Geotechnical Engineering I

Module 6: Soil Compaction

Instructor: Engr. Jerrick Wayne C. Vertudazo
 In geotechnical engineering, soil conditions at a site are often less than ideal, with issues like
weak strength, high compressibility, or excessive permeability. While relocating a structure may seem
logical, other considerations often dictate its placement, requiring engineers to work with existing
conditions. This can involve adapting the foundation or improving soil properties. Soil stabilization,
which can be mechanical, chemical, thermal, or electrical, is often the most economical solution.

Compaction
Compaction, a type of mechanical stabilization, involves densifying soil by removing air
through mechanical energy. It is crucial in projects where soil itself is the construction material, such
as earth dams and highway embankments. Proper compaction enhances stability and reduces
settlement. Historically, poorly compacted fills led to frequent embankment failures. Early compaction
methods relied on manual effort or animals like elephants, which proved inefficient.
Modern compaction achieves densification by adding water to soften soil particles, enabling
them to rearrange into denser formations. The degree of compaction is measured by dry unit weight,
which initially increases with moisture content. Beyond a certain point, known as the optimum
moisture content, further moisture reduces dry unit weight as water occupies spaces that could
otherwise hold soil particles.

The Proctor compaction test determines the maximum dry unit weight and optimum moisture
content for a soil.
Basic Concept
Dry unit weight (γd) is influenced by the void ratio and moisture content:
𝐺𝑠 𝛾 𝐺𝑠
𝛾𝑑 = ( ) 𝛾𝑤 = =( ) 𝛾𝑤
1+𝑒 1+𝑤 𝑤𝐺
1+ 𝑆𝑠
Reducing the void ratio or increasing the degree of saturation (S) maximizes dry unit weight.
The theoretical maximum is reached when S=100% (saturation), but achieving zero air voids through
compaction is rare.
A theoretical plot of dry unit weight versus moisture content shows that dry unit weight
decreases as water content increases, due to the higher density of soil solids compared to water. The
curve for S=100%, known as the saturation line or zero air voids curve, represents an idealized state
rarely attained in practice.

Theoretical maximum dry unit weight-water content relationship

for different degrees of saturation

Proctor Compaction Test (ASTM D 1140 and ASTM D 1557)

The Proctor compaction test determines the maximum dry unit weight of soil under a
standard compactive effort, simulating field compaction conditions.

Compaction apparatus
Standard Proctor Test
 Procedure: Dry soil is mixed with water and compacted in a cylindrical mold with a volume
of 9.44 × 10⁻⁴ m³. Compaction is done in three layers, with each layer receiving 25 blows from
a 2.5-kg hammer dropped from a height of 305 mm.
 Compaction Curve: Tests are performed at varying water contents. Results, plotted as dry
unit weight versus water content, typically form a bell-shaped curve for clays, while sands
may exhibit humps or irregularities.
 Optimum Water Content (wopt): The water content at which the maximum dry unit weight
(γd max) is achieved.

Dry unit weight – water content curves

Modified Proctor Test

For projects requiring higher compaction, such as runways:
 The hammer mass is increased to 4.54 kg, and the drop height to 457 mm.
 The soil is compacted in five layers, each receiving 25 blows.
 This test achieves a higher maximum dry unit weight and a lower optimum water content
compared to the standard test due to increased compactive effort.

Results and Observations

1. Below optimum water content (dry of optimum): Air is expelled, and water helps soil
particles rearrange into a denser configuration, increasing dry unit weight.
2. Above optimum water content (wet of optimum): Additional water displaces soil particles,
reducing the number of particles per unit volume, which decreases dry unit weight.
3. Degree of Saturation (S): At maximum dry unit weight, the soil is typically unsaturated (S <
1). The degree of saturation can be calculated from the Proctor test results using:
γd = (Gs / (1 + e)) γw = γ / (1 + w) = (Gs / (1 + wGs / S)) γw
If the specific gravity of soil solids (Gs) is unknown, a typical value of 2.7 can be used.

