Soil Compaction Handbook
Copyright © Multiquip Inc
compaction is defined as the method of mechanically increasing the density
of soil. In construction, this is a significant part of the building
process. If performed improperly, settlement of the soil could
and result in unnecessary maintenance costs or structure failure.
Almost all types of building sites and construction projects utilize
mechanical compaction techniques.
is formed in place or deposited by various forces of nature - such as
glaciers, wind, lakes and rivers - residually or organically.
Following are important elements in soil compaction:
- Soil type
- Soil moisture content
- Compaction effort required
There are five principle
reasons to compact soil:
- Increases load-bearing capacity
- Prevents soil settlement and frost damage
- Provides stability
- Reduces water seepage, swelling and contraction
- Reduces settling of soil
Types of Compaction
There are four types of
compaction effort on soil or asphalt:
These different types of effort are found in the two principle types of
compaction force: static and vibratory.
Static force is simply the deadweight of the machine, applying
downward force on the soil surface, compressing the soil 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.
Vibratory force uses a mechanism, usually engine-driven, to
create a downward force in addition to the machine's static weight.
The vibrating mechanism is usually a rotating eccentric weight or
piston/spring combination (in rammers). The compactors deliver a
rapid sequence of blows (impacts) to the surface, thereby affecting the
top layers as well as deeper layers. Vibration moves through the
material, setting particles in motion and moving them closer together for
the highest density possible. Based on the materials being
compacted, a certain amount of force must be used to overcome the cohesive
nature of particular particles.
Results of Poor
Both illustrations above show the
result of improper compaction and how proper compaction can ensure a
longer structural life.
Soil Types and
soil type behaves differently with respect to maximum density and optimum
moisture. Therefore, each soil type has its own unique requirements
and controls both in the field and for testing purposes. Soil types
are commonly classified by grain size, determined by passing the
soil through a series of sieves to screen or separate the different grain
sizes. Soil classification is categorized into 15
groups, a system set up by AASHTO (American Association of State Highway
and Transportation Officials). 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. A soil's makeup
determines the best compaction method to use.
The are three basic soil groups:
Organic (this soil is not suitable for compaction and will not be discussed here)
soils have the smallest particles. Clay has a particle size range of
.00004" to .002". Silt ranges from .0002" to
.003". Clay is used in embankment fills and retaining pond
Cohesive 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. Silt has a
noticeably lower cohesion than clay. However, silt is still heavily
reliant on water content.
soils range in particle size from .003" to .08" (sand) and
.08" to 1.0" (fine to medium gravel). Granular soils are
known for their water-draining properties.
Sand and gravel obtain maximum density in either
a fully dry or saturated state. Testing curves are relatively flat
so density can be obtained regardless of water content.
The tables that follow give a basic indication of
soils used in particular construction applications.
Desirability of Soils As Compacted Fill
to View Chart
|Guide to Soil Types
to look for
soils, fine sands and silts
can be seen. Feels gritty when rubbed between fingers
and soil are shaken in palm of hand, they mix. When shaking is
stopped they separate
or no plasticity
||Little or no
cohesive strength when dry. Soil sample will crumble easily.
soils, mixes and clays
be seen by naked eye. Feels smooth and greasy when rubbed between
and soil are shaken in palm of hand, they will not mix
sticky. Can be rolled
||Has high strength
when dry. Crumbles with difficulty. Slow saturation in
||Pavement Sub grade
Effect of Moisture
response of soil to moisture is very important, as the soil must carry the
load year-round. Rain, for example, may transform soil into a
plastic state or even into a liquid. In this state, soil has very
little or no load-bearing ability.
Moisture vs. Soil Density
Moisture content of the soil is vital to proper
compaction. Moisture acts as a lubricant within soil, sliding the
particles together. Too little moisture means inadequate compaction
- the particles cannot move past each other to achieve density. Too
much moisture leaves water-filled voids and subsequently weakens the
load-bearing ability. The highest density for most soils is at a
certain water content for a given compaction effort. The drier the
soil, the more resistant it is to compaction. In a water-saturated
state the voids between particles are partially filled with water,
creating an apparent cohesion that binds them together. This
cohesion increases as the particle size decreases (as in clay-type soils). l
Soil Density Tests
To determine if proper soil compaction is
achieved for any specific construction application, several methods were
developed. The most prominent by far is soil density.
