Understanding Soil Compaction: Strength, Settlement, & Geotechnical Impact

Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
BACHELOR OF SCIENCE IN CIVIL ENGINEERING
UNDERSTANDING SOIL COMPACTION AND IT’S IMPACT ON STRENGTH AND
SETTLEMENT
Submitted to:
ENGR. REYNALDO LUMABAN
(Instructor)
Submitted by:
MATEL, JAYRIV O.
BSCVE-4C
December, 2025
Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
ABSTRACT
This paper examines how soil compaction influences the strength and settlement
characteristics of soils used in civil engineering projects. Compaction increases soil density by
reducing air voids, which enhances shear strength, bearing capacity, and stability (Das & Sobhan,
2017). It also reduces settlement, preventing long-term deformation in structures such as
foundations, embankments, and pavements. This paper discusses basic concepts, laboratory
methods, and engineering applications, demonstrating that soil compaction is one of the most
critical processes in construction and geotechnical engineering.
INTRODUCTION
Soil compaction is a fundamental aspect of geotechnical engineering and is essential for
improving the performance and reliability of soil used as a construction material (Budhu, 2011).
Whether constructing buildings, highways, embankments, or dams, engineers rely heavily on
compaction to increase bearing capacity, reduce settlement, and ensure long-term stability.
Compaction is defined as the mechanical process of increasing soil density by reducing air voids
(Holtz et al., 2011). Understanding the relationship between compaction, strength, and settlement
is necessary for safe and cost-effective civil engineering design.
MAIN DISCUSSION
Basic Concepts
Soil compaction occurs when mechanical energy is applied to soil, causing particles to
rearrange into a denser configuration. Compaction, in general, is the densification of soil by
removal of air, which requires mechanical energy. The degree of compaction of a soil is measured
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
in terms of its dry unit weight. When water is added to the soil during compaction, it acts as a
softening agent on the soil particles. The soil particles slip over each other and move into a densely
packed position. The dry unit weight after compaction first increases as the moisture content
increases. (See Figure 1.) Note that at a moisture content w = 0, the moist unit weight is equal to
the dry unit weight, or
When the moisture content is gradually increased and the same compactive effort is used for
compaction, the weight of the soil solids in a unit volume gradually increases. For example, at w
= w1,
However, the dry unit weight at this moisture content is given by
Beyond a certain moisture content w = w2 (Figure 1), any increase in the moisture content tends
to reduce the dry unit weight. This phenomenon occurs because the water takes up the spaces that
would have been occupied by the solid particles. The moisture content at which the maximum dry
unit weight is attained is generally referred to as the optimum moisture content (Das & Sobhan,
2017).
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
The laboratory test generally used to obtain the maximum dry unit weight of compaction, and the
optimum moisture content is called the Proctor compaction test,
Figure 1 Principles of compaction
Note. “From Soil mechanics and foundations (3rd ed.).”
Soil compaction can be classified in several ways depending on the criteria used. One
common method is classification based on the type of compactive effort, which includes static,
kneading, impact, and vibratory compaction. Static compaction applies large, slow, direct
pressure—typical of smooth-wheel rollers—while kneading compaction (e.g., sheepsfoot rollers)
manipulates soil through shearing action. Impact compaction involves dropping a heavy weight to
densify the soil, and vibratory compaction relies on high-frequency vibrations, making it especially
effective for granular soils (Das & Sobhan, 2014). Another classification approach is based on soil
type, where coarse-grained soils (sands and gravels) respond best to vibration, whereas finegrained soils (silts and clays) are better compacted by kneading or impact methods (Holtz, Kovacs,
& Sheahan, 2011). Soil compaction is also classified according to compaction tests, such as the
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
Standard Proctor Test and Modified Proctor Test, which determine the optimum moisture content
(OMC) and maximum dry density (MDD) for a given soil under different levels of energy (Budhu,
2015). Finally, compaction can be categorized based on the degree of compaction, often expressed
as a percentage of MDD, which is commonly required in construction specifications—for
example, 90–95% for structural fill and up to 98% for highway subgrades (Craig & Knappett,
2012). These classification systems help engineers select appropriate compaction methods,
equipment, and quality control procedures to ensure soil performance in civil engineering projects.
Experimental Methods
PROCTOR COMPACTION TEST—ASTM D 1140 AND ASTM D 1557
The standard Proctor test is a laboratory test designed to measure the maximum dry unit
weight of a soil by applying a specified amount of mechanical energy (compactive effort). The
conventional Proctor test involves mixing a dry soil sample with water and compacting it in a
cylindrical mold with a volume of 9.44 x 10^-24 m^3 by repeatedly striking it with a 2.5 kg
hammer that is dropped freely from a height of 305 mm (Figure 2). Each of the three layers of
compacted earth receives 25 strikes.
The energy imparted by the hammer is
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
where mh is the mass of the hammer, g is the acceleration due to gravity, hd is the height of fall of
the hammer, V is the volume of compacted soil, Nb is the number of blows, and Nl is the number
of layers. Thus, the compaction energy of the standard Proctor test is
Figure 2 Compaction Apparatus.
Note. From “Principles of geotechnical engineering (9th ed.)”
The normal Proctor test is adequate for the majority of projects. A modified Proctor test
was created for projects involving enormous loads, like runways to support big airplane loads. The
hammer in this test weighs 4.54 kg and descends freely from a height of 457 mm. In the typical
Proctor mold, the dirt is compressed in five stages using 25 blows per layer. The modified Proctor
test has a compaction energy of 2695 kJ/m3, which is roughly 4.5 times that of the conventional
Proctor test.
