Soil’s Atterberg limits are essential to know for successfully constructing roads, buildings, and other projects. But, they’re about as clear as mud to most people. So, we’re here to share what the Atterberg limits are, why they matter, and how we test them in our lab.
The Atterberg limits are the critical water contents at which fine soils change from one state to another. Since that’s a highly technical definition, let’s break it down.
All soil contains water, and the amount of water in the soil is its water content—aka moisture content. Soil’s moisture content changes based on temperature, rainfall (or lack thereof), and other factors. That’s why wet soil gets muddy and dry soil gets dusty.
When it gains or loses enough water, soil’s consistency changes, and the point at which that happens is the critical water content. Soils change consistencies at three different critical water contents, which are the Atterberg limits.
There are two coarse soil types (gravel and sand) with large particles and two fine soil types (silt and clay) with small particles. The fifth soil, loam, contains a mix of other types; it varies from mostly coarse to mostly fine. All five soils contain moisture, but fine soils react more to changing moisture contents than coarse soils. These reactions significantly impact construction, so you must know if your jobsite’s soil is fine or coarse.
Coarse soils behave consistently regardless of moisture content. For example, wet gravel is as hard as dry gravel and compacts the same. So, coarse soils’ Atterberg limits are irrelevant because they don’t react to changing moisture.
Fine soils are the opposite. Wet clay swells and softens, while dry clay shrinks and hardens. Both can become unworkable. Clay and silt must have just the right water content to achieve a consistency that’s possible to shape and compact. At that consistency, fine soils become cohesive. They stick together, making them easy to shape and compact.
The Atterberg limits reveal when clay, silt, and mostly fine-grained loams will reach this ideal state.
When we refer to soil’s state, we mean its consistency based on its moisture content. Fine soil has four possible states:
Soil’s volume (the amount of space it takes up) changes along with its state, since water in the soil increases its volume. Thus, a solid soil sample occupies less space than the same soil in its plastic state.
Fun fact: Coarse soils can’t absorb enough water to become plastic or liquid… so their Atterberg limits don’t exist!
To sum it up, the Atterberg limits are lines that clay- or silt-based soils cross when they officially switch consistencies because they’ve gained or lost moisture.
There are three Atterberg limits:
The Atterberg limits test helps project planners and contractors classify soil, plan to stabilize it, and make the soil workable for construction.
Soils change states at different moisture contents, so Atterberg testing helps classify soils based on their plasticity and reactions to moisture. In other words, it tells contractors and engineers about their soil’s clay and/or silt content.
Typically, high-plasticity soils contain more clay, while low-plasticity soils contain more silt. That’s because clay usually retains more water and is more cohesive than silt, so it remains plastic under greater moisture variations.
Now, you might be wondering, What about loams? Great question! Loams tend to act like the soils they contain, in proportion. For instance, if a loam is predominately clay, it will act more like clay than anything else. Atterberg limits testing helps geotechnical engineers determine if loam contains more silt or more clay.
Most soil can’t naturally support the weight of buildings or traffic. Instead, it shifts—especially when changing moisture makes it expand or contract—and damages the structure. So, before contractors and engineers start building, they plan how to stabilize soil so it won’t move.
And remember, soil type dictates soil behavior. The Atterberg provides information about soil’s clay and silt content so builders can predict how it will behave during and after construction. Then, they can choose the right soil stabilizer to keep the structure safe. Knowing the Atterberg limits also helps them account for soil’s response to environmental conditions, like precipitation and groundwater, post-construction.
Plastic soils are easiest to work with during construction. Clay and silt will remain plastic at any moisture content between their liquid limit and plastic limit. This range of moisture contents is called the plasticity index. The Atterberg test reveals the plasticity index, so builders know when to add water or dry the soil to keep it plastic. This lets them make soil work on their schedule, not nature’s. (That would take forever!)
Atterberg limits testing also helps them compensate for environmental events during construction—like rain or extreme heat—that change soil’s moisture. Knowing how to maintain the proper consistency lets them avoid costly, time-consuming slowdowns.
At Substrata, we conduct Atterberg limits tests in our lab to make sure our customers’ soil will work with our eco-friendly, enzyme soil stabilizer, Perma-Zyme.