Soil testing accomplishes the following:
- Measures density of soil for comparing the
degree of compaction vs. specs
- Measures the effect of moisture on soil density
- Provides a moisture density curve identifying optimum moisture
Tests to determine optimum moisture content are
done in the laboratory. The most common is the Proctor Test, or
Modified Proctor Test. A particular soil needs to have an ideal (or
optimum) amount of moisture to achieve maximum density. This is
important not only for durability, but will save money because less
compaction effort is needed to achieve the desired results.
The Hand Test
A quick method of determining
moisture is known as the "Hand Test". Pick
up a handful of soil. Squeeze it in your hand. Open
your hand. 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. If the soil is
moldable and breaks into only a couple of
pieces when dropped, it has the right amount of moisture for proper compaction. If
is plastic in your hand, leaves traces of moisture on your fingers and
stays in one piece when dropped, it has too much moisture for compaction.
Proctor Test (ASTM D1557-91)
The Proctor, or Modified Proctor Test, determines
the maximum density of a soil needed for a specific job site. The
test first determines the maximum density achievable for the materials and
uses this figure as a reference. Secondly, it tests the effects of
moisture on soil density. The soil reference value is expressed as a
percentage of density. These values are determined before any
compaction takes place to develop the compaction specifications.
Modified Proctor values are higher because they take into account higher
densities needed foe certain typed of construction projects. Test
methods are similar for both tests.
A small soil sample is taken from the jobsite. A standard weight is dropped several times on the soil. The material weighed
and then oven dried for 12 hours in order to evaluate water content
|Modified Proctor Test
This is similar to the Proctor Test except a hammer is used to
compact material for greater impact, The test is normally preferred
in testing materials for higher shearing strength.
It is important to know and control the soil
density during compaction. Following are common field tests to
determine on the spot if compaction densities are being
|Field Density Testing
Balloon Dens meter
* Large sample
* Large sample
* Direct reading obtained
* Open graded material
* Deep sample
* Under pipe haunches
* Easy to redo
* More tests (statistical reliability)
||* Many steps
* Large area required
* Halt Equipment
* Tempting to accept flukes
* Balloon breakage
|* Small Sample
* No gravel
* Sample not always retained
|* No sample
* Moisture suspect
* Encourages amateurs
||* Void under plate
* Sand bulking
* Sand compacted
* Soil pumping
|* Surface not level
* Soil pumping
* Void under plate
* Rocks in path
* Plastic soil
* Rocks in path
* Surface prep required
Cone Test (ASTM D1556-90)
A small hole (6" x 6" deep) is dug in
the compacted material to be tested. The soil is removed and
weighed, then dried and weighed again to determine its moisture
content. A soil's moisture is figured as a percentage. The
specific volume of the hole is determined by filling it with calibrated
dry sand from a jar and cone device. The dry weight of the soil
removed is divided by the volume of sand needed to fill the hole.
This gives us the density of the compacted soil in lbs per cubic
foot. This density is compared to the maximum Proctor density
obtained earlier, which gives us the relative density of the soil that was
Nuclear Density (ASTM D2292-91)
Nuclear Density meters are a quick and fairly
accurate way of determining density and moisture content. The meter
uses a radioactive isotope source (Cesium 137) at the soil surface
(backscatter) or from a probe placed into the soil (direct
transmission). The isotope source gives off photons (usually Gamma
rays) which radiate back to the mater's detectors on the bottom of the
unit. Dense soil absorbs more radiation than loose soil and the
readings reflect overall density. Water content (ASTM D3017) can
also be read, all within a few minutes. A relative Proctor density
with the compaction results from the test.
