you removed, the prospective fill soil (the supplier might know this), and maybe even a good guess at a usable mix of them both while we are at it and if we have the time. Sometimes that is done in a machine that makes a tubular sample and keeps squashing it until it finally busts. But there isn't a thing wrong with being a bit primative. This isn't rocket science. Making some samples in dixie cups, drying them to some percentage in the oven, and then using a hydraulic jack under a PU bumper to bust the samples will get you in the neighborhood of a number and may even shed some light on the percentage to start with. From soil type, moisture, and that compression failure test you can figure a % mix of your local soil and the fill soil for strength. And from the moisture and clay percentage test you can look up on a chart to find out at what moisture content it needs to be to get to its maximum compaction....Too much or too little moisture won't work. Compaction proceeds in a series of those 8" lifts that the Clarksville report talks about on the bottom on page 5. Although I see that they neglected to also tell us the all-important moisture content to get to their magic 95% of dry density compaction number. Anyway, we don't need their number, with the tests above done you can easily find the required moisture content in any civil engineering reference book. Now that you know how to mix and compact your soil to be as good as it can be, and you know how much load it will handle when you do. So it's time to design your building, add up all the material weights of that building and that will give a rough idea of how much the building needs to weigh.....at least you can do that if you also have a scheme for keeping that soil at its compacted density. Hopefully somewhere close to that "95% of dry density" that the report mentions. Keep it dry; wet clay can move mountains. You can't think too much about good drainage.
Luck, rScotty

 

[wroughtn_harv]

In north Texas we have seven versions of gumbo. The latest greatest pad preparation involves digging down through a couple of layers of different clays. Then these clays are mixed with a tractor and put back down packing as they go. Then piers are installed and everyone keeps their fingers crossed.

What makes the clay move is the change in water content. A friend of mine has a house on a hill, well, big rise, and his elevation with change six inches between dry and wet. If you can keep the water content constant then there will be no movement. That's why one of the things that's recommended here in north Texas is watering the foundation during the dry season.

I hate the clay. In fact I have a saying, "when my ship comes in there will be no blackland port."

 

[EddieWalker]

Alan,

Are you sure you dug out the top soil to get down to red clay?

In my experience, areas with red clay don't have much, if any top soil. Your description of wet soil is totally different then top soil. Top soil is what grass grows in, gardens grow and is formed from decomposing materials. Clay is silt that's build up over the years. Because it is silt, it both holds water like a sponge, and also sheds it when saturated. Due to the various types of silt that can make up clay, along with sand, gravel and other types of clays, you get a variety of issues with it. Like Harvey mentioned, some forms of clay are very expansive and when they get wet, they swell dramatically, and when they dry out, they contract and crack.

Mixing stuff into clay is probably a huge waste of time and money. All soils have a load rating and when you build on any soil, you need to design your foundation based on the load capabilities of that soil. Red clay is pretty good to build residential type structures on. I don't know about larger stuff, but for a house, barn or shed, it's the desired material in my part of the world.

The reason I ask about the wet condition of your soil that you dug out is that you might be dealing with a moisture issue that will just continue when you fill in the hole with new material. Taking out the wet stuff is something you have to do if you want to build there, but in order to make sure the building is secure, the new material needs to be both compacted properly and the water in the ground needs to be dealt with. In most cases, just build up the pad enough to direct the surface water away is sufficient.

To know for sure, you have to know what's going on with your dirt. Why it's wet and will it get wet again?

As for your question as to measuring stability of the soil, you have to compact it, or go to it before it's desturbed and test it for it's strength. There is a tool that you use to push into the soil to see how much resistance it has. Imagine sand and how easy it would be to push a nail into it. Then think of hard pan clay. Each type of soil has a measurable density to it, but you can only test it when compacted to get it's real number. A soils engineer should have one of these tools with them, or if ther are high tech, a radioactive tester that is like something you saw in WWII movies with submarines sending off radar signals. They ping the ground and measure how long it takes for the signal to come back to determine how dense the material is. This is all part of how you pass your inspection when building up a pad back in CA, where I'm from.

For what you are doing, it is MUCH MORE IMPORTANT how you compact what you bring in then what you actualy bring in. If it's red clay, it's good stuff.

Another very amature test for pond building was to get the soil wet and squeeze it in your hands until the water oozes out of your fingers. Then try to pull it apart. If it holds together and doesn't cruble on you, it's going to compact nicely.

Hope this helps,
Eddie

 

[AlanB]

OK, maybe some pictures will help quantify what I am looking at and trying to decide on.

I want "red Clay" or whatever you care to call compactable stable stuff to put in a hole and build on. Rock would be nice but would get a tad expensive fast.

5701

Red Clay, mixed with a bit of Yellow clay, and some Gray (Grey) which is also called "Guthrie soil" here. Guthrie is a small town just north of here with notably terrible soil that I happen to sit on the line of where Guthrie soil ends (my backyard) and red clay starts (my front yard)

The question would be are these traces of other soils in with the red acceptable?

5703
The pile I happen to be moving at the moment, but really the question is generic as I go to other places and pick up and evaluate where I want to put it. Pretty clean red in center, Little mixed on Left and what I call Mid soil which is not good top soil but not good clay (what I have a lot of) on the right.

5707

The inspector came out to catch some frogs

5719

Pile again, Mixed on Left, Decent in Center and mid soil on Right.

Eddie, yes, I am very familiar with top soils and what I refer to as "mid soils" which are the soils of the area from around here that are not clays, but not really the nice rich in organic matter soils that you want as top soil. Unfortunately I have a couple inches of top soil, then lots of mid soil, then depending on where in the yard you are, either Guthrie soil or Red Clay base, sometimes pockets of each.

On our large construction projects around here, (and I assumed in other places but really have no experience to base the thought on) they will come in and pull off all the top soil and pile it, then pull out the mid soil's and pile it separate, often called the trash pile, then haul in clay to grade, then dress after completion with the top soil. The mid soil they will use to fill something like a sports field or something where there is no intention of building anything as it will not compact and firm up.

The Gray Guthrie soil I ran by a soils engineer I was working with on another project (pulling JP-4 and 8 out of Karst topography with a pump and treat station under the Airfield) and he came back and said something along the lines of "a highly aluminized something mumble jumble etc. with a strong polar charge, yadda yadda yadda" I looked at him and in my best redneckese said, "Means it really holds water right?" he said yes.

I will call around to the rental folks and see what is available for measuring screens and tools.

Oh, and when I drop that clay in the picture into my shop hole on top of the existing red clay, the new stuff looks more brown then red where in the front yard it looked red as could be against the brown backdrop of the other soils.