Soil Mechanics I

CE-225
Soil Compaction

1
 Introduction 2

• The Soil is used as a basic construction material in

many projects such as:
– Retaining walls,
– Highways, Embankments,
– Airports,
– Dams, Dikes, etc.

• The advantages of using soil are:

– It is generally available everywhere,
– It is durable and it will last for a long time,
– It has a comparatively low cost.
 Introduction 3

• The soils at a given site are often less than ideal for
the intended purpose.

• They may be weak, highly compressible, or have a

higher permeability then desirable from engineering
or economical point of view.

• Engineering properties of such soils can be improved

or stabilized.

• Stabilization is usually mechanical or chemical.

• Mechanical stabilization is called compaction.

 Compaction 4

• In most civil engineering projects, whenever soils are

imported or excavated and re-applied, they are compacted.
• The compaction is a ground improvement technique, where
the soil is densified through external compactive effort.
• The degree of compaction is measured by dry unit weight d.

Compactive
effort
+ water =
 Principle of Compaction 5

Compaction is the densification of soils by the application of

mechanical energy to reduce air void spaces in the three phase
soil model
• it reduces the air content, but not the water content
• can’t compact saturated soil (almost always true)

Loose soil Compacted soil

Air Air

Water Water

Solids Solids
 Compaction Advantages 6

• As compaction increases, the following

occurs:
– Increase in soil strength
– Increase in bearing capacity
– Decrease in potential for settlement
– Control of undesirable volume changes
– Reduction in hydraulic conductivity
Compaction vs Consolidation 7
 Compaction – General Principles 8

When water is added Optimum

moisture
 The particles content
develop larger and
larger films
around them.
 Water lubricates d
the particles
 Water helps
moving particles
and orient them = d (at w = 0)
into denser
configuration

When peak density is reached

 Water starts to replace soil particles
 Since w << s, the dry density curve start to fall down
 Compaction – General Principles 9

• Water acts as a lubricant

• Too much water
– takes up space
– does not allow bonding

• Too little water

– same compactive effort, lower compaction

• Optimum moisture content (OMC): The moisture

content of the soil at which maximum density can be
achieved for a given amount of compactive effort.
• OMC of fine-grained soils is higher than coarse-
grained soils.
 Standard proctor test 10

• Developed by R.R. Proctor (1933).

• The compaction is a function of
– Dry density ( d)
– Water content (w)
– Compactive effort (energy E)
– Soil type (gradation, presence of clay
etc.)
• Equipment & methods
– ASTM D 698
– 1/30 ft3 (943.3 cm3) mold, dia 4-in
(101.6 mm)
– 5.5 lb (2.45 kg) hammer
– 12-in (305 mm) drop
– 3 layers of soil
– 25 blows per layer
 Standard Proctor Test - Procedure 11

• The soil is mixed with varying amounts of water to

achieve different water contents.

• For each water content,the soil is compacted by

dropping a hammer 25 times onto the confined soil.

• The soil in mold will be divided into three lifts.

• Each Lift is compacted 25 times.

• This is done 4-6 times from dry-to-wet.

Layer or lift # 3
Layer or lift # 2
Layer or lift # 1
25 Blows/Layer
Standard Proctor Compaction 12
Test results
 Standard Proctor Compaction 13
Test results
Zero-air-void (ZAV)
curve corresponds to S<100%
100% saturation

All compaction points

should lie to the left
of ZAV curve
(because S > 100% is
not possible)
 14

20

Gs = 2.69
19

Dr y un i t wed i(kgNh /t m
3)
18

17
S = 100%
60%
16 70%
80%
90%

15
5 10 15 20 25
Moisture content, w (%)
15
 Effect of Compaction Energy 16

 No of   No   Weight   Height 
       
 blows    of    of    of 
 per layer  layers  hammer  drop 
E        
Volumeof mold

25 3  5.51
E  12,375ft  lb/ft3  592.5 kJ/m3
1 30
 Effect of Compaction Energy 17

 As energy of
compaction increased,
max d of compaction
is also increased.

 As the energy of
compaction is
increased, the
optimum moisture
content is decreased to
some extent.
 Common compaction curves encountered 18

Bell-shaped One & one-half peaks

Clayey soils (LL = 30~70) LL <30
d
Dry unit weight

Double-peaked
LL <30 or LL>70 Odd-shaped
LL>70

Water content (w)

 Example 1 19

Given
1. A borrow pit’s soil is being used as earth fill at a construction
project.
2. The in situ dry unit weight of the borrow pit soil was
determined to be 17.18 kN/m3.
3. The soil at the construction site is to be compacted to a dry
unit weight of 18.90 kN/m3.
4. The construction project requires 15,000 m3 to compacted fill.