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
The soil is subjected to four or more tests with varying water contents. When more water
results in a decrease in the bulk unit weight of the soil, this is the final test. The findings are
displayed as water content (abscissa) versus dry unit weight (ordinate). Figure 3 displays typical
dry unit weight–water content graphs.
Bell-shaped curves are typically produced by clays. Sands often show an early fall in dry
unit weight, attributable to capillary tension that restrains the free movement of soil particles,
followed by a hump. Some soils—those with liquid limit less than 30% and fine, poorly graded
sands—may form one or more humps before the maximum dry unit weight is obtained. The
optimal water content (wopt) is the water content at which the greatest dry unit weight is attained.
Air is released when the water content is below optimal (dry of optimum), and water helps the soil
grains reorganize into a denser structure, increasing the number of soil grains per unit volume of
soil (Budhu, 2011).
Figure 3 Dry unit weight-water content curves
Note. From “Principles of geotechnical engineering (9th ed.)”
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
INTERPRETATION
For building specifications of soil improvement via compaction, it is crucial to understand
the ideal water content and the maximum dry unit weight of soils. A minimum of 95% of Proctor
maximum dry unit weight is typically required by specifications for earth structures (footings,
embankments, etc.). Two water contents can be used to achieve this level of compaction: one
before reaching the maximum dry unit weight, known as dry of optimal, and the other after
reaching the maximum dry unit weight, known as wet of optimum (Figure 4). Compaction of the
dry soil is standard procedure. Soil liners for solid waste landfills, projects where soil volume
changes due to unacceptable moisture conditions, and swelling (expansive) soils should all be
compacted.
Figure 4 Illustration of compaction specification of soils in the field
Note. From “Soil mechanics and foundations (3rd ed.)”
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
When a heavily compacted soil mass (near to maximum dry unit weight) is sheared, it tends
to expand (dilate) and gets looser. Usually this expansion is not uniform; some parts of the soil
mass are looser than other parts. The flow rate of water in the soil will increase as water can easily
(compared to the intact one) flow through the looser parts, possibly leading to catastrophic failure
(Budhu, 2011). Heavily compacted soils tend to show sudden decrease in strength when sheared.
In engineering, if failure is to occur, we prefer that it occurs gradually rather than suddenly so that
mitigation measures can be implemented. In some earth structures (for example, earth dams) you
should try to achieve a level of compaction that would cause the soil to behave ductile (ability to
deform without rupture). This may require compaction wet of optimum at levels less than 95% of
the maximum dry unit weight (approximately 80% to 90% of maximum dry unit weight).
Applications in Civil Engineering
Soil compaction is a fundamental process in civil engineering because it enhances the
overall performance and stability of soil used in construction. Properly compacted soil increases
load-bearing capacity, which is essential for supporting building foundations, pavements,
highways, and airfield runways (Holtz, Kovacs, & Sheahan, 2011). By reducing the void ratio,
compaction also minimizes both immediate and long-term settlements, thereby preventing
structural damage such as cracking and uneven surface deformation (Das & Sobhan, 2014).
Additionally, compaction improves shear strength, leading to greater slope stability and safer
construction of embankments and retaining structures. It also decreases soil permeability, which
is crucial in applications such as landfill liners, earth-dam cores, and canal linings where
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
controlling seepage is necessary (Budhu, 2015). Compaction helps mitigate the swelling and
shrinkage behaviors of expansive clays used in subgrades, and it enhances resistance to frost action
in colder regions, contributing to more durable pavement systems. Overall, soil compaction is
indispensable for ensuring the strength, durability, and long-term serviceability of civil
engineering infrastructure.
CONCLUSION
Soil compaction is one of the most important steps in preparing the ground for any
construction project. By increasing the density of the soil and reducing the amount of air within it,
compaction improves key properties such as strength, stability, and resistance to settlement. These
improvements help ensure that structures, from buildings and roads to embankments and dams,
can safely withstand the loads placed on them over time.
Laboratory tests like the Standard and Modified Proctor tests play a crucial role in
identifying the optimum moisture content and maximum dry unit weight of a soil. These values
guide engineers in choosing the right compaction methods and equipment in the field.
Understanding how different soils behave during compaction allows engineers to avoid problems
such as excessive settlement, sudden strength loss, or water-related failures.
Overall, soil compaction is more than just a routine procedure, it is a critical part of making
sure that the foundations of our infrastructure are safe, reliable, and built to last. By applying proper
compaction techniques, engineers can create stable ground conditions that support long-term
performance and reduce the risk of structural issues in the future.
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Philippine Christian University
College of Engineering and Technology
Aguinaldo Highway, Sampaloc 1, Dasmariñas City, Cavite 4114
REFERENCES
Bowels, J. E. (1996). Foundation analysis and design (5th ed.). McGraw-Hill.
Budhu, M. (2011). Soil mechanics and foundations (3rd ed.). John Wiley & Sons.
Craig, R. F., & Knappett, J. (2012). Craig’s soil mechanics (8th ed.). CRC Press.
Das, B. M., & Sobhan, K. (2017). Principles of geotechnical engineering (9th ed.). Cengage
Learning.
Holtz, R. D., Kovacs, W. D., & Sheahan, T. C. (2011). An introduction to geotechnical engineering
(2nd ed.). Pearson.
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