Key Differences
 Standard Proctor Test: Suitable for general applications.
 Modified Proctor Test: Designed for heavy-load applications, offering higher compaction
levels.
This test is essential in geotechnical engineering for evaluating and ensuring soil stability under
various load conditions.
Sample Problem 1
The wet mass of one of the standard Proctor test samples is 1806 grams at a water content of 8%.
The volume of the standard Proctor test sample is 9.44 x 10-4 m3. Determine the bulk and dry unit
weights.
Bulk unit weight
1806
𝑊 (1000) 9.81 𝑁 𝑘𝑁
𝛾= = −4
= 18767.86 3 = 18.768 3
𝑉 9.44 × 10 𝑚 𝑚
Dry unit weight
𝛾 18.768 𝑘𝑁
𝛾𝑑 = = = 17.378 3
1+𝑤 1.08 𝑚

Sample Problem 2
The results of a standard compaction test are shown in the table below.
Water content (%) 6.2 8.1 9.8 11.5 12.3 13.2
Bulk unit weight (kN/cu.m.) 16.9 18.7 19.5 20.5 20.4 20.1
(a) Determine the maximum dry unit weight and optimum water content.
(b) What is the dry unit weight and water content at 95% standard compaction, dry of optimum?
(c) Determine the degree of saturation at the maximum dry density.

Solution
Water Content (%) Bulk unit weight (kN/m3) Dry unit weight (kN/m3)
𝛾
𝛾𝑑 =
1+𝑤
6.2 16.9 15.91337
8.1 18.7 17.2988
9.8 19.5 17.75956
11.5 20.5 18.38565
12.3 20.4 18.16563
13.2 20.1 17.75618

19

18.5
Dry Unit Weight (kN/cu.m.)

18

17.5

17

16.5

16

15.5
6 7 8 9 10 11 12 13 14
Moisture Content (%)

(a) From the graph, maximum dry density = 18.4 kN/cu.m and optimum moisture content = 11.7%
(b) At 95% compaction, dry unit weight = 0.95(18.4) = 17.48 kN/cu.m. and w = 8.5%.
(c)
𝐺𝑠 𝛾𝑤 2.7(9.81)
𝛾𝑑 = → 18.4 = → 𝑒 = 0.44
1+𝑒 1+𝑒
𝑤𝐺𝑠 0.117(2.7)
𝑆𝑒 = 𝑤𝐺𝑠 → 𝑆 = = = 0.72 = 72%
𝑒 0.44

Sample Problem 3
Aggregates from a material storage site are required for the embankment of a roadway. The porosity
of the aggregates at the storage site is 80%, and the desired porosity of the compacted aggregates in
the embankment is 20%. For a section of the embankment 7.6 m wide x 0.61 m compacted
thickness x 305 m long, calculate the volume of aggregates required.
Solution:
𝑉𝑒𝑚𝑏𝑎𝑛𝑘𝑚𝑒𝑛𝑡 = 7.6(0.61)(305) = 1413.98
𝑒𝑠𝑠
𝑛𝑠𝑠 = = 0.80 → 𝑒𝑠𝑠 = 4
1 + 𝑒𝑠𝑠
𝑒𝑒𝑚𝑏
𝑛𝑒𝑚𝑏 = = 0.20 → 𝑒𝑒𝑚𝑏 = 0.25
1 + 𝑒𝑒𝑚𝑏
𝑽𝒔𝒕𝒐𝒓𝒂𝒈𝒆 𝒔𝒊𝒕𝒆 𝟏 + 𝒆𝒔𝒕𝒐𝒓𝒂𝒈𝒆 𝒔𝒊𝒕𝒆
=
𝑽𝒆𝒎𝒃𝒂𝒏𝒌𝒎𝒆𝒏𝒕 𝟏 + 𝒆𝒆𝒎𝒃𝒂𝒏𝒌𝒎𝒆𝒏𝒕
𝑉𝑠𝑡𝑜𝑟𝑎𝑔𝑒 𝑠𝑖𝑡𝑒 1+4
= =4
𝑉𝑒𝑚𝑏𝑎𝑛𝑘𝑚𝑒𝑛𝑡 1 + 0.25
𝑉𝑠𝑡𝑜𝑟𝑎𝑔𝑒 = 4 ∗ 𝑉𝑒𝑚𝑏 = 4(1413.98)
𝑽𝒔𝒕𝒐𝒓𝒂𝒈𝒆 = 𝟓𝟔𝟓𝟓. 𝟗𝟐 𝒄𝒖. 𝒎.

