Compaction is the process of pressing soil particles together to reduce the space between them.
When topsoil is compacted it can have a significant negative impact on crop yield.
On the other hand, compaction of the subsoil has a range of benefits in construction.
In this blog, we’ll walk through everything you need to know about soil compaction and all the factors that impact it.
1. What is soil compaction?
Soil compaction is the practice of applying mechanical compaction effort to densify soil by reducing the void space between soil particles.
In short, compaction occurs when particles are pressed together to reduce the space between them.
Highly compact soils (those with very few spaces) have a higher unit weight than non-compacted soils.
This compaction decreases the likelihood of settlement after a building, roadway, runway, or parking lot is constructed.
Settlement could result in premature pavement failure, costly maintenance, or repairs.
Soil compaction provides a strong working platform and is an essential part of the construction process.
2. Why is soil compaction important?
Soil compaction is important because it increases the bearing capacity and stiffness of in-situ (natural state) or chemically modified soils.
As noted above, soil compaction also decreases the likelihood of settlement after construction.
You’ll want to keep soil compaction in mind for buildings, roadways, runways, or parking lots.
You can decrease the future settlement of soils by increasing the stiffness and eliminating voids.
Removing voids reduces the chance of soil settling, shrinking, or expanding.
It also decreases water seepage that would contribute to the deleterious shrinking and swelling of soil.
If you’re asking yourself the simple question of “why compact?” here’s your quick answer in five points:
It increases the load-bearing capacity
It prevents soil settlement and frost damage
It provides stability
It reduces water seepage, swelling, and contraction
It reduces the settling of soil
3. What are the four types of soil compaction?
There are four types of compaction on both soil and asphalt:
Vibration
Impact
Kneading
Pressure
4. What’s the difference between soil compaction and soil consolidation?
Soil compaction occurs when stress is applied to soil and air is displaced from the pores between the soil grains.
Soil consolidation occurs when stress is applied and causes densification due to water (or other liquid) being displaced between soil grains.
5. What factors affect soil compaction?
Type of soil
Not all types of soil will respond the same way to compaction.
Soils are classified by their particle size and (in some soil categories) critical water content value (also known as Atterberg limits).
Certain types of soil, such as well-graded granular soils, are more easily compacted and thus eliminate voids due to their interlocking particles and resistance to moisture absorption.
On the other hand, poorly graded soils contain a narrow range of particle sizes and are harder to work with for construction purposes.
They do not typically have interlocking particles because of their similar sizes, and this can make them difficult to compact.
Moisture content
Water content is highly important in soil compaction.
In fact, the maximum dry density is only achieved when the water content is at its ideal level.
This is known as OMC or optimum moisture content.
When soil is too dry water trucks can be deployed to spread water and raise the water content to the acceptable range of optimal moisture content.
When soil is too wet, it may need to be dried prior to compaction.
Types of compactors
Soil compaction rollers are available in a variety of styles, including single or double drums, vibratory mechanisms, or dozer blades.
Here are just a few of the types of compactors available to you:
- Smooth rollers
- Padfoot and tamping foot rollers
- Pneumatic rollers
Thickness of lift
Soil compaction can involve compacting “multiple lifts” or layers of soil.
This is done until the overall thickness is achieved.
Each lift’s stability will depend on the one below it.
The proper soil compaction of each layer is critical and must be monitored.
Lift thickness typically ranges from 8 to 14-inches depending on the specifications.
Be sure to establish the correct lift thickness as having layers that are too small or large can result in issues like poor compaction, compromise stability, excessive costs, and unnecessary time.
Contact pressure
Another factor related to soil compaction includes the contact pressure between the soil and equipment.
Contact pressure is impacted by the overall weight of the compaction equipment and the area of soil contacted by the equipment.
When there is higher contact pressure, more soil compaction is achieved.
Speed of rolling
There are two ways to go about soil compaction.
You can compact soil quickly, which allows a greater area to be compacted.
Or you can compact soil slowly, which allows time for additional vibrations at a given point resulting in better compaction.
Number of roller passes
This is one of the factors that can impact whether something is over compacted, under compacted, or just right.
The number of passes needed to achieve the desired compaction depends on the contact pressure and speed of the equipment.
Other factors that affect this include soil type, moisture level, lift thickness, and type of compactor.
A lighter piece of equipment that has less contact with the soil will need a higher number of passes over the same soil to achieve the desired density.
On the other hand, a heavier piece of equipment with a larger contact area will require fewer passes to achieve the desired density.
That said, you’ll reach a point where more weight and/or lower travel speed will have diminishing returns.
Thus, using a heavy compactor slowly is not always the most efficient option.
We recommend a test section to determine the roller pattern that works best.
6. How does soil classification affect soil compaction?
Soil types are classified by a couple of different factors, including particle size and Atterberg limits.
Different soil types react differently to compaction efforts with particle size and critical water values playing the largest role in soil compaction.
In terms of the best soils for construction, look for well-graded soils that contain a wide range of particles.
These can be easily compacted because they have interlocking particles and better moisture resistance and absorption.
On the other hand, poorly graded soils generally contain a narrow range of particle sizes and are less conducive for construction purposes.
7. What are some myths about soil compaction?
There are two common myths regarding soil compaction in an agricultural context.
The first myth is that freeze-thaw cycles will alleviate a majority of the soil compaction created by machinery.
This belief originated years ago when compaction would have been relatively shallow.
The second myth is that deep tillage or subsoiling will take care of over-compacted soils without additional assistance.
8. What are the biggest soil compaction mistakes?
Even for an experienced contractor, soil compaction mistakes are common.
