Strong roads start with strong soil. When soil stays in place, the road’s surface lasts longer and stays safer. The trouble is that the soil in many parts of the world contains expansive clays that are difficult to build on. We’ll explore what these soils are and the problems with building on them. Then, we’ll compare different clay stabilizers to help you build better, stronger roads.
Clay is one of the four main soil types (the others are silt, sand, and gravel). Loam soil contains a mix of these types; most clayey soils are actually clay loams, meaning clay is the dominant soil type in the mixture. The more clay that a loam contains, the more that loam will take on the clay’s properties and behavior.
Clay has the smallest particle size of all soil types, but those particles are packed with nutrient-rich organic matter from dead plants and animals. The minerals and organic matter inside the clay help determine its color. Several subtypes of clay soil are named for their color, such as blue, red, and yellow clay.
Clays are plastic—aka stretchy and easy to mold into shapes. They swell and shrink due to changing moisture. Highly plastic clay soils are called expansive clays because they swell the most when wet. They can become extremely soupy and sticky, hence their nickname gumbo clay. You may also hear people call them fat clay or heavy clay because the particles weigh more when they absorb lots of water.
Clay’s plasticity sounds good for construction. It should make roads easy to shape and build, right? Not quite! Unfortunately, all that plasticity makes construction more challenging for both unpaved roads and paved roads. In this section, we’ll cover six of the main problems with building on clay soil.
Clay soils are so sensitive to water that they can change shape any time their moisture content changes during the construction process. And if those moisture changes are inconsistent or “spotty,” the results will be, too.
For example, the crew may start by wetting the clay to reach its optimum moisture content (the amount of water soil must contain to fully compact). But as the day goes on, air and sunlight begin to dry the clay, causing it to contract unevenly on different parts of the jobsite. That makes it difficult for the crew to compact expansive clays uniformly, so the soil may not retain a consistent shape when it dries.
Expansive clays are heavy, clumpy, and sticky when wet. If the construction crew oversaturates them, these soils can clog or stick to trucks and heavy equipment. That can damage the machine—or at the very least, force operators to spend extra time cleaning out the gunk instead of working. Both of those things can slow down the project timeline significantly.
Expansive clay soil under roads will get wet due to groundwater, rain, and other moisture seeping in. That makes its particles change shape—and some changes could be permanent. And when expansive clays dry out, they often contract and change shape again. They can even harden into a concrete-like material that’s extremely difficult to break up.
That’s bad news for road construction. Any soil that shifts under the roadway will cause structural failures and safety issues on the roadway. (Hello, potholes and collapsing asphalt!)
Clay soils drain poorly because they absorb water rather than letting it filter through their particles. That can cause problems. For example, if the soil is waterlogged when the temperature drops, the water in the soil can freeze and damage the road’s subgrade. Poor drainage also means clay takes longer to thaw in the spring than other soils because water warms slower than minerals.
Think about a time when it didn’t rain for days, and you saw soil that developed large cracks. That soil probably contained large amounts of clay. When it dries, clay becomes extremely brittle and cracks apart. That makes unpaved clay roads difficult to drive on. They also become even more vulnerable to potholes, ruts, and freeze-thaw damage because water can seep into those cracks—deteriorating the road from the inside out.
Because clay particles are so fine, they dry quickly. And it only takes a little breeze to send them soaring into the atmosphere as road dust. Road dust poses serious health and safety risks. It causes low visibility for drivers, creates air-quality issues for nearby communities, pollutes water, and coats plants (and solar panels) so they can’t absorb sunlight. So, people who build and manage unpaved clay roads find themselves locked in a constant battle against dust.
The type of soil stabilizer you choose will depend on whether you’re building a dirt road or a paved road. Let’s take a look at some traditional methods for each road type.
When you build on expansive clay, you’ll need a soil stabilizer that’s durable enough to hold clay soil particles together. There are only a few options for treating unpaved clay roads. Check ‘em out.
A layer of gravel filters out water, so less of it soaks into the road. This decreases the clay’s potential expansion, as well as the likelihood of impassable mud or ruts. This option is widely available to most people: quarries worldwide produce over 16.5 billion tons of aggregate each year!1
The downside is that gravel doesn't last forever. Storms can wash away gravel, and traffic can fling it off the road or work it into the clay. Most applications only last a few months—maybe a year if you’re lucky. The cost of applying gravel to clay roads adds up over time, making it expensive long-term.
Polymers are chemical soil stabilizers that can either work like glue to bind soil particles together for greater strength or work like soap to lubricate the soil for better compaction. You can use them as a topical spray or inject them straight into problem areas.
These soil stabilizers work best on coarse soil. Since clay is anything but coarse, you’ll need a clay loam that contains plenty of sand or gravel to make them work. Polymers also tend to be pricey and may begin to break down after a year or two.
Chlorides are road salts that pull moisture from their surroundings and wet the clay particles. This makes them heavier, so they can’t fly up into the air as dust. And since chlorides form a crust over the soil’s surface, they also help stabilize unpaved clay roads to prevent erosion.
Chlorides are effective—especially at dust control—but they do have significant drawbacks. They only last three to six months, so you’ll have to reapply them multiple times a year. They only work in humid climates. They’re corrosive to cars and concrete bridges. And they can build up in water and soil, permanently contaminating them. Clay soils are especially prone to chloride contamination due to their absorptive nature.
As you well know, paved roads contain several layers: a soil subgrade on the bottom, an aggregate subbase in the middle, and an asphalt or concrete topcoat. But, building on a clay soil subgrade can cause the pavement to crack and fail because of the clay’s constant shrink-swell cycles. Roadbuilders must stabilize the clay before adding any more layers to the road. So, let’s take a look at the traditional soil stabilizers for the job.
During over excavation, the construction crew removes most or all of the expansive clay from the jobsite. Then, they import more stable materials like sand and Type II aggregate to fill the void. Over excavation is common because it removes all the problems of building on expansive clay. After all, the soil can’t give you any trouble if it isn’t there!
However, over excavation is expensive and time-consuming because it requires digging up and hauling out the old material, then hauling in and compacting the new material. So, you’ll pay the trifecta of major road construction costs: material, hauling, and labor.
Cement and lime are common clay stabilizers. They’re both binders, meaning they hold expansive clay together when you add them to the soil. This process often increases the soil’s strength and adds a chemical bond that helps decrease the likelihood of the soil expanding or contracting later on.
The largest downside to lime and cement-treated base is the cost, as it usually takes a lot of material to stabilize soil with these options. Additionally, prices and local availability can be unpredictable and fluctuate from region to region. That’s why these methods are most common for paved roads; they’re usually cost-prohibitive for unpaved ones.
The best way to prepare clay for road construction is to use a purpose-made soil stabilizer that’s designed to treat clay’s unique properties, like Perma-Zyme. Since 1972, Perma-Zyme has been stabilizing unpaved clay roads and clay subbases for paved roads.
Perma-Zyme is a powerful, enzyme-based soil stabilizer that doesn't just counteract expansive clay; it works with clay to stabilize roads. The enzymes in Perma-Zyme contain positively charged cations that chemically react with the negatively charged anions found in clay particles. This chemical reaction permanently binds soil particles together, creating a hard, concrete-like surface that’s up to 13 times stronger than untreated soil alone.
Applying Perma-Zyme is so simple that it can take as little as one day, using the equipment you have on hand. Here’s how to do it:
You can also use Perma-Zyme to lock in a gravel topcoat on unpaved roads, giving drivers more traction and reducing maintenance by preventing material loss.
Because it’s made to work with clay, Perma-Zyme has significant advantages when you compare it to traditional soil stabilizers:
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