Sorry, we do not sell Nuclear Density Meters
Modulus (soil stiffness)
This field-test method is a very recent
development that replaces soil density testing. Soil stiffness is
the ratio of force-to-displacement. Testing is done by a machine
that sends vibrations into the soil and then measures the deflection of
the soil from the vibrations. This is a very fast, safe method of
testing soil stiffness. Soil stiffness is the desired engineering
property, not just dry density and water content. This method is
currently being researched and tested by the Federal Highway
The desired level of compaction is best achieved
by matching the soil type with its proper compaction method. Other
factors must be considered as well, such as compaction specs and job site
- Cohesive soils - clay is
particles stick together.*
Therefore, a machine with a high
impact force is required to ram the soil and force the air out,
arranging the particles. A rammer is the best choice, or
vibratory roller if higher production is needed.
*The particles must be sheared to compact.
- Granular soils - since granular soils are not cohesive and the particles
require a shaking or vibratory action to move them, vibratory plates
(forward travel) are the best choice.
Reversible plates and smooth
drum vibratory rollers are appropriate for production work.
Granular soil particles respond to different frequencies (vibrations)
depending on particle size. The smaller the particle, the higher the
frequencies and higher compaction forces.
Normally, soils are mixtures
of clay and granular materials, making the selection of compaction
equipment more difficult. It is a good idea to choose the machine
appropriate for the larger percentage of the mixture. Equipment
testing may be required to match the best machine to the job.
Asphalt is considered granular due to its base of mixed aggregate sizes
(crushed stone, gravel, sand and fines) mixed with bitumen binder (asphalt
cement). Consequently, asphalt must be compacted with pressure
(static) or vibration.
Compaction Machine Characteristics
factors are important in determining the type of force a compaction
machine produces: frequency and amplitude.
Frequency is the speed at which an
eccentric shaft rotates or the machine jumps. Each compaction
frequency machine is designed to operate at an optimum frequency to supply
the maximum force. Frequency is usually given in terms of vibration
per minute (vpm).
Amplitude (or normal amplitude) is the
maximum movement of a vibrating body from its axis in one direction.
Double amplitude is the maximum movement of a vibrating body from its axis
in one direction. Double amplitude ins the maximum distance a
vibrating body moves in both directions from its axis. The apparent
amplitude varies for each machine under different job site
conditions. The apparent amplitude increases as the material becomes
more dense and compacted.
height and Machine Performance
Soil can also be
over-compacted if the compactor makes too many passes (a pass is the
machine going across a lift in one direction). Over-compaction is
like constantly hitting concrete with a sledgehammer. Cracks will
eventually appear, reducing density. This is a waste of man-hours
and adds unnecessary wear to the machine.
height (depth of the soil layer is an important factor that effectsmachine performance and compaction cost. Vibratory and rammer-type
equipment compact soil in the same direction: from top to bottom and
bottom to top. As the machine hits the soil, the impact travels to
the hard surface below and then returns upward. This sets all
particles in motion and compaction takes place.
As the soil becomes compacted, the impact has a
shorter distance to travel. More force returns to the machine,
making it lift off the ground higher in its stroke cycle. If the
lift is too deep, the machine will take longer to compact the soil and a
layer within the lift will not be compacted.
A word about meeting job site
specifications. Generally, compaction performance parameters are
given on a construction project in one of two ways:
- Method Specification - detailed instructions
specify machine type, lift depths, number of passes, machine speed and
moisture content. A "recipe" is given as part of the job
spec to accomplish the compaction needed. This method is outdated,
as machine technology has far outpaced common method specification
- End-result Specification - engineers indicate
final compaction requirements, thus giving the contractor much more
flexibility in determining the best, most economical method of meeting the
required specs. Fortunately, this is the trend, allowing the
contractor to take advantage of the latest technology available.
Rammers deliver a high impact force ( high
amplitude) making them an excellent choice for cohesive and semi-cohesive
soils. Frequency range is 500 to 750 blows per minute. Rammers
get compaction force from a small gasoline or diesel engine powering 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.
Rammers cover three types of compaction: impact, vibration and kneading.
Vibratory plates are low amplitude and high
frequency, designed to compact granular soils and asphalt. Gasoline
or diesel engines drive one or two eccentric weights at a high speed to
develop compaction force. The resulting vibrations cause forward
motion. The engine and handle are vibration-isolated from the
vibrating plate. The heavier the plate, the more compaction force it
generates. Frequency range is usually 2500 vpm to 6000 vpm.
