Soil Compaction

Chapter (6)

Instructor : Dr. Jehad Hamad

2017- 2016 1

What is Compaction?

• In most instances in civil engineering

and/or construction practice,

whenever soils are imported or

excavated and re-applied, they are compacted.

• The terms compaction and

consolidation may sound as though

they describe the same thing, but in

reality they do not.

Definition:

Soil compaction is defined as the method of mechanically increasing the

density of soil by reducing volume of air.

Solids

Water

Air

Solids

Water

Air

Compressed

soil

Load

Soil

Matrix

gsoil (1) = WT1

VT1 gsoil (2) =

WT1

VT2

gsoil (2) > gsoil (1)

Densification of soil by removing air voids using mechanical equipment Without changing its

water content

Increase

• STRENGTH

• STIFFNESS

• DURABILITY

Decrease

• PERMEABILITY

Soil Compaction

What is Consolidation

• When a Static loads are applied to saturated soils, and over a period of time the increased stresses are transferred to the soil skeleton, leading to a reduction in void ratio.

• Depending on the permeability of the soil and the magnitude of the drainage distance, this can be a very time-consuming process.

• Typically applies to existing, undisturbed soil deposits that has appreciable amount of clay.

Compaction - Consolidation • Compaction means

the removal of air-filled porosity.

• Consolidation means the removal of water-filled porosity.

Principles of Compaction

uncompacted compacted uncompacted compacted

Compaction of soils is achieved by reducing the volume of voids. It is assumed that the compaction process does not decrease the volume of the solids or soil grains·

What Does Compaction Do?

1. Increased Shear Strength Larger loads can be applied to stronger more compacted Soils

3. Reduced Compressibility

This also means that larger loads can be applied

to compacted soils since they will produce smaller settlements.

2. Reduced Permeability soils’ ability to absorb water, and therefore reduces the tendency to expand/shrink and potentially liquefy

5. Reduce Liquefaction Potential

4. Control Swelling & Shrinking

Water Role in Compaction Process

Water lubricates the soil grains so that they slide more easily over each other and can thus achieve a more densely packed arrangement.

A little bit of water facilitates compaction

Too much water inhibits compaction.

Compaction Effect

Factors Affecting on Compaction

There are 4 control factors affecting the extent of compaction:

1.Compaction effort;

Soil Compaction

Effect of Water on Compaction

• In soils, compaction is a function of water content

• Water added to the soil during compaction acts as a

softening agent on the soil particles

– Consider 0% moisture -Only compact so much

– Add a little water -compacts better

– A little more water -a little better compaction

– Even more water –Soil begins to flow

• What is better compaction?

– The dry unit weight (γd) increases as the moisture content

increases TO A POINT – Beyond a certain moisture content,

any increase in moisture content tends to reduce the dry unit

weight

Principle of Compaction

Standard Proctor Compaction Test

1

• The standard was originally developed to simulate field compaction in the lab.

2

• Purpose: Find the optimum moisture content at which the maximum dry unit weight is attained

3 • ASTM D 698

4 • Equipments;

Standard Energy

• Compactive (E) applied to soil per unit volume:

mold of Volume

drop) of(height * ight)(hammer we * layers) of (# * r)blows/laye (#E

33

SP /375,12(1/30)ft

ft) (1.0 * lbs) (5.5 * layers) of (3 *  ayer)(25blows/lE ftlbft

Standard Proctor 1/30

ft3mold

12”drop

25 blows / layer

3 layers of soil

5.5 lb hammer

Effort Compaction Effort is calculated with the following parameters

• Mold volume = 1/30 cubic foot

• Compact in 3 layers

• 25 blows/layer

• 5.5 lb hammer5.5 lb hammer

• 12" drop

Soil Compaction in the Lab:

1- Standard Proctor Test

2- Modified Proctor Test

Standard Proctor Test Modified Proctor Test

Soil Compaction in the Lab:

1- Standard Proctor Test

wc1 wc2 wc3 wc4

wc5

gd1 gd2 gd3 gd4

gd5

Optimum

Water

Content

Water

Content

Dry Density

gd max

Zero Air Void Curve

Sr =100%

Compaction

Curve

1

2

3

4

5

(OWC)

4 inch diameter compaction mold.

(V = 1/30 of a cubic foot)

5.5 pound hammer

25 blows

per layer

H = 12 in

Wet to

Optimum Dry to

Optimum

Increasing Water Content

e

G wsdry

1

gg

gdry = gwet

Wc 100

% 1+

gZAV = Gs gw

Wc Gs 1+

Sr

gdry max

Optimum

Water

Content

The Shape of the Compaction Curve

Zero air

voids line

Dry Density

Moisture content

d max

OMC

air = 5%?