Sample Problem 4
The detailed results of a standard compaction test are shown in the table below. Determine the
maximum dry unit weight and optimum water content.
Diameter of mold = 101.4 mm
Height of mold = 116.7 mm
Mass of mold, M = 4196.50 grams
Unit weight data Water content data
Mass of wet soil and Mass of can and Mass of can and dry Mass of can
mold (grams) wet soil soil (grams) (grams)
5906 114.92 111.48 46.5
6056 163.12 155.08 46.43
6124 190.43 178.64 46.2
6156 193.13 178.24 46.5
6103 188.77 171.58 46.1

Solution
𝜋
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑚𝑜𝑙𝑑 = ∗ 0.10142 ∗ 0.1167 = 9.424 ∗ 10−4 𝑐𝑢. 𝑚.
4
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑛 𝑎𝑛𝑑 𝑤𝑒𝑡 𝑠𝑜𝑖𝑙 − 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑛 𝑎𝑛𝑑 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙
𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 = ∗ 100%
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑛 𝑎𝑛𝑑 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙 − 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑛
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑤𝑒𝑡 𝑠𝑜𝑖𝑙 (𝑖𝑛 𝑘𝑁)
𝑀𝑜𝑖𝑠𝑡 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑚𝑜𝑙𝑑 (𝑖𝑛 𝑐𝑢. 𝑚. )
𝑀𝑜𝑖𝑠𝑡 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡
𝐷𝑟𝑦 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 =
1+𝑤
 Water content calculation Unit weight calculation
Mass of Mass of can Mass of Moistur Mass of wet Mass of Moist unit Dry unit
can and and dry soil can e soil and mold wet soil weight weight
wet soil (grams) (grams) content (grams) (grams) (kN/cu.m.) (kN/cu.m.
(grams) (%) )
114.92 111.48 46.5 5.3 5906 1709.5 17.8 16.9
163.12 155.08 46.43 7.4 6056 1859.5 19.4 18.0
190.43 178.64 46.2 8.9 6124 1927.5 20.1 18.4
193.13 178.24 46.5 11.3 6156 1959.5 20.4 18.3
188.77 171.58 46.1 13.7 6103 1906.5 19.8 17.5

19.0

18.5

18.0

17.5

17.0

16.5

16.0
4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0

From the graph, max dry weight = 18.5 kN/m3, optimum moisture content = 9.7%

Benefits of Soil Compaction

Soil compaction, achieved through mechanical effort with or without water, is a widely used method
to improve soil properties. The process densifies the soil by expelling air and rearranging particles,
offering several advantages:
Benefits:
1. Increased Strength: Compaction enhances the soil's ability to resist applied forces,
improving stability.
2. Higher Load-Bearing Capacity: Compacted soil can support greater loads, making it
suitable for construction foundations and pavements.
3. Reduced Settlement: Lower compressibility minimizes both total and differential
settlements, preventing structural issues.
4. Decreased Water Seepage: Compaction reduces soil permeability, limiting water flow and
mitigating seepage-related problems.
5. Controlled Soil Swelling and Contraction: Proper compaction reduces the potential for
expansion (swelling) and shrinkage (contraction) of soils.
6. Improved Stability: The denser soil fabric resists deformation under load, contributing to
long-term stability.
7. Reduced Frost Damage: Compact soil minimizes the effects of freeze-thaw cycles, preventing
frost heaving.
Consequences of Improper Compaction:
1. Structural Distress: Excessive settlement can lead to structural instability and failure.
2. Cracking: Pavements, floors, and basements are prone to cracks when underlying soil is not
compacted correctly.
3. Damage to Subsurface Structures: Poorly compacted soil can compromise buried pipes,
conduits, and utility systems.
4. Erosion: Loose soil is more susceptible to erosion by water or wind, undermining the
integrity of the area.

Essential Points:
1. Definition of Compaction: Compaction is the densification of soil by expelling air and
rearranging soil particles.
2. Role of the Proctor Test: It determines the maximum dry unit weight and optimum water
content, serving as a reference for field compaction specifications.
3. Impact of Compactive Effort: Increased compactive energy results in a higher maximum
dry unit weight and lower optimum water content.
4. Key Benefits of Compaction: Enhanced strength, reduced compressibility, and lower
permeability are achieved, leading to improved soil performance.
By ensuring proper compaction practices, geotechnical stability, structural integrity, and long-term
performance of projects are significantly enhanced.

Field Compaction
Field compaction is a critical step in achieving stable and durable soil structures. The process
involves using specialized equipment to densify soil to meet specified compaction standards.
Compaction Equipment
1. Smooth-Wheel Rollers (Smooth-Drum Rollers):
o Ideal for proof rolling subgrades and finishing fills with sandy and clayey soils.
o Provide 100% ground coverage.
o Ground contact pressures: 310–380 kN/m².
o Not effective for achieving high unit weights in thick layers.

