This document defines soil compaction and discusses methods for testing and achieving compaction. It describes how compaction decreases soil volume by expelling air, increasing density. Laboratory tests like standard and modified Proctor determine maximum dry density and optimum moisture content for a given soil by applying successive blows and measuring density at different moisture levels. Specifications then require achieving a percentage of maximum density in the field, controlling layer thickness and moisture during compaction with equipment. Proper compaction increases soil strength and reduces settlement/permeability.

1. COMPACTION OF SOILS
Definition of Compaction:
Compaction is the artificial and mechanical process of decreasing the volume of the soil
rapidly by the expulsion of air voids in the soil resulting in the increase in density.
Densification of soil also occurs naturally due to consolidation of foundation soils by
expulsion of pore water due to loads from the structure. This is a rather long-term process
compared to compaction.
Soil compaction changes pore space size, distribution and soil strength. One way to
quantify the change is by measuring the bulk density. As the pore space decreases within
a soil, the bulk density increases. Soils with a higher percentage of clay and silt, which
naturally have more pore space, have a lower bulk density than sandier soils.
Importance Compaction:
1. Artificial process caused by mechanical means such as rollers.
2. Decrease in volume and increase in the density of soil occurs by expulsion of air from the
voids.
3. Compaction occurs in partially saturated soils.
4. Compaction is completed within minutes-and hence is a short-term process.
5. Compaction is effective in well-graded soils containing gravel and sand, and to a less
extent in silts and clays.
6. Compaction is caused by short-term dynamic load, which are removed after compaction.

2. Principle of Compaction:
The principle of compaction was developed by R. R. Proctor in 1933 during construction of
earth dams in California. The objective of compaction is to achieve maximum possible dry
density of the compacted soil. The water content used for compaction controls the dry density
achieved. Figure 12.1 shows the variation of the dry density with water content.
At low water content, the soil is stiff and the particles offer resistance to come closer,
resulting in low dry density. As the water content is increased, water forms a lubricating film
around particles causing them to be compacted to a closer state of contact resulting in higher
dry density. The dry density increases with increase in the water content until maximum dry
density (MDD) is reached.
At this stage, the soil particles come to the closest possible state of contact. On increase of
water content beyond optimum moisture content (OMC), the volume of soil does not
decrease further by compaction and water starts to occupy additional space causing an
increase in the volume of voids and the total volume, and resulting in a decrease in dry
density.
The water content corresponding to maximum dry density is determined, which gives
optimum water content. For the compaction of soil in the field, water equal to OMC, or less
(dry of OMC) or more (wet of OMC) water is used depending on the objective of compaction
and type of construction. Same compaction energy per unit volume of soil, as used in the
laboratory compaction test, is used to compact the soil in the field.

3. The following are some of the objectives of compaction:
i. Increase the shear strength of soil.
ii. Decrease the undesirable settlement of structures.
iii. Control undesirable volume change.
iv. Decrease permeability of soil.
v. Increase the bearing capacity of foundations.
vi. Increase the stability of slopes.
Factors Affecting Compaction:
The MDD achievable by the compaction depends on the following factors:
1. Water content.
2. Type of soil and its gradation.
3. Gradation of Soil
4. Compaction energy.
5. Method of compaction.
1. Effect of Water Content:
Increase of water content used for compaction increases the dry density initially until the dry
density reaches its maximum. After reaching MDD, further increase in the water content
decreases the dry density.
2. Type of Soil:
The type of soil used for compaction primarily decides MDD achievable by the compaction.
Figure 12.8 shows the compaction curves for different types of soil.

4. Coarse-grained soils can be compacted to a higher dry density than fine-grained soils.
Cohesive soils usually have high air voids content. The void ratio of cohesive soils increases
with increase in plasticity index. Thus, coarse-grained soils have higher MDD and lower
OMC than fine-grained soil. The MDD decreases and OMC increases for low plastic silt,
high plastic silt, and high plastic clay.
3. Gradation of Soil:
For a given soil, a well-graded soil has higher MDD and lower OMC then a poorly graded
soil. This is because a well-graded soil contains particles of all sizes and the finer size
particles fill the void space between the coarser particles resulting in lower air voids and
higher MDD.
Addition of small amount of fines to a coarse-grained soil increases its MDD for the same
reason. However, when the amount of fines added is more than that needed to fill the voids of
coarse-grained soil, the MDD again decreases.
4. Compaction Energy
The compaction energy applied to the soil during compaction has a significant influence on
the MDD. In general, the higher the compaction energy or compaction effort, the higher will
be the MDD and lower will be the OMC. This is the reason why the subgrades of airfield
pavements are compacted using heavy compaction. Thus, the compaction curve of a modified
Proctor test, which uses more compaction effort on the soil, is above and to the left of that of
standard Proctor test or IS light compaction test as shown in Fig. 12.9.

