Optimum Soil Compaction: What, Why & How

The first thing that comes to most contractor’s mind when they think about soil preparedness is compaction. Compaction is defined according to the Asphalt Institute Handbook as "the process used to densify, or reduce the volume of a mass of material.” Contractors do it for the sole purpose of reducing the void ratio. As a result soils becomes stronger, letting water and air penetrate through these air voids. We will dive deeper in detail into the basic steps for proper compaction to help understand why soil compaction is essential.

One of the many purposes of this construction task is to increase the density of the soil by removing the air with the appropriate mechanical equipment. Keep in mind compaction depends on the permeability and type of the soil. To obtain optimum compaction requires knowing the composition of the soil with which you are dealing with - which are most commonly solids in soil grains, water, and air. These last two elements make up the soil matrix. By filling the remaining space with water makes soil denser.

Why compaction?

There are three main reasons compaction is key for soil preparedness:

  1. Increases stability and bearing capacity of soils.

  2. Prevents soil settlement and frost damage.

  3. Reduces contraction, swelling, and settling of soil.


Acquiring the proper equipment will help you achieve the desired compaction level. The compression caused from heavy equipment facilitates your construction results. Tamping rammers or jumping jacks, vibratory plate compactors, rollers, and more can all be found at job sites around the world to compact various soils. With these pieces of equipment, two different types of force, static force and vibratoryforce , can be utilized to achieve proper compaction.

Static force: Found in the deadweight of machines, static force applies pressure downward on soil surfaces. As a result, soil particles compress in the topsoil layer. 

Vibratory force: This force is engine-driven creating a downward force, in addition to the machine's of static weight. Vibrations compress the soil material closer together to increase density. 

The type of force produced by a compaction machine is determined through its frequency and amplitude. The frequency is known as the speed at which the machine jumps to provide maximum force. Frequency can be designated as vibrations per minute (VPM). Amplitude is defined as the maximum movement of a vibrating body from its axis in one direction, and varies for each type of compaction equipment. The more amplitude increases, the more compacted and denser soils can become.

Types of compaction: There are four types of compaction that can be applied to soils or asphalt. Each one takes place using one of the two types of the forces explained above (static or vibratory). 

1. Vibration: Periodic motion of the particles with rotating weight in alternately opposite directions from a position of equilibrium.
2. Impact: An action of one object coming into contact with another.

3. Kneading: Shear is applied by alternating movement in adjacent positions. 
4. Pressure: The process of continuous physical force against buried solid materials by something in contact.

To achieve optimum compaction there are three easy guidelines:

Soil preparedness: Soil preparedness refers to the “wetness” the dirt or soil. Soil needs to be 50% dry and 50% wet, before starting compaction. A simple “hand test” can determine this. Pick up a handful of soil with your hand and squeeze the dirt. Observe whether the soil is powdery or if it breaks apart when dropped. If the soil does break apart, it means that it is too dry. If the soil keeps together in one piece when dropped, it is ready for compaction.

Testing: The function of this step is to measure the density of an aggregate of matter to ensure density increases when driving out air. At a low moisture content level, there are more soil particles assembling together. In order to determine if the soil is compacted properly, there are several methods.

Test strips are useful to determine the method of compaction, and understand how many passes are needed to achieve the optimum compaction. Every layer of compacted soil meets a specific percentage on the proctor curve. Through soil testing, it is possible to identify optimum moisture. Soil testing measures the soil density compared to the degree of compaction specifications, as well as the effect of the moisture.

A common laboratory method called the Proctor compaction test can be used to determine the optimal moisture content for a given soil type. The goal of this method is to understand the soil’s maximum dry density. A second method of soil testing is known as the California Test 216 and is used to find the relative compaction of untreated and treated soils. Moreover, there are tools used in civil construction industry such as the nuclear density gauge that provide density and moisture measurement to define the percent of soil compaction.

Compaction specifications: Four factors account for optimum compaction including, lift thickness, contract, pressure, and soil moisture content. During the compaction process, the soil moisture adds density and lubricates soil grain by joining soil moisture with content. Finally you will reach the ideal water content of the soil, until there is a maximum dry unit weight without voids in the soil. The table below explains the different outcomes and properties of fill materials such as, gravel, sand, silt and clay. Download our guide of properties of different fill materials, here

Compaction VS Consolidation:

Compaction becomes necessary in construction, but should not be confused with consolidation. There are plenty of differences between these tasks that are in most cases mixed up. Compaction is employed mainly for sandy soils, but consolidation applies to clay like soils. Soil compaction is a dynamic process and needs to be instantaneous. This task needs to be done before building a structure with unsaturated soil. On the other hand, consolidation is a static process with saturated soil and drives away water from air voids.

Soil Types:

The desired result of compaction is better achieved by understanding soil types.  

Cohesive soils: Clays and mixes have a particular particle size of less than .003” or .002” are typically classified as cohesive soils. This type of soil is primarily used for retaining pond beds and mound fills. These soils are dense due to the strongly bound molecular attraction. Cohesive soils and water will not mix effortlessly when saturation in water, but only once the soils are moist it will feel sticky.

Granular soils: These soils have particle sizes of .003” or greater, like sand. One of granular soils properties is that water drains easily through the soils particles. The larger the particles, the larger the equipment needed to achieve lower frequencies and higher compaction force. Plate compactors are the typically the best option for compacting granular soils, but depending on the vibration frequency and particle size, reversible plates and double drum rollers are appropriate for this type of work production as well.

Mixed soils: Sometimes soils can be a mixture of both types, clay and granular materials, thus choosing the appropriate compaction equipment is more difficult. We recommend testing equipment to match the best machine to the desired job.

Before even worrying about the soil itself, follow these guidelines to achieve optimum pavement performance. Compaction is key for soil improvement and precise preparedness. Therefore, behind every soil there is a designated test method to achieve your desired results.


  • Dead weight of machine: A dead load or unrelieved weight of an object or machine.

  • Soil: The loose surface material of the earth’s crust.

  • Optimum moisture content: the percent of moisture at which the greatest density a soil can be obtained through compaction.

  • Proctor curve: Laboratory method determining the optimal moisture content. It is almost universally used to determine the maximum density of any soil so that specifications may be properly prepared for field construction requirements.

  • Test strips: A strip of material containing chemicals that react to certain substances.


Compaction equipment

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