Plates used for asphalt have a water tank and sprinkler system to prevent
asphalt from sticking to the bottom of the base plate. Vibration is
the one principal compaction effect.
Reversible Vibratory Plates
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 is possible the best compaction buy
dollar for dollar. Unlike standard plates, the reversible forward
travel may be stopped and the machine will maintain its force for
Rollers are available in several categories:
walk-behind and ride-on, which are available as smooth drum, padded drum,
and rubber-tired models; and are further divided into static and vibratory
A popular design for many years, smooth-drum
machines are ideal for both soil and asphalt. Dual steel drums are
mounted on a rigid frame and powered by gasoline or diesel engines.
Steering is done by manually the machine handle. Frequency is
around 4000 vpm and amplitudes range from .018 to .020. Vibration is
provided by eccentric shafts placed in the drums or mounted on the frame.
Padded rollers are also known as trench rollers
due to their effective use in trenches and excavations. These
machines feature hydraulic or hydrostatic steering and operation.
Powered by diesel engines, trench rollers are built to withstand the
rigors of confined compaction. Trench rollers are either skid-steer
or equipped with articulated steering. Operation can be by manual or
remote control. Large eccentric units provide high impact force and
high amplitude (for rollers) that are appropriate for cohesive
soils. The drum pads provide a kneading action on soil. Use
these machines for high productivity.
Configured as static-wheel rollers, ride-ons are
used primarily for asphalt surface sealing and finishing work in the
larger (8 to 15 ton) range. Small ride-on units are used for patch
jobs with thin lifts. The trend is toward vibratory rollers.
Tandem vibratory rollers are usually found with drum widths of 30" up
to 110", with the most common being 48". Suitable for
soil, sub-base and asphalt compaction, tandem rollers use the dynamic
force of eccentric vibrator assemblies for high production work.
single-drum machines feature a single vibrating drum with pneumatic drive
wheels. The drum is available as smooth for sub-base or rock fill,
or padded for soil compaction. Additionally, a ride-on version of
the pad foot trench roller is available for very high productivity in
confined areas, with either manual or remote control operation.
These rollers are equipped with 7 to 11 pneumatic
tires with the front and rear tires overlapping. A static roller by
nature, compaction force is altered by the addition or removal of weight
added as ballast in the form of water or sand. Weight ranges vary
from 10 to 35 tons. The compaction effort is pressure and kneading,
primarily with asphalt finish rolling. Tire pressures on some
machines can be decreased while rolling to adjust ground contact pressure
for different job conditions.
Safety and General
all construction equipment, there are many safety practices that should be
followed while using compaction equipment. While this instructional
guide is not designed to cover all aspects of job site safety, we wish to
mention some of the more obvious items in regard to compaction
equipment. Ideally, equipment operators should familiarize
themselves with all of their company's safety regulations, as well as any
OSHA, state agency or local agency regulations pertaining to job
safety. Basic personal protection, consisting of durable work
gloves, eye protection, ear protection, approved hard hat and work
clothes, should be standard issue on any job available for immediate use.
In the case of walk-behind compaction equipment,
additional toe protection devices should be available, depending on
applicable regulations. All personnel operating powered compaction
equipment should read all operating and safety instructions for each piece
of equipment. Additionally, training should be provided so that the
operator is aware of all aspects of operation.
No minors should be allowed to operate
construction equipment. No operator should run construction
equipment when under the influence of medication, illegal drugs or
alcohol. Serious injury or death could occur as a result of improper
use or neglect of safety practices and attitudes. This applies to
both the new worker as well as the seasoned professional.
Trench work brings a new set of
safety practices and regulations for the compaction equipment
operator. This section does not intend to cover the regulations
pertaining to trench safety (OSHA Part 1926, Subpart P). The
operator should have knowledge of what is required before
compacting in a trench or confined area. Be certain a
"competent person" (as defined by OSHA Part 1926.650 revised
July 1, 1998) has inspected the trench and follows OSHA guidelines for
inspection during the duration of the job. Besides the obvious
danger of a trench cave-in, the worker must also be protected from falling
objects. Unshored (or shored) trenches can be compacted with the use
of remote control compaction equipment. This allows to operator to
stay outside the trench while operating the equipment.