5. The increase in dry density due to the increase in compaction effort is more at water content
less than OMC (dry of optimum) than that on the wet of optimum.
It may be noted that the increase in compaction effort does not go on increasing the MDD
indefinitely. When compaction effort is increased in equal increment, the increment in MDD
becomes smaller and smaller with each increment of compaction effort. Finally, a stage is
reached where further increase of compaction effort does not bring any significant increase in
MDD.
Care should be taken to see that the compaction effort does not cause a stress on the soil
particles beyond their crushing strength, in which case the higher compaction effort crushes
the individual particles, causing a reduction in MDD in some soils..
5. Method of Compaction:
Compaction of soils is done on both in the field and laboratory can be done by a variety of
compaction equipment. Depends on method of compaction soil will show MDD and OMC.
Methods of Testing Compaction of Soil
1. Standard Proctor’s Test for Compaction of Soil
 The test provides a relationship between the water content and the dry density.
 The water content at which the maximum dry density is attained is obtained
from the relationship provided by the tests.
 Proctor used a standard mould of 4 inches internal diameter and an effective
height of 4.6 inches with a capacity of 1/30 cubic foot. The mould had a
detachable base plate and a removable collar of 2 inches height at its top. The
soil is compacted in the mould in 3 layers; each layer was given 25 blows of
5.5 pounds rammer filling through a height of 12 inches.

6.  IS: 2720 part VII recommends essentially the same specification as in
Standard Proctor test, some minor modifications. The mould recommended is
of 100mm diameter, 127.3 mm height and 1000 ml capacity.
 The rammer recommended is of 2.6 kg mass with a free drop of 310mm and a
face diameter of 50mm. The soil is compacted in three layers. The mould is
fixed to the detachable base plate. The collar is of 60mm height.
2. Modified Proctor Test for Compaction of Soil
 The modified Proctor test was developed to represent heavier compaction
than that in the standard Proctor test.
 The test is used to simulate field conditions where heavy rollers are used. The
test was standardized by American association of State Highway Officials
and is, therefore also known as modified AASHO test.
 In this, the mould used is same as that in the Standard Proctor test. However,
the rammer used is much heavier and has a greater drop than that in the Std
Proctor test. Its mass is 4.89 kg and the free drop is 450mm.
 The soil is compacted in five equal layers, each layer is given 25 blows. The
comp active effort in modified Proctor test is 4.56 times greater than in the
Std Proctor test. The rest of the procedure is same

7. Ratio of MPT to SPT

8. Field Compaction
To control soil properties in the field during earthwork construction, it is usual to specify
the degree of compaction (also known as the relative compaction). This specification is
usually that a certain percentage of the maximum dry density, as found from a laboratory
tests (Light or Heavy Compaction), must be achieved. For example, it could be specified that
field dry densities must be greater than 95% of the maximum dry density (MDD) as
determined from a laboratory test. Target values for the range of water content near the
optimum moisture content (OMC) to be adopted at the site can then be decided, as shown in
the figure.
For this reason, it is important to have a good control over moisture content during
compaction of soil layers in the field. It is then up to the field contractor to select the
thickness of each soil lift (layer of soil added) and the type of field equipment in order to
achieve the specified amount of compaction. The standard of field compaction is usually
controlled through either end-product specifications or method specifications.
End-Product Specifications
In end-product specifications, the required field dry density is specified as a percentage of the
laboratory maximum dry density, usually 90% to 95%. The target parameters are specified
based on laboratory test results.
The field water content working range is usually within ± 2% of the laboratory optimum
moisture content.
It is necessary to control the moisture content so that it is near the chosen value. From the
borrow pit, if the soil is dry, water is sprinkled and mixed thoroughly before compacting. If
the soil is too wet, it is excavated in advance and dried.
In the field, compaction is done in successive horizontal layers. After each layer has been
compacted, the water content and the in-situ density are determined at several random