Required
Volume of soil required to be excavated from the borrow pit to
provide the necessary volume of compacted fill.
Example 4-5, Page 106
Soils and Foundations by Liu & Evett, 6th Ed.
 Example 2 20

You are a Project Engineer on a large dam project that has a volume
of 5×106 yd3 of select fill, compacted such that the final void ratio in
the dam is 0.80. Your boss, the Project Manager delegates to you
the important decision of buying the earth fill from one of three
suppliers.
Supplier A sells fill at Rs. 50/yd3 with e = 0.90
Supplier B sells fill at Rs. 33/yd3 with e = 2.00
Supplier C sells fill at Rs. 44/yd3 with e = 1.60
Which one of the three suppliers is the most economical, and how
much will you save?
 Example 3 21

Based on the previous problem data, if the fill dumped into the
truck has an e = 1.2, how many truck loads will you need to fill the
dam? Assume each truck carries 10 yd3 of soil.
 Example 2 22

Given
1. The in situ void ratio, e of a borrow pit’s soil is 0.72
2. The borrow pit soil is to be excavated and transported to fill a
construction site where it will be compacted to a void ratio of
0.42.
3. The construction project requires 10,000 m3 of compacted fill.

Required
Volume of soil that must be excavated from the borrow pit to
provide the required volume of fill.

Example 4-6, Page 107

Soils and Foundations by Liu & Evett, 6th Ed.
Structure of compacted clay soil 23
 24

Orientation against
moisture content
for Boston blue
clay
(after Lambe 1958)
 25

Change in
permeability
with molding
water content
 26

Shrinkage as a function
of water content and
type of compaction
Effect of w.c & compaction type on Shrinkage
27
 28

Strength as a function
of water content and
type of compaction
 29

Strength as measured
by CBR and the dry
density versus water
content for laboratory
impact compaction
 30

California Bearing
ratio (CBR) is a
penetration test
for evaluation of
the mechanical
strength of road
subgrades and
base courses
p
CBR  100
ps
p: measured pressure
for site soils
ps:pressure to achieve
equal penetration
on standard soil
 Modified Proctor Test 31

• The modified test was developed to simulate larger

compaction effort to more serious loads and bigger
equipment.
• ASTM D698 Modified E=E2

Dry Density ( d)
• Equipment
– 1/30 cu. ft mold
– 10 lb hammer
– 18 inch drop Standard E=E1

– 5 layers of soil
Water Content (w)
– 25 blows per layer
 Standard vs Modified Proctor Test 32

Standard Modified
Proctor Test Proctor Test
Mold size (ft3) 1/30 1/30
Height of drop (inch) 12 18
Hammer weight (lb) 5.5 10
No. of layers 3 10
No. of blows per layer 25 25
Energy (ft.lb/ft3) 12,375 56,250
Effect of soil type 33

• Soil type influence

the max. dry unit
weight and
moisture content
• Factor influencing
soil type:
– Grain size
distribution
– Shape of soil grains
– Specific gravity of
soil solids
– Amount and type
of minerals
 34

Effect of soil type

 35

Field compaction equipment

and procedures
 Field compaction equipment 36

A family of heavy fill movement and compaction equipment

 Field compaction equipment 37

Motor-scarifier cuts and lays fills in 8 to 24 inch lifts for compaction

Field compaction equipment 38

Motor grader
 Field compaction equipment 39

Water truck used for attaining optimum moisture for compaction

of the subgrade
 Field Compaction 40

• Most of field compaction is done with rollers.

• The most common types of rollers are

– Smooth-wheel rollers (or smooth-drum rollers)
– Pneumatic rubber-tired rollers
– Sheepfoot rollers
– Tamping foot rollers
– Vibratory rollers
– Impact rollers
– Grid rollers
 Smooth-wheel rollers 41

• Coverage: 100% under the wheel

• Contact pressure: up to 380 kPa (55 psi)
• Use: all soil types except rock soils.
• Most common use: proofrolling subgrades and compacting
asphalt pavements

• Not suitable for

compacting thick
layers, as it result in
low unit weight.
 Pneumatic rubber-tired rollers 42

• Coverage: 80% (i.e. 80% of the total area is covered by tires)

• Contact pressure: upto 700 kPa (100 psi)
• Use: may be used for both granular and cohesive highway fills,
as well as for earth dam construction.
 Sheep-foot rollers 43

• Coverage: 8 to 12%
• Contact pressure: 1400 to 7000 kPa
(200 to 1000 psi)
• Use: fine grained soils; sands and
gravels, with >20% fines; good for
breaking down soil ‘clods’
 Tamping foot roller 44

• Coverage: about 40%

• Contact pressure: 1400 to 8400 kPa (
• It is best for compacting fine-grained soils (silt and clay).
• Compactive effort: static weight and kneading.
 Impact roller 45

• Compaction by static pressure, combined with the impact of

the 3 or 5-sided roller
• Higher impact energy breaks up soil clods, achieving better
compaction (like a sheeps-foot roller in some ways)
Impact roller 46
 Vibratory rollers 47