2. Pneumatic Rubber-Tired Rollers:

o Feature several rows of closely spaced tires (4–6 per row).
o Contact pressures: 600–700 kN/m².
o Coverage: 70–80%.
o Suitable for sandy and clayey soils, offering compaction through pressure and
kneading action.
3. Sheepsfoot Rollers:
o Equipped with drums containing projections (area: 25–85 cm²).
o Best for compacting clayey soils.
o Contact pressures: 1400–7000 kN/m².
o Initially compacts the lower lift portions, followed by the top and middle layers.

4. Vibratory Rollers:
o Highly effective for granular soil compaction.
o Vibrations are generated by rotating off-center weights.
o Variants include smooth-wheel, pneumatic rubber-tired, and sheepsfoot rollers with
vibratory effects.
o Handheld vibrating plates and gang-mounted machines are available for limited or
restricted areas.
Specifications for Field Compaction
Field compaction standards require achieving a specific percentage of the maximum dry unit weight
determined in the laboratory through standard or modified Proctor tests.
 Relative Compaction Formula:
R (%) = (γd field / γd max lab) * 100
where:
R = Relative compaction (in percentage)
γd field = Dry unit weight achieved in the field
γd max lab = Maximum dry unit weight from laboratory tests

Field Compaction Testing Equipment

To measure field compaction, the following equipment is commonly used:
1. Sand Cone Method: Measures the in-situ density of soil by determining the volume of a hole
filled with sand.
2. Balloon Method: Uses a water-filled balloon to measure the volume of excavated soil.
3. Nuclear Density Meters: Employ radioactive isotopes to quickly determine soil density and
moisture content on-site.
Effective field compaction ensures soil stability, reduces settlement, and improves the performance
of the soil in supporting structural loads.

Sample Problem 5
A field compacted of sandy loam was found to have a wet density of 2181 kg/cu.m. at a water
content of 10%. The maximum dry density of the soil obtained in a Standard Proctor Test was 2000
kg/cu.m. Assume specific gravity (Gs) to be 2.65. Compute the following:
a. Dry unit weight (γd); b. Dry unit weight at zero air voids (γz); c. Degree of saturation (S); d.
Porosity (n); e. Percent compaction of the field sample
Solution
a.
𝜌 2181 𝑘𝑔 1982.7 𝑘𝑔 9.81 𝑁 1 𝑘𝑁 𝑘𝑁
𝜌𝑑 = = = 1982.7 → 𝛾𝑑 = ∗ ∗ = 19.45
1 + 𝑤 1 + 0.10 𝑐𝑢. 𝑚. 𝑐𝑢. 𝑚. 1 𝑘𝑔 1000 𝑁 𝑐𝑢. 𝑚.
b.
𝐺𝑠
𝛾𝑑 = ( ) 𝛾𝑤 ; 𝑓𝑜𝑟 𝑧𝑒𝑟𝑜 𝑎𝑖𝑟 𝑣𝑜𝑖𝑑𝑠, 𝑠𝑒𝑡 𝑆 = 100%
𝑤𝐺
1+ 𝑆𝑠
𝐺𝑠 2.65 ∗ 9.81
𝛾𝑧𝑎𝑣 = ( ) 𝛾𝑤 = = 20.55 𝑘𝑁/𝑚3
1 + 𝑤𝐺𝑠 1 + 0.10 ∗ 2.65
c.
𝐺𝑠 𝛾𝑤 2.65 ∗ 9.81
𝛾𝑑 = ; 19.45 = → 𝑒 = 0.337
1+𝑒 1+𝑒
0.10 ∗ 2.65
𝑆𝑒 = 𝑤𝐺𝑠 → 𝑆 = = 0.787 = 78.7%
0.337
d.
𝑒 0.337
𝑛= = = 0.252
1 + 𝑒 1.337
e.
𝛾𝑑(𝑓𝑖𝑒𝑙𝑑) 𝜌𝑑(𝑓𝑖𝑒𝑙𝑑) 1982.7
𝑅(%) = ∗ 100 = ∗ 100 = ∗ 100 = 99.14% 𝑐𝑜𝑚𝑝𝑎𝑐𝑡𝑖𝑜𝑛
𝛾𝑑(𝑚𝑎𝑥−𝑙𝑎𝑏) 𝜌𝑑(𝑚𝑎𝑥−𝑙𝑎𝑏) 2000