Most people think about their metal building from the roof down. The panels, the doors, the color, the size. What they do not think about nearly enough is what is happening from the ground up — specifically, what kind of soil their building is sitting on.

Soil conditions affect everything: how your concrete slab performs, how your anchor points hold, how your gravel pad drains, and how long your building lasts without problems. The good news is that most soil issues are manageable if you identify them before you build.

The Four Soil Types You Will Encounter

Clay soil is the most common problem soil in the Mid-South. It expands when it absorbs water and contracts when it dries out. This movement — called shrink-swell — can crack concrete slabs, shift anchor points, and cause doors and frames to rack over time. If you have clay, you need to address it before you pour.

Signs of clay soil: sticky when wet, hard and cracked when dry, dark gray or reddish-brown color, slow to drain.

Sandy or loamy soil is generally good for building. It drains well, compacts well, and does not have the shrink-swell problem of clay. The main concern with sandy soil is that it can shift under load if it is not properly compacted.

Signs of sandy soil: loose and granular, drains quickly after rain, light tan or brown color.

Rocky soil has excellent bearing capacity — it will support a concrete slab or anchor points without any issues. The downside is that it is expensive to excavate if you need to run conduit, utilities, or footings deeper than the rock surface.

Fill soil is any soil that was brought in and placed to raise grade or fill a depression. Fill soil is unpredictable because you do not know what is in it or how well it was compacted. Always treat fill soil with extra caution — it may need to be removed and replaced, or at minimum tested for compaction before you build on it.

How to Do a Basic Soil Test

You do not need to hire a geotechnical engineer for a basic soil assessment. A hand auger or post-hole digger and 20 minutes will tell you a lot.

Dig a hole about 18–24 inches deep in the area where your building will sit. Look at:

• Color and texture: Is it clay, sand, or rock? Is it uniform or layered?
• Moisture: Is it wet even though it has not rained recently? That suggests a high water table or poor drainage.
• Organic material: Is there dark, rich topsoil? Organic material must be removed before building — it compresses and decomposes over time.
• Compaction: Does the soil feel firm or does it give easily? Loose soil needs to be compacted before you place a pad.

What to Do About Problem Soils

For clay soil:

• Remove the top 6–12 inches and replace with compacted crushed stone or a gravel base
• For concrete slabs, consider adding a vapor barrier and thickening the slab edges
• In severe cases, lime stabilization (mixing agricultural lime into the clay) can reduce shrink-swell behavior
• Make sure drainage is excellent — clay problems get worse when water is present

For loose or sandy soil:

• Compact in layers using a plate compactor or roller
• Use geotextile fabric between the native soil and any gravel base to prevent mixing
• Do not skip the compaction step — loose soil under a slab will settle unevenly

For fill soil:

• Have it tested for compaction before you build on it
• If it was placed recently (within the last few years), it may still be settling
• When in doubt, remove it and replace with engineered fill that is placed and compacted in controlled lifts

For organic topsoil:

• Remove it. All of it. Organic material under a building pad will compress and decompose, causing uneven settlement.
• Typically you need to remove 6–12 inches of topsoil before placing your base material

The Bearing Capacity Question

Every soil has a bearing capacity — the amount of weight it can support per square foot without compressing. For most residential metal buildings on a concrete slab, you need a minimum of 1,500–2,000 pounds per square foot of bearing capacity.

Most undisturbed native soils in the Mid-South meet this requirement. Fill soils, organic soils, and very wet clay soils may not. If you have any doubt, a simple soil bearing test from a local geotechnical firm costs a few hundred dollars and gives you certainty before you pour thousands of dollars of concrete.

The Bottom Line

Good soil preparation is not glamorous, but it is the foundation — literally — of a building that performs well for decades. Take the time to understand what you are building on before you build on it.

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