Perma-Zyme reacts with clay particles in the soil to create a hard, concrete-like surface that can last 10+ years, and it requires thorough compaction to achieve maximum strength and durability. So, we conduct several soil tests, including an Atterberg test to check for high plasticity—which indicates that the soil contains clay and will compact properly.
Thanks to Terzaghi and Casagrande, we follow a standardized Atterberg testing process called the ASTM D4318. (ASTM stands for American Society of Testing and Materials; they standardize soil tests in the U.S. D4318 is the Atterberg test’s identification number.)
The five basic steps we follow during an Atterberg test are:
The Atterberg test begins with 250 grams of soil that passes through the #40 sieve. We usually air-dry the soil, but if it’s really wet or needs fast results, we oven-dry it at 150°F.
We put the dry soil in a glass bowl and gradually mix in water until it reaches optimum moisture, the point at which soil can retain a shape without crumbling or oozing. It should now have a peanut buttery consistency, so we can fold it with a spatula.
Once the soil is the right consistency, we split it into two portions: half for the liquid limit test, half for the plastic limit test.
First, we place the soil in a brass cup affixed to an Atterberg testing machine called a motorized Casagrande device (after Arthur Casagrande). The soil should cover about half the cup. Then, we draw a 13-millimeter groove down the middle with a grooving tool.
Here’s where things start to get fun—and loud. We turn on the Casagrande device so the brass cup repeatedly drops onto a metal plate at a controlled rate of speed. When these blows make the soil collapse into at least half an inch of the groove, it has reached its liquid limit.
The more strikes soil can tolerate before it collapses, the more plastic it is. Typically, the machine strikes the cup 15 to 30 times before most soils collapse.
Now, we must calculate the soil’s moisture content at its liquid limit. We scoop a portion of the soil from edge to edge of the bowl—including soil that fell into the groove—and place it in an empty container called a moisture tare. We weigh the soil and tare. Then, we subtract the tare’s empty weight (which we already know) from that figure to find the wet soil’s weight.
Once we weigh the wet soil, we put it in an oven to dry.
After the first trial, we clean the testing machine and grooving tool. We take fresh soil from the glass bowl and repeat the liquid limit test. Then, we do it a third time. Running three trials helps us pinpoint the liquid limit more accurately.
During each subsequent trial, we increase the soil’s moisture. The rising water content should decrease the number of blows the brass cup must strike to collapse the soil. Each trial should look something like this:
If the soil always collapses before the cup drops 25 times, it isn’t plastic. At that point, we record these results and stop testing it. No sense looking for a plastic limit on a non-plastic soil!
When the soil collapses at 25 blows, it’s at the liquid limit. As you can see from the number of blows listed above, the soil could reach its liquid limit during any of the three trials. It’s essential to conduct all three trials—even if the groove closes at 25 blows in the first or second trial—to make absolutely sure that we have found the liquid limit.
Once we’ve completed the three trials, it’s time to calculate the soil’s moisture content when it’s at the liquid limit.
To finish the liquid limit test, we must determine the moisture content at which the soil became liquid. So, we weigh the three oven-dried samples and calculate each one’s moisture content with this formula:
So, as you can see in this example, the soil reaches its liquid limit when it contains 17.6% water.
Now, it’s time to grab the soil that we set aside for the plastic limit test. We press this soil with paper towels to squeeze out water (super high tech, we know!) until it becomes dry enough to roll into a shape without sticking to our hands.
Next, we roll 1.5 to two grams of soil into a ball. We set the ball on a glass plate and use our palms and fingers to roll it into a ribbon one-eighth of an inch, or three millimeters, thick. (Some lab techs spread the moistened soil onto the glass plate without rolling it into a ball first and use a thin, metal bar to roll it into the ribbon, but we like playing in the dirt with our hands—it’s more fun!)
Whichever technique you choose, highly plastic soil can take on this ribbon shape. But if the soil’s plasticity is too low, it’ll break before the three-millimeter mark. Non-plastic soil won’t roll at all.
Once the ribbon reaches the right diameter, we break it into pieces, squeeze them together, and re-roll them into another ribbon. This process dries the soil slightly, bringing it closer to its plastic limit. We repeat this until the thread crumbles under pressure and won’t roll to the three-millimeter diameter. Then, we collect those broken pieces in a moisture tare of known weight and cover them.