As a result, you’ll want to take your time to avoid any problems.
The two most common issues can be split into two categories: over-compaction and under-compaction.
Over-compaction occurs when an operation makes too many passes in one direction with a compaction machine.
This will start to break down the soil and change its composition.
Under-compaction occurs when the operator does not make enough passes with a compaction machine.
As a result, the particles in the soil will be too soft and won’t get the cohesion they need to create the right amount of density.
Another common mistake that can occur pertains to the amount of lift or depth of the soil layer.
If you’re filling a trench with soil, you must place the correct amount of dirt in the trench to provide the necessary amount of compactive force to the area.
With an incorrect lift (too much dirt and too much depth), there will be a loose layer of dirt created, which results in under-compaction.
Finally, many of the common mistakes made in soil compaction can be traced back to the equipment.
Thus, it’s essential that you regularly service your equipment.
Soil compaction equipment hits the ground with greater force than its static weight.
For best performance, stay up to date on maintenance.
9. What should you do when soil is too compact?
While soil compaction is an important element of any construction project, you can always have too much of a good thing.
For example, one reason you wouldn’t want your soil to be too compact is that your plants wouldn’t be able to grow well.
This is something that gardeners must be keenly aware of and take steps toward improving if they want their garden to flourish.
The best way to handle over-compaction is to avoid tilling your soil when it’s too wet or dry.
Additionally, don’t till your soil more than once a year.
If you can help it, avoid tilling your soil at all and keep foot and vehicle traffic to a minimum.
Loosening compacted soil can be done in numerous ways.
Here are some ways you may consider if your soil is overly compacted:
Use an aerator
Work in organic materials like compost, peat moss, and other organic soils
Add earthworms to garden beds that have problems with soil compaction
They will eat their way through compacted soil and leave behind burrows and droppings that help to aerate and fertilize the ground.
10. What does 95 percent compaction mean?
You’ll often hear 95 percent soil compaction as the target compaction threshold to ensure that construction projects are erected on a solid platform.
This means that the soil has been compacted to 95 percent of the possible density of the soil through compactive efforts.
Both maximum dry density and optimum moisture content are determined in the laboratory and provided as the target for field compaction.
In general, the design engineer will determine the compactive threshold based on the bearing capacity required for the final load to be structurally stable.
11. How should you select your soil compaction equipment?
The type of soil compaction equipment you use is based on the type of soil that you are trying to compact.
Soil is generally categorized into three groups: organic soils, coarse-grained soils, and fine-grained soils.
Organic soils are unsuitable for compaction and construction purposes. They won’t be discussed further in this blog.
Coarse-grained soils are essentially granular. They are gritty to the touch and don’t retain water. They can be broken down into sand and gravel.
Fine-grained soils are cohesive in nature and can be broken into silt and clay. Clay and silt both feel smooth when moist and retain high amounts of water compared to other soils.
Here are some of the best soil compaction equipment based on the type of soil.
Smoother rollers: Effective on granular soils, including gravel and sand, smooth rollers use static pressure (occasionally combined with vibration and impact) to compact soil.
These rollers are not the only type of compactor utilized during the process of compaction, and they are likely to be used in the final step to smooth the surface.
Padfoot and tamping foot rollers: These rollers compact soils with more cohesive content.
Using tapered feet, they use manipulative force to break the natural bonds between fine-grained soil particles.
These tapered feet also prevent “fluffing” of the soil, which ultimately decreases the soil’s ability to absorb additional rainwater.
Pneumatic rollers: These rollers work well on small to medium compaction jobs.
The staggered rubber tires on these vehicles have varying air pressure and additional weights or ballasts can be added to meet compaction goals.
Tamping rammers: This equipment is recommended for small or confined workspaces as they’re light and portable.
They also work well on cohesive soils.
12. What are the different types of soil compaction tests?
Soil compaction tests use one of the numerous methods to measure the dry density and moisture content of the soil in an area.
The most common tests are as follows.
Sand Cone Test
Rubber Balloon Test
Moisture Content of Soil and Unit Weight Tests
Nuclear Density Test
Read more about each of these tests in more detail in Soil Density Testing: 3 Test Methods You Can Count On.
13. Was proper soil compaction done in residential construction?
As you begin learning about soil compaction, you may begin to see mistakes in your own neighborhood.
Some indications of incomplete compaction around houses include buckling sidewalks and driveways, settling cracks in foundations, and puddles in yards.
These often result in poor drainage and are just a few of the common negative results of incomplete or improper soil compaction.
Remember, compacting soil early in the construction process is significantly easier than correcting the effects of a settling road later.
Don’t skip this step in the process!
Final thoughts
The compaction of soil puts soil under mechanical stress and densifies it in the process.
This is useful because it increases the strength of the soil and decreases the compressibility and permeability of the soil.
Soil compaction is done to increase the bearing capacity and stiffness of the soil — typically ahead of construction.
This is a critical step if you want to reduce the risk of settling.
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Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.
Prior to starting Gokce Capital, Erika received a Bachelor of Architecture from the University of Southern California and a graduate degree in Urban Policy from Columbia University. She worked as both an architectural designer and engineer in New York before joining the New York City Department of Housing Preservation and Development.
Erika currently lives in the New York Metropolitan area with her spouse, daughter and cat. She is originally from Chicago and still considers herself a midwesterner at heart.
Erika also loves to read, write and travel (fun fact, she has visited all 50 states and more than 30 countries!). Her new book, Land Investing Mistakes: 11 True Stories You Need To Know Before Buying Land, is now available on Amazon.