Working with clay soil is tough for any project—from county roads to multi-acre drilling pads to small home gardens. To succeed, you must understand clay’s characteristics and how to work with them.
We’ll walk you through the basic traits and types of clay. Then, we’ll explore its pros and cons and teach you how to work with clay soil more effectively. Let’s dig in!
The five main soil types are gravel, sand, silt, clay, and loam. Most soils are loam, a mix of two or more other types. Clay loam contains 27-40% clay. For soil to count as “pure” clay, it must contain at least 40% clay particles.1 These clay particles are the smallest of all soil types, measuring less than two one-thousandths of a millimeter, providing a fine, smooth texture.
Clay forms when air or water breaks down rocks and organic matter. It accumulates in deposits, which geologists name for where they form:
Many countries worldwide—including Germany, France, Turkey, Ukraine, and Russia—have large clay deposits. America’s Midwest and Southeast are especially clay-rich.
Clay contains organic matter, such as decomposing plants and animals. Its negative ionic charge helps it retain positively charged nutrients like potassium and magnesium. However, clay often lacks phosphates. Most clays have an alkaline pH of 7.5 to 10, but some are neutral or mildly acidic.
As the most plastic soil, clay is easiest to shape and compact. Most clays retain water easily, so they swell and clump when wet and shrink when dry. Highly expansive, or “heavy,” clays change shape the most.
Clay comes in many colors. Check out how they occur:
Fun fact: Blue and green clays can be antibacterial.2
We’ll discuss clay’s numerous uses throughout this blog. First, let’s start with the highlights:
The many types of clay soils each have their own characteristics. We’ll explore them by particle size and texture first, then formation and behavior. For bonus points, we’ll explain the difference between clay soils and clay minerals.
The Unified Soil Classification System (USCS) categorizes soils by particle size and texture. Clays are fine-grained soils, with over half their grains passing the #200 sieve. Each clay has a two-letter abbreviation based on its texture and plasticity:
|
USCS Clay Type |
Description |
|
CL |
Lean and inorganic clays with low to medium plasticity |
|
OL |
Silty clays with low plasticity |
|
CH |
Inorganic high plasticity clays or fat clays |
|
OH |
Organic medium-to-high plasticity clays |
Soil orders group soils by how they form and behave. Of the 12 soil orders, seven contain clay:
Together, these seven soil orders account for two-thirds of America’s ice-free soil surface.5
Clay soils differ from clay minerals. A mineral is solid, inorganic matter. Soil is the upper layer of earth, containing both minerals and organic matter. Clay soils contain clay minerals; in fact, you may hear people call soil by mineral names. Geologists have categorized clay minerals into nine groups. We’ll cover how a few of these affect clay soil.
Kaolinite is one of the softest, smoothest clays. It’s good for making paper, ceramics, tiles, and cosmetics. It has a low shrink-swell capacity because it retains less water than other clays, so it’s most common in well-drained tropical and subtropical regions.
Fun Fact: Kaolinite is a kaolin-serpentine mineral; this group also includes the humorously named lizardite.
Smectite excels at retaining water. Drainage… not so much! It has high shrink-swell potential (like the puffer fish from Finding Nemo). It gets sticky when wet and cracks when dry; you’re more likely to see cracked smectite in the wild since it usually forms in dry regions. People use it in oil drilling mud, pond sealant, landfill liner, and cat litter.
One common smectite mineral is montmorillonite. (Say that 10 times fast!) Another is bentonite, which is currently popular in detoxifying skincare products.
Illite is a mica clay mineral. It has good water retention with moderate shrink-swell capacity, making it ideal for agriculture. Its fine, flaky particles offer a good texture for pottery and brickmaking, but they can be dusty and difficult to compact. You’ll find this mineral in temperate climates, where people use it to construct adobe or earthen homes.
Chlorite has a low shrink-swell capacity and gets its unique green color from its high magnesium and iron content. (Chloro comes from the Greek word for green.) Chlorite resists high temperatures and can improve soil fertility thanks to its structure and silicate content. It occurs in areas with volcanic ash and rock.
Mixed-layer clays are what they sound like: layers of minerals or soils containing at least two types of clay. Their behavior and uses depend on the dominant material. You’ll typically find mixed-layer clays, like rectorite and corrensite, in transition zones between different soil types.
Each clay behaves differently based on its unique traits, so geotechnical testing is the best way to learn what to expect from your soil. That said, understanding clay’s basic pros and cons will help you make informed decisions for your projects.
Clay’s primary benefit is its ability to hold things, such as nutrients, water, or its own shape. Here’s why this trait is useful.
Clay retains nutrients well, which plants can easily draw from to grow and yield produce. Consider illite: its high potassium content is ideal for grains and legumes. Meanwhile, chlorite can fix magnesium deficiencies in other soils when you blend them.