9. locations. These are then compared with the laboratory OMC and MDD using either of these
two methods: the sand replacement method, or the core cutter method.
Method Specifications
A procedure for the site is specified giving:
 Type and weight of compaction equipment
 Maximum soil layer thickness
 Number of passes for each layer
They are useful for large projects. This requires a prior knowledge of working with the
borrow soils to be used.
Field Compaction Equipment
There is a wide range of compaction equipment. The compaction achieved will depend on the
thickness of lift (or layer), the type of roller, the no. of passes of the roller, and the intensity
of pressure on the soil. The selection of equipment depends on the soil type as indicated.
Equipment Most suitable soils Least suitable soils
Smooth steel drum rollers (static
or vibratory)
Well-graded sand-gravel, crushed
rock, asphalt
Uniform sands, silty
sands, soft clays
Pneumatic tyred rollers Most coarse and fine soils Very soft clays
Sheepsfoot rollers Fine grained soils, sands and gravels
with > 20% fines
Uniform gravels, very
coarse soils
Grid rollers Weathered rock, well-graded coarse
soils
Uniform materials, silty
clays, clays
Vibrating plates Coarse soils with 4 to 8% fines
Tampers and rammers All soil types
Engineering Behaviour of Compacted Soils
The water content of a compacted soil is expressed with reference to the OMC. Thus, soils
are said to be compacted dry of optimum or wet of optimum (i.e. on the dry side or wet
side of OMC). The structure of a compacted soil is not similar on both sides even when the
dry density is the same, and this difference has a strong influence on the engineering
characteristics.
Soil Structure
For a given comp active effort, soils have a flocculated structure on the dry side (i.e. soil
particles are oriented randomly), whereas they have a dispersed structure on the wet side (i.e.

10. particles are more oriented in a parallel arrangement perpendicular to the direction of applied
stress). This is due to the well-developed adsorbed water layer (water film) surrounding each
particle on the wet side.
Swelling
Due to a higher water deficiency and partially developed water films in the dry side, when
given access to water, the soil will soak in much more water and then swell more.
Shrinkage
During drying, soils compacted in the wet side tend to show more shrinkage than those
compacted in the dry side. In the wet side, the more orderly orientation of particles allows
them to pack more efficiently.
Construction Pore Water Pressure
The compaction of man-made deposits proceeds layer by layer, and pore water pressures are
induced in the previous layers. Soils compacted wet of optimum will have higher pore water
pressures compared to soils compacted dry of optimum, which have initially negative pore
water pressure.
Permeability
The randomly oriented soil in the dry side exhibits the same permeability in all directions,
whereas the dispersed soil in the wet side is more permeable along particle orientation than
across particle orientation.
Compressibility
At low applied stresses, the dry compacted soil is less compressible on account of its truss-
like arrangement of particles whereas the wet compacted soil is more compressible.

11. The stress-strain curve of the dry compacted soil rises to a peak and drops down when the
flocculated structure collapses. At high applied stresses, the initially flocculated and the
initially dispersed soil samples will have similar structures, and they exhibit similar
compressibility and strength.
Compaction Quality Control
Compaction Quality Control mainly depends on
 Dry Density
 Water Content
To assess the degree of compaction, it is necessary to use the dry unit weight, which is an
indicator of compactness of solid soil particles in a given volume. The laboratory testing is
meant to establish the maximum dry density that can be attained for a given soil with a
standard amount of compaction effort.
In the test, the dry density cannot be determined directly, and as such the bulk density and the
moisture content are obtained first to calculate the dry density as , where =
bulk density, and w = water content.
A series of samples of the soil are compacted at different water contents, and a curve is drawn
with axes of dry density and water content. The resulting plot usually has a distinct peak as
shown. Such inverted “V” curves are obtained for cohesive soils (or soils with fines), and are
known as compaction curves.
Dry density can be related to water content and degree of saturation (S) as
thus, it can be visualized that an increase of dry density means a decrease of voids ratio and a
more compact soil.
Similarly, dry density can be related to percentage air voids (na) as

12. The relation between moisture content and dry unit weight for a saturated soil is the zero air-
voids line. It is not feasible to expel air completely by compaction, no matter how much
compaction effort is used and in whatever manner.
Effect of Increasing Water Content
As water is added to a soil at low moisture contents, it becomes easier for the particles to
move past one another during the application of compacting force. The particles come closer,
the voids are reduced and this causes the dry density to increase. As the water content
increases, the soil particles develop larger water films around them.
This increase in dry density continues till a stage is reached where water starts occupying the
space that could have been occupied by the soil grains. Thus the water at this stage hinders
the closer packing of grains and reduces the dry unit weight. The maximum dry
density (MDD) occurs at an optimum water content (OMC), and their values can be
obtained from the plot.
Effect of Increasing Compaction Effort
The effect of increasing compaction effort is shown. Different curves are obtained for
different compaction efforts. A greater compaction effort reduces the optimum moisture
content and increases the maximum dry density.
An increase in compaction effort produces a very large increase in dry density for soil when it
is compacted at water contents drier than the optimum moisture content. It should be noted
that for moisture contents greater than the optimum, the use of heavier compaction effort will
have only a small effect on increasing dry unit weights.
It can be seen that the compaction curve is not a unique soil characteristic. It depends on the
compaction effort. For this reason, it is important to specify the compaction procedure (light
or heavy) when giving values of MDD and OMC.