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Factors to Consider When Compacting Soil

Soil compaction can be an easy process once you understand a few variables. Of course experience is everything, but a little knowledge of the soil type, compaction equipment, and proper operating procedure can make any job successful. This article will help you gain an understanding of what it takes to solve soil compaction questions.


Types of Soil

Before we begin to compact, we need to better understand how different types of soils affect how we compact, and the type of equipment we should use. 

The American Association of State Highway and Transportation Officials has classified soil, found in nature, into 15 groups. Within these groups, there are three basic soil classifications: cohesive, granular, and organic. Since organic soil is unsuitable for compaction, we won't go into details on this type of soil classification. 

Cohesive and granular each act differently due to their particle size, density, and moisture. Accordingly, each has unique requirements and controls for testing and compacting. 

  • Cohesive Soil - This includes clay and silt. This soil has the smallest particles, and its particles usually stick together. Clay is commonly used in embankment fills and retaining pond beds. However, silt has a much lower cohesion than clay; therefore, it’s much more reliant on water content. 

Characteristics: These soils are dense and tightly bound together. When wet, they can be molded and are plastic, but when dry, they will become very hard. For perfect compaction, it’s important to have the appropriate water content evenly distributed. 

Equipment: Use a high-impact machine for small areas and very narrow trenches that will ram the soil and force the air out. A vibratory rammer is the best choice, and for higher production, try a sheepsfoot drum roller. 

  • Granular Soil: This includes gravel and sand, which are not cohesive, meaning they do not stick together. Granular soil creates a very strong compacted foundation and is made of varying sized particles. This is referred to as well-graded soils. These particles require shaking or vibratory forces to interlock them. The smaller the particle size, the higher the frequency (vibration) needed to move it. When compacting large particles, use larger equipment with lower frequencies. These coarse granular soils are known for their water-draining properties and ability to hold material together.

Characteristics: Like cohesive soil, moisture content is key for granular soil to achieve maximum compaction effort from either centrifugal, static, or impact force. 

Equipment: Single direction and reversible vibratory plates are a great choice, and you may also try smooth drum rollers with vibration on or static rolling. 

  • Asphalt: Though not classified as a soil because it is not a natural material, we believe this is a good spot to discuss asphalt. There are three phases to rolling asphalt:

    1. Breakdown which provides the needed density.

    2. Intermediate rolling adds more density and seals the surface.

    3. Finish Rolling removes roller marks and blemishes from earlier passes.

Characteristics: Compaction starts on the bottom and works its way up to the surface by using vibratory forces that radiate downward and bounce up after hitting a stable base. 
Equipment: Use vibratory plates in confined areas and for patch jobs and vibratory smooth drum rollers for most asphalt compaction.

Guide to Soil Types


Equipment Application Chart

How Water Affects Soil and Compaction

Moisture has a huge influence on soil, including how it is compacted and the compaction results. Compacted soil must support a heavy load through all seasons and all weather conditions. For example, rain can transform soil into a plastic state or even a liquid. When that happens, sand loses its load-bearing abilities. 

Perfect compaction relies on perfect moisture content. Moisture acts as a lubricant within the soil, sliding the particles together. Each type of soil will react to different moisture content.

Not enough moisture = inadequate compaction; no load-bearing ability.

Too much moisture = inadequate compaction; weak load-bearing ability.


Soil Materials Chart


Soil Density Tests

Before compacting, test the soil for moisture and density. Several tests are available, and most tests provide density measurement, moisture, and identify optimum moisture. The results provide a comparison between the compaction and your project specs. 

The most widely used test is the Proctor Test or Modified Proctor Test. A particular soil needs to have an ideal moisture amount to achieve maximum density. This is important for durability, and it saves money because less compaction effort is needed to achieve the desired specified results. This type of test is typically performed in a laboratory. 

The Hand Test is a quick way of determining the moisture density and soil type. But only use this as a quick soil identification type in the field.
Soil Density Hand Test
  1. Pick up a handful of soil.

  2. Squeeze it in your hand.

  3. Open your hand.

  • Too Dry for Compaction: If the soil is powdery and will not retain the shape made by your hand, it is too dry. If it shatters when dropped, it is too dry for compaction.

  • Too Moist for Compaction: If the soil is plastic in your hand, leaves small traces of moisture on your fingers, and stays in one piece when dropped, it is too moist for compaction.

  • Compaction Ready: If the soil is moldable and breaks into only a couple of pieces when dropped, it has right amount of moisture for proper compaction.

Close Up of Wacker Neuson Rammer

Types of Compaction

We know the basic soil types and how to test soil, and now, we’re ready to begin compacting.

To get the best compaction, you need to match the soil type with its corresponding compaction method. See “Types of Soil” above.

Compaction Actions
There are four types of compaction actions, and each action uses either a static or dynamic force or a combination of both:  

  1. Vibration (through centrifugal force) is a periodic motion of the particles with rotating weight in alternately opposite directions from a position of equilibrium.

  2. Impact is an action of one object coming into contact with another.

  3. Kneading is alternating movement in adjacent positions. 

  4. Pressure is the process of continuous physical force against buried solid materials by something in contact.

Static and Dynamic Compaction Forces
The different types of compaction actions described above use static or dynamic forces, or a combination.

  • Static Force uses the deadweight of a compaction machine to apply a downward force on the soil, squeezing the air and water out, compressing the soil together into a denser state. The machine’s weight can be increased or decreased to change the compaction effectiveness. Static compaction is used on upper soil layers and limited to any appreciable depth. Kneading and pressure are two forms of static compaction.

  • Dynamic Force is using an engine-driven machine to create a downward force in addition to the machine’s static weight. Examples are the vibrating actions and rotating weights of plate compactors or the powerful piston/spring combination on rammers. Both of these compactors deliver a rapid sequence of impacts to the layer’s surface.

Lift Depth and Number of Passes

Left depth (aka: lift thickness) is dependent on the energy imparted from the machine and soil type. Lighter machines that give less force through any means will give good compaction results with shallower lifts of 4-6 inches. Larger machines can yield compaction results of 8-20-inch lift depths. 

The number of passes is also critical to the compaction effort. It is very common to only run one pass, which is the machine in one direction. This leaves room for more settling in the future and potential failure and costly repairs. Most manufacturers will recommend at least two and upwards of five passes to achieve the best compaction effort from the product. 

One of the most common misconceptions is the relationship between compaction force and compaction effectiveness. In other words, is a machine that hits harder going to do a better job? 

“Quality of compaction should be prioritized over compaction force. A compactor’s force is only one variable in compaction quality, which is a balance between a compactor’s static weight, force, frequency, amplitude, and shape. Additionally, stated compaction force is the maximum force that can be generated and was tested and achieved under ideal conditions in controlled testing with optimum soil mix and moisture. All jobsites have different types of soils, and soil mixtures, with various moisture content that will ultimately change how much lift can be compacted with the recommended passes.”

–       Stevan Garcia, Product Manager, Compaction Products, Multiquip 


Static vs Dynamic Force Chart

Keep this information in mind when purchasing compaction equipment for your projects. If you want to dig deeper, the next article “Types of Compaction Equipment and Their Applications” is a great guide on the various types of compaction machinery you can consider before you buy.
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