Soil too dry

and brittle Soil too wet &

deformable

1- Standard Proctor Test

ASTM D-698 or AASHTO T-99

2- Modified Proctor Test

ASTM D-1557 or AASHTO T-180

Energy = 56,520 foot-pounds per

cubic foot

Energy = 12,375 foot-pounds

per cubic foot

Moisture

Content

Dry Density

gd max

Compaction

Curve for Standard

Proctor

(OMC)

gd max

(OMC)

Zero Air Void Curve

Sr < 100%

Zero Air Void Curve

Sr =100%

Zero Air Void Curve

Sr = 60%

Compaction

Curve for

Modified

Proctor

Example: The laboratory test for a standard proctor is shown below. Determine the optimum water

content and maximum dry density. If the Gs of the soil is 2.70, draw the ZAV curve.

Solution:

gdry = gwet

Wc

100

% 1+

Volume of

Proctor Mold

(ft3)

1/30

1/30

1/30

1/30

1/30

1/30

Weight of wet

soil in the

mold (lb)

3.88

4.09

4.23

4.28

4.24

4.19

Water

Content (%)

12

14

16

18

20

22

Volume of

Mold

(ft3)

1/30

1/30

1/30

1/30

1/30

1/30

Weight of wet

soil in the

mold (lb)

3.88

4.09

4.23

4.28

4.24

4.19

Water

Content

(%)

12

14

16

18

20

22

Wet Unit

Weight

(lb/ft3)

116.4

122.7

126.9

128.4

127.2

125.7

Dry Unit

Weight

(lb/ft3)

103.9

107.6

109.4

108.8

106.0

103.0

gZAV = Gs gw

Wc Gs 1+

Sr

INFLUENCE OF SOIL TYPE ON COMPACTION

CURVE

Zero air voids

line

Sand,some fines

Clay

Dry

Density

Moisture

content

d max

OMC Constant compaction energy

2.0

1.9

1.8

1.7

1.6 0 5 10 15 20 25

Water content w (%)

Dry

dens

ity

( M

g /

m )

3

Line of

optimums

"Zero Air Voids"

ZAV:

The curve represents the fully

saturated condition (S=100%).

ZAV cannot be reached by

compaction.

Line of Optimum: A line drawn

through the peak points of

several compaction curves at

different compactive efforts for

the same soil will be almost

parallel to a 100 % S curve

Entrapped Air: is the distance

between the wet side of the

compaction curve and the line of

100% saturation.

Zero-Air-Void

Points from the ZAV curve can

be calculated from:

Holtz and Kovacs, 1981

Degree of Saturation Degree of Saturation

Standard Proctor

Modified Proctor

100% 60% 80%

Wet Side

Dry Side

Results-Explanation

Dry

Dens

ity (

gd)

Water Content (w)

OMC

Below womc

Dry of Optimum

•As the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them easier to be moved about and reoriented into a denser configuration.

Hammer Impact

•Air expelled from the soil upon impact in quantities larger than the volume of water added.

At womc

The density is at the maximum, and it does not increase any further.

Above womc

Wet of Optimum

Water starts to replace soil particles in the mold, and since

w<<s the dry density starts to decrease. Hammer Impact

Moisture cannot escape under impact of the hammer. Instead, the entrapped air is energized and lifts the soil in the region around the hammer.

Holtz and Kovacs, 1981

Effects of Soil Types on Compaction

2.2

2.1

2.0

1.9

1.8

1.7

1.6

5 10 15 20 25

Water content w (%)

Dry

dens

ity

(M

g /

m3)

1

2

3

4

5

6

7

8

Zero air voids, S= 100 %

The soil type-that is, grain-size distribution, shape of the soil grains, specific gravity of soil solids, and amount and type of clay minerals present

Holtz and Kovacs, 1981; Das, 1998

Dry

Density

Moisture content

Standard

Compaction

Modified

Compaction

Influence of Compaction Energy

Modified Proctor Test Same as the Standard Proctor Test with the following

exceptions:

3

MP

3MP

/250,56E

(1/30)ft

ft) (1.5 * lbs) (10 * layers) of (5 *  ayer)(25blows/lE

ftlbft

The soil is compacted in five layers Hammer weight is 10 Lbs or 4.54 Kg

Drop height h is 18 inches -

Then the amount of Energy is calculated

Remember Standard Proctor Energy

55.4/375,12

/250,56

E

E3

3

SP

MP

ftlbft

ftlbft

Comparison-Summary Standard Proctor Test

• Mold size: 1/30 ft3

• 12 in height of drop

• 5.5 lb hammer

• 3 layers

• 25 blows/layer

• Energy 12,375 ft·lb/ft3

Modified Proctor Test

• Mold size: 1/30 ft3

• 18 in height of drop

• 10 lb hammer

• 5 layers

• 25 blows/layer

• Energy 56,250 ft·lb/ft3

Compaction and permeability D

ry D

en

sit

y

or

perm

eab

ilit

y

Moisture content

A

B

C

kmin

Field Density

Sand Cone method

Rubber balloon

Method Nuclear Density method

35

Checking Soil Density in the Field:

1- Sand Cone (ASTM D1556-90)

2- Balloon Dens meter The same as the sand cone, except a rubber

balloon is used to determine the volume of the

hole

3- 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 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 just compacted.