• Extremely efficient in
compacting granular
soils.
• Vibrators can be
attached to smooth-well,
pneumatic rubber-tired,
or sheepfoot rollers to
provide vibratory effect
to soil.
 Grid roller 48

• Coverage: 50%
• Contact pressure: 1400 to 6200 kPa (200 to 900 psi)
• Use: rocky soils, gravels, and sands.
 Plate and Rammer Compactors 49

• Vibrating plate
compactors used for
smaller confined areas
• Rammer compactors
used for backfilling
(trenches)
Equipment – summary 50
51
52
 Factors affecting field compaction 53

• Characteristics of the compactor

– Mass, size
– Operating frequency and frequency range

• Characteristics of the soil

– Initial density
– Grain size and shape
– Water content

• Construction procedures
– Number of passes of the roller
– Lift thickness
– Frequency of operation vibrator
– Towing speed
 Factors affecting field compaction 54

Effect of number of passes

on dry unit weight

Soil type: Silty clay

Roller wt: 84.5 kN (19 kip)

Layer thickness: 229 mm Relationship between dry unit

(9 in) loose weight and number of passes

w: 11.6 & 17.8 %

 Factors affecting 55
field compaction

Variation of dry unit weight with

depth and number of passes

Soil type: sand

Roller wt: 55.6 kN (12.5 kip)

Compaction with vibration

Layer thickness: 2.45 m (8 ft)

loose

No of passes: 2, 5, 15, 45
 56

Field compaction control

and specification
 Control parameters 57

• Dry density and water content correlate well with

the engineering properties, and thus they are
convenient construction control parameters.

• Since the objective of compaction is to stabilize soils

and improve their engineering behavior, it is
important to keep in mind the desired engineering
properties of the fill, not just its dry density and
water content. This point is often lost in the
earthwork construction control.
 Design-construct procedures 58

• Laboratory tests are conducted on samples of the

proposed borrow materials to define the properties
required for design.

• After the earth structure is designed, the compaction

specifications are written. Field compaction control
tests are specified, and the results of these become
the standard for controlling the project.
 Specifications 59

• End-product specifications: This specification is used

for most highways and building foundation, as long
as the contractor is able to obtain the specified
relative compaction, how he obtains it doesn’t
matter, nor does the equipment he uses. Care the
results only !

• Method specifications: The type and weight of roller,

the number of passes of that roller, as well as the lift
thickness are specified. A maximum allowable size of
material may also be specified. It is typically used for
large compaction project.
 Relative compaction 60

Relative compaction, or percent compaction is defined as the

ratio of the ( d)field to ( d max)lab. The ( d max)lab is determined
according to some specified standard (e.g. standard Proctor or
modified Proctor).

If fines < 12%, relative density should be used instead of relative

compaction.

Based on statistical
study of 47 different
granular soils
 Relative compaction 61

Relative compaction or percent compaction

Correlation between relative compaction (R.C.) and the relative

density Dr

R.C.  80  0.2D r It is a statistical result based on

47 soil samples.
As Dr = 0, R.C. is 80

Typical required R.C. = 90% ~ 95%

 62
Water content for the 100% saturation
Line of
field compaction optimums

d max
Control
(1) Relative compaction
90% R.C.

Dry density, d
(2) Water content (dry side or 
wet side)


Increased
compaction
energy

a wopt b c
Water content w %
 Determination of relative compaction 63

• Where and When: First, the test site is selected. It should be

representative or typical of the compacted lift and borrow
material. Typical specifications call for a new field test for
every 1000 to 3000 m2 or so, or when the borrow material
changes significantly. It is also advisable to make the field test
at least one or maybe two compacted lifts below the already
compacted ground surface, especially when sheepsfoot rollers
are used or in granular soils.

• Method: Field control tests, measuring the dry density and

water content in the field can either be destructive or
nondestructive.
 Determination of field unit weight 64

• When compaction work is progressing in field,

knowing whether the specified unit weight has been
achieved is useful.

• The standard procedures for determining the field

unit weight of compaction include
– Sand cone method
– Rubber balloon method
– Nuclear method
Determination of field unit weight 65
 66

A
Determination of field unit weight 67
Determination of field unit weight 68
 Example 69

Result of a Standard Proctor Test

Determine the max. dry 20
unit wt. and optimum
Gs = 2.70
moisture content from
the graph. 19

h t/ m
Dr y un i t we di(kg N 3)
What is the appropriate 18
moisture range for
attaining 98% of
Standard proctor in the 17
S = 100%
field?
70%
16
Determine the degree 80%
90%
of saturation at 60%
maximum dry unit 15
weight and at optimum 0 5 10 15 20 25
water content.
Moisture content, w (%)
 Problems 70

Problem 5.8 (DAS)

 Problems 71

Problem 5.9 (DAS)

 Problems 72

Problem 5.10 (DAS)

Problem 5.11 (DAS)