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Soil Compaction

Starting at the base of the problem

Experienced contractors know that producing quality concrete flatwork begins with the proper preparation of the slab support system. Using proper compaction techniques, contractors efficiently transform raw land into an engineered system of uniform support. Reducing exposure to structural cracking, improper slab thicknesses, and uneven joints is easy when contractors employ proper compaction tools to construct the slab support system. In this article we’ll do a deep-dive on best practices for compaction, how to determine which equipment and materials to use for various applications, and some of the pitfalls to watch out for.

Contractors use compaction in slab construction for four reasons.

  1. They want to increase the soil’s load-bearing capacity allowing the slab to support larger weights.
  2. Compaction reduces any settling or shrinkage that could cause voids in the support system after the slab’s placement, that could cause weakness.
  3. By compacting the support system, contractors reduce water seepage into the base course prior to construction.
  4. A firm base course provides better stability during concrete placement and aids in meeting thickness tolerances.

Improper compaction techniques can result in costly tear out, and replacement costs often range in the thousands of dollars. These costs don’t include the loss of time and missed opportunity for additional business.

What’s beneath the slab?

ACI documents indicate that slab support systems may have up to three components: the base, sub-base and sub-grade. The thickness of the entire slab support system should be at least 4 inches. For slabs designed to support heavier loads the engineer may extend the thickness and specify the materials that should be within each component layer.  Since each component may have different materials, contractors may be required to use different compaction techniques.

The base course is the layer directly beneath the slab and on which the fresh concrete is placed. On most commercial floor projects, the engineers specify the base course be made with crushed rocks and gravels. ACI 302, “Concrete Floor and Slab Construction,” recommends that the material should be “compactible, easy to trim, granular fill that will remain stable and support construction traffic.” The document also recommends the granular material be clean of fines with no clay, silt, or organic materials.

The sub-base (middle layer) is the layer beneath the base course. The sub grade (bottom layer) is often comprised of the site’s original soils. Many engineers believe that properly preparing the sub-base is the most important task when preparing the slab support system. In very general terms, the thicker the subbase, the more load the slab can support.

Not all slab support systems require granular fill in the base course or sub-base layers. If the natural soil is relatively clean and compactable, contractors can place a slab directly on its surface following compaction and over any specified vapor barrier and under-slab insulation. But there are potential risks associated with this decision. These risks could include poor drainage and poor levelness. Typically a slab’s sub grade is not treated or compacted.

Read the requirements

Contractors should pay close attention to contract requirements relating to the slab support system.  Engineers often include a slab support design unique to each job site. Generally, engineers outline compaction performance parameters for the support system with one of two approaches:

  1. Some engineers will use a prescriptive approach. They provide detailed instructions on how to perform the compaction on each component. Their instructions include the desired soil moisture content, the type of compactor to be used, the lift depth of each material thickness, the number of passes by the compactor, and even the compactor’s speed.
  2. The second option is called the performance approach. Engineers simply indicate the final compaction requirements for the support system. Contractors have much more flexibility in determining the best, most economical method of meeting the required compaction specification. With improvements in compaction technology, contractors prefer this approach.

Whether there’s an engineer involved or not, the contractor’s selection of the compaction technology begins prior the job. They often hire technicians to conduct soil testing to determine the type of soil. These results predict the material’s physical ability to be properly compacted. When the soils beneath the base course are determined to be of extremely poor quality, engineers may require removal and construction of the sub-base using crushed rock, gravels, or treated soils to create a stiffened support system.

ACI 302.1R 4.1.4 – CHAPTER 4—Site Preparation And Placing Environment states:

The base material should be a compactible, easy to trim, granular fill that will remain stable and support construction traffic. The tire of a loaded concrete truck mixer should not penetrate the surface more than 1/2 in. (13 mm) when driven across the base.

Soil Types

Soil types are commonly classified by grain size. Samples are passed through a series of sieves to separate the different grain sizes. Soils found in nature are almost always a combination of soil types. A well-graded soil consists of a wide range of particle sizes with the smaller particles filling voids between larger particles. The result is a dense structure that lends itself well to compaction.

There are two ranges of common compactable soil types.

  1. At one end of the range are Cohesive Soils. These soils are dense and tightly bound together by molecular attraction. They are plastic when wet and can be molded, but become very hard when dry. Proper water content, evenly distributed, is critical for proper compaction. Cohesive soils usually require a force such as impact or pressure.

Compaction occurs by using a machine that uses an action to force the particles closer together. For example, ramming is an action that moves the particles together by forcing out air.

  1. Granular Soils (also called Non-Cohesive Soils) are not comprised of particles that have the natural affinity to join together. Rather than a direct downward force, it’s necessary to shake these particles to eliminate void spaces between the particles.

