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Yazoo Clay, and Why It Breaks Drainfields

Everything else on this site is downstream of one document: the log a licensed evaluator writes after putting holes in your ground.

Rankin County sits on the Jackson Prairie, and a great deal of it sits on Yazoo clay. Yazoo is a smectitic clay: it swells as it takes on water and shrinks as it dries, hard enough to crack foundations and split driveways across the Jackson metro. For a septic system it is close to the worst ground there is. Wet, it is effectively impermeable and effluent has nowhere to go. Dry, it opens into deep shrinkage cracks that will carry untreated water straight down a fissure without the soil ever treating it. Neither state is any use, and the ground spends the year moving between them.

Typical soil boring log for the area A vertical boring log showing each soil horizon in order with its depth, texture and what it means for a septic system, as a hand auger advances down through them. Typical boring log, THE JACKSON PRAIRIE WHAT A LICENSED EVALUATOR IS ACTUALLY LOOKING AT. YOURS WILL DIFFER. BORED TO FIVE FEET, OR TO A RESTRICTIVE HORIZON IF THAT COMES FIRST. 0 IN SILTY CLAY LOAM THE WORKABLE SKIN, AND THERE IS NOT MUCH OF IT. 10 IN CLAY PLASTIC WHEN WET. LOADING RATES COLLAPSE. 23 IN MOTTLED CLAY CHROMA 2 OR LESS. A PERCHED TABLE SITS HERE. 35 IN YAZOO CLAY SHRINK-SWELL. IMPERMEABLE WET, FISSURED DRY. 60 IN
This is the single document everything else follows from. The horizon that decides your system is usually not the one at the top; it is the first layer down that water will not move through, and how far below grade it sits.

What the evaluator is actually looking for

Not "is this good soil". The questions are narrower and more mechanical than that, and they are the same questions every time:

  1. How deep is the first layer water will not move through

    Bedrock, a fragipan, a dense clay: whatever it is, it puts a floor under the design. Everything above it is the soil you get to work with, and if there is not enough of it, a conventional trench is off the table before anything else is discussed.

  2. How high does the water table come in a wet season

    Not today. In February. Soil keeps a record of this, and a trained eye reads it off the colour of the profile long after the water has gone.

  3. What is the texture of each layer

    Sand, silt and clay in proportion, judged by hand against a standard method. Texture sets the loading rate, and the loading rate sets how much trench you need, which is most of what you are about to pay for.

  4. Is there anywhere legal to put it

    Slope, flooding, and the separation distances from wells, water, buildings and lot lines. A perfect soil profile in a place the rules will not allow a system is not a usable answer.

  5. Is there room to do it twice

    Good practice, and in some designs a requirement, is to keep a reserve area that could take a replacement system. A lot with no reserve is a lot with no plan for the day the first system reaches the end of its life.

Do this before you build, not after

The evaluation decides where the system can go. If the house, the drive and the shop are already placed, the system has to fit around them, and on a tight lot that is how a straightforward job turns into an expensive one. Have local pros quote the evaluation →

Why a dry-day walkover tells you nothing

Ground that looks perfect in a dry spell can have a water table sitting a foot down for three months of the year. That is the single most common reason a homeowner feels blindsided by an evaluation result: the lot looked fine to them, because they were standing on it in the wrong season.

The profile is what settles it. A layer that spends part of the year saturated does not look like a layer that never does, and the difference does not wash out in August.

Last reviewed September 28, 2026 against the Mississippi State Department of Health and the Mississippi State Department of Health published rules. How we research this site.