Sand Replacement

1. Cylindrical hole cut in soil

2. Soil kept and weighed (M), then moisture content

(w) obtained by drying the soil

3. Obtain dry sand (SP) of known density

(sand) when poured from a funnel

4. Pour dry sand from container into the hole

5. Loss in mass of container and sand

M volume of hole (V = M/sand)

38

Sand Cone

Hole prepared - plan

Testing with clean dry sand - elevation

Nuclear Density

Nuclear Density Meter

“The rate at which gamma rays pass

through the soil to reach a detector tube

at the ground surface can be converted to

wet density of the soil”.

Take a sample for moisture content and so

obtain Dry Density

Hole prepared

source

sand pad

direct path

1. SET - UP 2. MEASURING - g photon count

g detector

indirect path

Soil Compaction in the Field:

2- Vibratory Plates

3- Smooth Rollers

4- Rubber-Tire

5- Sheep foot Roller

6- Dynamic Compaction

1- Rammers

Heavy Compaction Equipment

Smooth steel rollers, with or

without vibration

Suitable for finishing operation of

fills with sandy or clayey soil

- double or single drum

Pneumatic roller

Tires are closely Spaced

Used for Sandy and clayey soils

Sheepsfoot roller

Drum with large number of projections.

Most Suitable for clayey soils

Common Compaction Curves Encountered in Practice

Bell-shaped One & one-half peaks

Double-peaked

Odd-shaped

Water content (w)

Dry

unit

weight

gd

Structure of Compacted Clay Dry

Dens

ity

Water Content

High Compactive Effort

Flocculated Structure or Honeycomb Structure or Random

Dispersed Structure or parallel

Intermediate structure

Lambe and Whitman, 1979

Particle Arrangement Dry side more random

Water DeficiencyDry side more deficient; thus imbibes more water,

swells more, has lower pore pressure

Structure

Effect of Compaction on permeability P

erm

eab

ilit

y

10-9

10-7

Permeability at constant compactive effort

decreases with increasing water content and

reaches a minimum at about the optimum.

If compactive effort is increased, the

permeability decreases because the void

ratio decreases.

Water content

Den

sity

Magnitude Dry side more permeable

PermanenceDry side permeability reduced much more by

permeation

Permeability

Pressure, natural scale

Void

ratio

, e

0

Low pressure consolidation Pressure, log scale

Void

ratio

, e

0

High-pressure consolidation

Dry compacted or undisturbed sample

Compressibility of compacted clays is function of stress level.

Low stress level: Clay compacted wet of optimum are more

compressible.

High stress level: The opposite is true

Effect of Compressibility

Dry compacted or undisturbed sample

Rebound for both samples

Wet compacted or Remolded sample

Wet compacted or Remolded sample

MagnitudeWet side more compressible in low pressure range,

dry side in high pressure range

Rate Dry side consolidates more rapidly

Compressibility

Compaction and Shrinkage

• samples compacted wet of optimum have the highest shrinkage

1.80

1.75

1.70

1.65

1.60

Kneading

Vibratory

Static

Molding water content (%)

Dry

dens

ity

( M

g /

m3

)

Wet of

optimum

Dry of

optimum

S = 100%

12 16 14 18 20 22 24

OMC

Vibratory compaction

Kneading compaction

Static compaction

Legend

Factor Affecting Soil Compaction:

1- Soil Type

2- Water Content (wc)

3- Compaction Effort Required (Energy)

Why Soil Compaction:

1- Increase Soil Strength

2- Reduce Soil Settlement

3- Reduce Soil Permeability

4- Reduce Frost Damage

5- Reduce Erosion Damage

Types of Compaction : (Static or Dynamic) 1- Vibration

2- Impact

3- Kneading

4- Pressure

Water Content

Dry Density

Effect of Energy on Soil Compaction

Higher

Energy

Increasing compaction energy Lower OWC and higher dry density

In the field

increasing compaction

energy = increasing

number of passes or

reducing lift depth In the lab

increasing compaction

energy = increasing

number of blows

Field Soil Compaction

Because of the differences between lab and field

compaction methods, the maximum dry density in the

field may reach 90% to 95%.

Moisture

Content

Dry Density

gd max

(OMC)

ZAV

95% gd

max

Compaction Specifications:

Compaction performance parameters are given on a construction project in one

of two ways:

1- 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 specifications to accomplish the compaction needed.

2- End-result Specification

Only final compaction requirements are specified (95% modified or

standard Proctor). This method, gives the contractor much more flexibility in

determining the best, most economical method of meeting the required specs.