Normally, soils are mixtures of cohesive and granular materials. Testing can determine the percentage of each soil type that aids the selection of compaction equipment.

Moisture’s Effect on Compaction

As demonstrated by the field test just described, the proper amount of moisture in the base and sub base helps compaction to be more efficient. Each soil type has an optimum moisture content that yields maximum density from compaction. The amount of a soil’s densification potential is determined by a laboratory test.

Too little moisture often means inadequate compaction. Even with the force from the compactors, particles cannot move past each other to achieve the required density.

If there is too much moisture, the voids between the particles become water-filled and withstand normal compaction efforts. Without proper testing, water-saturated soils can wrongly appear to be at the proper density. The excess moisture in the voids between particles create an apparent cohesion that binds soils together. When the soils dry, the unconsolidated particles are still separated. The result is an unconsolidated base that subsequently weakens the slab support system’s load-bearing ability.

For these soil conditions, contractors often use compactors equipped with sheepsfoot rollers. This equipment provides a kneading action to soils. The sheepsfoot rollers have ground-penetrating nubs with vibration devices that force out excess moisture. The combined action of kneading and vibration force the soil particles closer together and allow the soils to achieve the optimum moisture content.

What is Compaction?

Compaction is the mechanical action that decreases the void content of layers of soils or aggregates. These action force soil particles closer together to create a denser support system. Contractors can use compaction to create stable slab support systems for almost every type of material, with the exception of organic soils.

There are four types of compaction actions used on soils and aggregates.

These actions use a combination of static and dynamic forces.


A static force is simply the deadweight of the machine, applying downward force on the soil surface, compressing the particles. The only way to change the effective compaction force is by adding or subtracting the weight of the machine. Static compaction is confined to upper soil layers and is limited to any appreciable depth. Kneading and pressure are two examples of static compaction.


Compaction actions that use dynamic forces use mechanisms, usually engine-driven, to create a downward force in addition to the machine’s static weight on the component’s surface. Plate Compactors utilize a vibrating mechanism and rotating eccentric weights, while Rammers use a powerful piston/spring combination. These compactors deliver a rapid sequence of blows (impacts) to the layer’s surface.

The action affects the top layers, but with additional weights, rammers can compact deeper layers. A vibration pulse moves through the material, setting particles in motion and moving them closer together for the highest density possible. Rammers deliver a high impact force (high amplitude) with a frequency range of about 500 to 750 blows per minute. These units have a small gasoline engine or electric battery that powers a large piston set with two sets of springs. The rammer is inclined at a forward angle to allow forward travel as the machine jumps.

Some soils require more than a single downward pulse to compact particles. Some directional forces are used to overcome the cohesive nature of certain particles. Vibratory plates are often used to travel forward and backward to provide these dynamic forces. Vibratory plates create forces that are low amplitude and high frequency and work best on granular soils. A gasoline engine or battery pack drives one or two eccentric weights at a high speed to develop compaction force. The resulting vibrations cause forward motion. The engine and handle are usually isolated from the vibrating plate to protect the operator. In general terms. the heavier the plate, the more compaction force it delivers.

In addition to some of the standard vibratory plate features, reversible plates have two eccentric weights that allow smooth transition for forward or reverse travel, plus increased compaction force as the result of dual weights. Due to their weight and force, reversible plates are ideal for semi-cohesive soils.

A reversible vibratory plate is considered to be the most versatile tool. Unlike standard plates, an operator can throttle the reversible plate’s forward travel allowing energy to be positioned over areas that may require more attention.

Matching equipment to the project size

Another important consideration is the scope of the project. Many compactors are designed for contained areas, such as trenching or small slab tear outs. Large slab areas often require larger production machines. These ride-on compactors are effective in creating a stable base.

When working on large slabs, contractors must select compaction equipment that not only reduces the void content across the middle of the slab but also all edges. Proper compaction at the slab’s edges and at any joints can prevent cracking and joint spalling. These slab locations need to be supported so they don’t behave like a cantilever and bend into the sub base.

Proper grade means accurate thickness

When compacting a slab’s base, contractors must pay close attention to the final grade elevation. A very serious, yet common problem, is that a poured slab thickness often varies too much.

On some projects, contractors may add bedding material to the base to achieve proper compaction. If not closely monitored, and reworked, these areas may cause later problems.

Concrete researchers Ward Malisch and Bruce Suprenant measured the as-built slab thickness of more than 200 projects. They found that the contractors had placed slab with areas that were much thinner than specified. Based on 30,000 measured data points they concluded the average slab thickness was about 3/8 in. less than the specified thickness.

Even with the best compacted support system, a slab that is too thin may not withstand its design load without cracking.

Follow these links for more info on soil compaction:

Multiquip Soil Compaction Handbook
Wacker Neuson Equipment Guide

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