Since clay stores water, plants that grow in it are less likely to suffer from drought. Check out how different clays’ water retention impacts their use:
Plasticity describes soil’s ability to stretch and hold shapes without breaking. Clays are more plastic—and thus better at holding their shape—than other soils. That’s why they’re useful for making construction materials like bricks and tiles.
Clay compacts thoroughly during construction because it holds moisture and shape. Plus, its tiny particles can pack closer together than larger soil particles, preventing air voids for more structural integrity, load-bearing capacity, and erosion resistance. Once compacted, clay generally stays put under consistent moisture conditions.
Some of the same traits that make clay useful also make it challenging to work with. Let’s take a look.
Water retention causes poor drainage, especially for heavy clays like smectite. Prolonged saturation weakens the soil, delays construction, and causes root rot in plants. Meanwhile, new water—i.e., rain or snowmelt—pools on the surface or runs off the side, causing erosion and floods.
Another downside: water moves through clay slowly. Clays are therefore slow to warm up in spring because water heats slower than minerals. In winter, they’re prone to frost boils as water freezes and thrusts the soil upward. All told, clay’s poor drainage creates an endless maintenance cycle.
Soggy, swollen clay sticks to machines and is more difficult to compact. After construction, clay keeps shrinking and swelling—and it may permanently deform. This can cause uneven surfaces, potholes, cracked pavement, foundation settling, and structural instability.
Some good news: kaolinite and chlorite swell less, so they’re somewhat more stable. However, chlorite works better as a supplemental material because its lower plasticity makes it harder to shape.
Clay is prone to all types of erosion. For example, wind sweeps away clay dust, heavy traffic washboards steep grades, and rain washes away soggy slopes in landslides. In every case, erosion creates structural instability that’s difficult to repair.
As easily as clay can retain water, it can also dry out. Clay soils are prone to cracking, damaging plant roots. They’re also dust-prone because their minuscule particles are light and blow into the air.
Clays are the weakest soils. They have low shear strength and score poorly on r-value and CBR tests. Foundations built on clay may settle over time. Unpaved roads are prone to ruts, potholes, and washboarding under traffic, and paved roads may shift or crack if the clay fails to support them.
Clay may compact well at optimum moisture, but if it’s too wet, it won’t compact at all. And sometimes, it over compacts. It becomes unworkable, suffers more surface erosion, and suffocates plant roots. Heavy traffic—even animal traffic—can over compact clay. In fact, some ranchers remove their livestock from high-clay areas during wet seasons to keep this from happening.
As we mentioned, clay can take on positively charged nutrients. That’s great—but positively charged contaminants, like pesticides, can also enter and stick to clay. Not good!
Here at Substrata, we specialize in stabilizing unpaved clay roads and surfaces. These best practices will help you make clay stronger and easier to work with.
Since clay is sensitive to environmental conditions, it’s best to build during optimal weather. Skip rainy or cold days when possible, and stick with dry weather with a temperature over 40°F.
Uneven grading lets water pool in low areas, exacerbating clay's natural tendencies to swell and erode. Laser and GPS-guided technology come in handy here, but even with old-school tech, well-trained operators using well-maintained machines can get the right grade.
Compacting clay in thick layers can lead to unevenness and voids, making it unstable. We recommend using six-inch lifts to achieve maximum compaction in every layer. Another tip: Take advantage of clay’s fine, cohesive particles by using a padfoot compactor for aggressive compaction.
At optimum moisture, clay contains enough water to compact fully for greater load-bearing capacity. Check your soil’s moisture often during construction and adjust it as needed.
Overcompaction can make soil too hard, causing water runoff and drainage issues. Generally, clay has reached maximum compaction when tiny rocks on the surface break under a roller instead of pressing into the soil.
Loosely compacted clay can absorb more water. More water = more swelling. So, compact your clay thoroughly to reduce permeability. Then, add a crown, raised turns, and ditches to keep water from pooling on the surface.
Soil stabilization alters clay’s properties to make it more suitable for its intended use. Because clay is so unstable, people often remove it from jobsites entirely or use soil stabilizers to harden clay to the desired load-bearing capacity and reduce shrink-swell potential.
With the right soil stabilization, your clay soil will be stronger, support heavier loads, resist erosion, shrink and swell less, and drain better—all so your structures can last longer. Of course, the key is finding a cost-effective soil stabilizer that gives real, lasting results.
That’s where Perma-Zyme comes in. This all-natural enzyme soil stabilizer permanently binds clay particles together into a hard, concrete-like surface that lasts 10+ years. Say goodbye to erosion and endless maintenance! With Perma-Zyme, our customers reduce their yearly maintenance costs to as low as $0, and they save an average of 60% on construction compared to traditional methods.
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