The most essential part of construction is compaction. Compacting soil ensures the particles get close enough to form a strong, load-bearing surface that lasts over time. For soil to compact properly, it must be thoroughly moist, but not so moist it turns into mud. This state is called optimum moisture or optimum moisture content.
Let’s explore why optimum moisture content matters for compaction, helpful terms to know, and how to check your soil’s optimum moisture content.
Compaction increases soil’s load-bearing capacity by improving its density—how loosely or tightly packed together soil particles are. Low-density soil is loose and unstable for building. High-density soil is packed tight, with few air pockets. It’s more stable, with greater shear strength and load-bearing capacity. It’s also less water-permeable, leading to fewer surface problems like potholes.
Soil must be at the right density to support infrastructure—and that rarely happens naturally. You must compact the soil, and compaction requires water. Water helps the soil particles move closer to each other to achieve the right density.
Let’s visualize how optimum moisture makes compaction possible.
Imagine you’re making bread. You start with flour, but it won’t hold a shape because it's dry and powdery. You can’t knead it or sit anything on it; it won’t support weight. So, you add wet ingredients like water, oil, and eggs until you can make the flour into a loaf. But be careful—too much water makes a gooey mess!
Soil works the same way. Dry soil particles won't stick together, and neither will mud. You can't compact soil that's too dry or too wet. And if you can't compact it, it won’t support the weight of structures, vehicles, equipment, or pavement. You must add the right amount of water (optimum moisture), so the soil particles will stick to themselves during compaction. Then, you can create a strong, long-lasting foundation.
When calculating soil’s current and optimum moisture content, engineers measure the soil’s wet density and dry density.
All soil naturally contains some water due to precipitation, groundwater, and humidity. This natural water content varies based on type of soil, location, and climate. Geotechnical engineers must calculate soil’s natural moisture content so they can determine the optimum moisture content, as well as how much water to add to achieve it.
The first step to determining optimum moisture is finding the soil’s wet density. That’s the combined amount of water and soil in a given sample, divided by how much space the sample occupies. Engineers use this equation:
Mass ÷ Volume = Wet Density of the Soil
Next, they calculate the soil’s dry density.
The dry density of soil shows how loosely or tightly packed particles are when they're completely dry. Engineers use this equation:
Dry Mass ÷ Dry Volume = Dry Density of the Soil
Let’s revisit our bread example to see why dry density matters. When baking, you add wet ingredients to the dough so you can shape it. The oven dries some of that moisture while baking, but the bread still holds its shape. Likewise, you add water to reach optimum moisture and compact soil. After compaction, the soil dries out while still holding the shape you gave it.
Shaping soil is most effective when you compact it near or at maximum dry density, which is the most compact it can possibly be. Sometimes, people call this number MDD or just dry density. Once they know the soil’s dry density, engineers can determine:
Percent compaction is another name for maximum dry density. Percent compaction tells you how much to compact your soil so it’s dense enough to, for example, make a dirt road that withstands heavy vehicle traffic.
Engineers talk about percent compaction on a scale from 0-100. At 100% compaction, the soil is as dense as possible. Most construction projects require soil to be at 90-95% compaction, or even higher for some heavy-traffic roads. Remember, your soil must be at optimum moisture to achieve the proper percent compaction.
Because each soil’s wet and dry densities vary, optimum moisture varies, too. The two primary ways to determine your soil’s optimum moisture content are the proctor test and the hand test.
Geotechnical engineers determine the optimum moisture content of soil by performing a proctor test, which establishes the maximum unit weight at which you can compact the soil.
A one-point proctor test can give approximate results in the field, but you’ll get more detailed results with a laboratory proctor test. Lab proctors can be standard or modified. Standard proctors work better for dirt embankments and building pads, while modified proctors are better for heavily loaded pavements.
We’ll cover the basic steps of a standard proctor, also called an ASTM D-698 or AASHTO T-99. (ASTM and AASHTO are American organizations that set engineering standards for soil tests to ensure safety and durability.)
During proctors, engineers:
Compaction curves visualize the soil's maximum dry density and optimum moisture. By looking at your geotechnical firm’s compaction curves, you can see that overly wet or dry soil won’t achieve maximum dry density and, therefore, will not have optimum structural properties.
Another advantage of the proctor test is that it determines the soil’s current moisture content and optimum moisture content, so during construction, you can calculate how much water to add with this equation:
Soil Weight x (Optimum Moisture Content - Current Moisture Content) =
Water Weight to Add
Laboratory proctor tests are ideal because they give you highly accurate data. But in the field, you can’t stop mid-construction to send soil samples back to the lab. It’s vital to be able to quickly identify the soil’s moisture content on your own.
You can determine the soil’s approximate moisture content by performing a hand test. Simply reach down and grab a handful of soil. Then, try shaping the soil into a ball. You’ll get results almost instantly.
Shaping a moist sample of soil into a ball is easy when the soil’s at optimum moisture. When performing a hand test, think like Goldilocks: you don’t want soil that’s too wet or too dry. You want soil that’s just right!
Performing hand tests in various locations around the jobsite quickly tells you the soil’s moisture content. Just be sure you get a realistic sample. Sometimes, the soil’s top layer is dry because of the climate or sun, but underneath it’s well-saturated. It’s okay to dig into the soil a couple inches to check the moisture at varying levels.
Most construction requires you to adjust your soil’s moisture content multiple times throughout the project—not just at the beginning. That’s because soil conditions are constantly changing due to weather; plus, soil begins drying out as you work with it.
Overly dry soil leads to poor compaction. The soil is not lubricated enough for the particles to move close together; instead, they create friction and repel each other. That leaves air voids in the soil, inhibiting the extremely dense surface you need.
Additionally, dry soil makes it difficult to mix in soil stabilizers like Perma-Zyme, an enzyme-based formula that hardens soil to make surfaces last longer with less maintenance. Water acts as Perma-Zyme's vehicle to move through the soil; it won't mix adequately with dry soil, producing unsatisfactory results.
Thankfully, adjusting dry soil is easy. Simply fill a water truck and evenly spray the dry area. For extra dry soil or far-away water sources, it’s wise to have multiple water trucks onsite. One or two trucks can spray the surface while the other truck refills.
When soil is above optimum moisture, it turns into mud. At this point, water has displaced the soil particles, making them hard or impossible to compact. The soil particles become like a bunch of inner tubes floating in a pool: they may get close, but they won’t stick together.
In this case, the soil must dry out before compaction. You can let the soil air dry, or speed things up by using a motor grader to windrow the soil side to side. This exposes it to air and helps it dry faster.
To recap, optimum moisture content is the ideal amount of water soil must contain so you can compact it into a strong, safe, load-bearing surface.
You can determine your soil’s optimum moisture content by hiring a geotechnical firm to perform proctor tests in a lab and monitor the soil’s moisture content onsite with hand tests. Maintaining the moisture content of the soil throughout construction is vital to creating long-lasting paved roads, unpaved roads, and other projects.
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