We then repeat the plastic limit test for two to three more soil samples—enough so that the moisture tare contains at least six grams of used soil. We weigh this wet, used soil. Next, we oven-dry it, weigh the dry soil, and calculate its moisture content using the formula from earlier to find its plastic limit.
The “ideal” result varies because different projects need different things from their soil. For Perma-Zyme customers, we generally recommend a plastic limit of 7% or more, but even this may vary by project.
Finally, we calculate the soil’s plasticity index, or PI—the range of moisture contents at which it remains plastic and is ideal for construction. This process is super simple because we just subtract the plastic limit from the liquid limit: LL - PL = PI.
So, what about non-plastic soil? If the soil is non-plastic, we report the PI as 0. No plasticity means no plasticity index.
If you’re wondering why we skipped the shrinkage limit test, great question! This test helps builders determine soil’s potential for cracking and how it could affect their projects. But, we don’t perform it because it’s not important for our customers. (Sorry, shrinkage limit.)
Our customers need soil to remain plastic during construction, without becoming liquid (liquid limit) or semi-solid (plastic limit). As long as it stays between those two lines, it’s good. After construction, Perma-Zyme cures into a solid surface that won’t shift or crack, rendering the shrinkage limit irrelevant.
However, if you were to perform a shrinkage limit test, you’d do so after the plastic limit test. Here’s a basic overview of how it’s done:
Every geotechnical firm shares test results with their customers, and we do the same here at Substrata. We provide a written report of the Atterberg results to people who want to use Perma-Zyme, and then, we take it a step further.
We meet with our customers individually to make sure you understand your results and how they’re going to affect your project. We’ll even provide tailored recommendations to make sure you get the best possible results from your Perma-Zyme treatment so you can enjoy maximum durability and soil stabilization for years to come.
And that’s that! Now you know how the Atterberg test works, why it matters, and how we conduct these tests for customers who stabilize their soil with all-natural, eco-friendly Perma-Zyme.
Note: Due to U.S. importation laws and high demand, we can only conduct Atterberg tests for U.S.-based public and commercial customers.
Atterberg testing came along at a crucial time when construction—and society—were changing. And while you don’t have to know the history to understand how this test works, you may find it useful for understanding modern applications. (Don’t worry; we won’t quiz you!)
During the Industrial Revolution in the mid to late 1800s, manufacturers mass-produced goods for the first time. Unskilled laborers moved from the countryside to cities to work in factories, bringing their families along. As population density increased, people constructed bigger buildings and more railroads and roads.
By 1891, the first U.S. towns started paving those roads to prevent erosion and make them more passable.1 Unfortunately, neither paved nor unpaved roads could support the increased traffic weight, and newspaper editorials chronicled the dilapidated conditions.2
Equally problematic, many buildings were too heavy for the soil they were built on. Chicago contractors built the city’s first skyscrapers on hard clay with a layer of soft, oozing clay underneath. The lower layer shifted, taking the hard clay and buildings with it. The post office sank into the ground. City Hall collapsed and caught fire. The Board of Trade developed a 10-inch crack in an exterior wall.3 And that was just in one city! These problems occurred alongside industrialization and migration worldwide.
In 1911, a Swedish scientist named Albert Atterberg defined the four states of soil and discovered that only clay and silt could become plastic or liquid. He also identified the limits at which they change states. Then, in a fit of humility, he named them after himself—hence, the Atterberg limits.
Fun fact: Atterberg also suggested that any soil with particles smaller than 0.002 millimeters in size should be classified as clay, a classification we still use today.
In the late 1920s and early 1930s, two engineers at MIT—Karl Terzaghi and his assistant, Arthur Casagrande—worked to refine and standardize Atterberg’s tests. We still use their methods today. These standardized procedures ensure consistent soil classifications, regardless of location or who’s performing the test. And correct soil classification is crucial for knowing how to achieve optimal results during construction.
The Atterberg limits test doesn’t provide a magic formula to tell us how to solve our soil problems, but it does something better: give people information. Precise information about soil type and behavior gives builders the freedom to try various solutions and develop new soil stabilization techniques. And at the end of the day, that’s the best thing that could happen to construction!