CHAPTER 6 SOIL COMPACTION€¦ · Stabilize or improve the properties of the soil The third alternative may be in most cases the most economical alternative. There are different techniques

CHAPTER 6

SOIL COMPACTION

Omitted Sections

6.6, 6.7, 6.8

SOIL COMPACTION

In Geotechnical engineering practice, the soils at a given site

are often less than desirable for the intended purpose. They

may be:

Weak (strength)

Highly compressible

Have a high permeability

Solution

Relocate the project

Articulate design for structure members

Stabilize or improve the properties of the soil

The third alternative may be in most cases the most economical

alternative. There are different techniques for improvement of soils

(This subject is covered in details in CE 486 “Improvement of

Geotechnical Materials”).

We will consider in this course only compaction.

SOIL COMPACTION

Compaction is also very important when soil is used as an

engineering material, that is the structure itself is made of soil.

Ex.

Earth dams

Highways

Airfields

etc.

Compaction is the densification of soils by removal of air

through the application of mechanical energy.

Definition

The degree of compaction is measured in terms of its dry unit

weight.

SOIL COMPACTION

Increases unit weight

Increases shear strength

Increases bearing capacity

Increases stability of slopes of embankments

Decreases settlement of structures

SOIL COMPACTION

Air

Water

Solid

Air

Water

Solid

Compaction

reduced

You remember well-graded

SOIL COMPACTION

Soil Solid

Soil Solid

water

gd(max)

The degree of compaction of soil is measured by its dry unit weight.

When water is added during compaction it acts as a softening agent

on the soil particles.

When the moisture content is

gradually increased, the weight

of the soil solids in a unit

volume gradually increases.

Optimum moisture content (OMC) is the water content at

which the maximum dry unit weight is attained. (max)dg

General Principle

SOIL COMPACTION

Soil Solid

Soil Solid

water

SOIL COMPACTION

Types of Compaction Methods in the Laboratory

•Impact or dynamic (The most common type)

•Kneading

•Static

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.

It is named after R. R. Proctor (1933) (engineer in LA). He

established that compaction is a function of:

1.Moisture Content

2.Compactive Effort

3.Soil Type

Standard Proctor test (ASTM D-698 & AASHTO T-99)

Modified Proctor test (ASTM D-1557 & AASHTO T-180)

There are two methods or tests:

• Mold 1/30 ft3 in volume

• 3 layers

• 25 blows

• 5.5 lb hammer

• 12 inch drop

Mold Hammer

The procedure for the standard Proctor test is elaborated in ASTM

Test Designation D-698 (ASTM, 2007) and AASHTO Test

Designation T-99 (AASHTO, 1982).

Standard Proctor Test


Process of Compaction

Several samples are mixed at different water contents

mold

moistV

Wg

W = Weight of compacted soil in the mold

Vmold = Volume of the mold = (1/30 ft3)

w

moistd

1

gg

(max)dg

Compact according to the compaction test (standard or modified).

For each test find the moisture

content of the compacted soil.

The dry unit weight is given by

From the plot, find OMC and

Plot vs. w dg


In order to avoid a large number of compaction tests, it is

desirable to begin the first test at a moisture content that is about

4 to 5% below the approximate optimum moisture content.


REMARKS

1. Each data point on the curve represent a single compaction test.

2. Four or five tests are required

3. The curve is unique for:- A given soil type- Method of compaction- (constant) compactive effort

4. gd(max) is only a maximum for a specific compactive effort and method ofcompaction. This does not necessarily reflect the maximum dry unit weightthat can be obtained in the field.

5. Typical OMC are between 10% and 20%. Outside maximum range 5% to40%.


8. For clay soils gd(max) tends to decrease as plasticity increases.

9. The approximation to field is not exact because the lab. test is a dynamicimpact type, whereas field compaction is essentially a kneading-typecompaction.

7. In practice less amount of water is used but higher compactive effort orvise versa.

10. In the field, compactive effort is the number of passes or “coverage” of theroller of a certain type and weight on a given volume of soil.

6. Increasing the compactive effort tends to increase the maximum drydensity, as expected, but also decrease the OMC. (This is why the curvenever be to the right of zero curve).


The maximum is obtained when no air in the voids (i.e. s =100%) (max)dg

ws

de

Ggg

1

sewGs but

swGe 100%Sfor

wwG

G

sG

ww

s

s

avz

11

ggg

Where gzav = zero air void unit weight.

The relationship between gzav and w can be obtained as shown in the figure across.

Compaction curve is always to the left of the zero-air-void curve.

No matter how much water is added, the soil never

becomes completely saturated by compaction.

Theoretical d(max) g


To obtain the variation of gzav with moisture content, use

the following procedure:

Under no circumstances should any

part of the compaction curve lie to

the right of the zero-air-void curve.


Besides moisture content, other important factors that affect

compaction are: 1) Soil type; 2) Compaction effort.

1. Effect of Soil Type

Grain Size Distribution

Shape of the soil grains

Gs

Amount of clay minerals

Type of clay minerals

Fine grain soil needs more water

to reach optimum.

FACTORS AFFECTING COMPACTION


Effect of Soil type and gradation

Fine grain soilneeds more waterto reach optimum.


Gs is constant, therefore increasing maximum dry unit weight is associated

with decreasing optimum moisture contents.

Do not use typical values for design as soil is highly variable.

Effect of Soil type and gradation

Typical Values

(kN/m3) OMC (%)

Well graded sand SW 22 7

Sandy clay SC 19 12

Poorly graded sand SP 18 15

Low plasticity clay CL 18 15

Non plastic silt ML 17 17

High plasticity clay CH 15 25

(max)dgwwG

G

sG

ww

s

s

avz

11

ggg


•The bell-shaped compaction

curve is typical for most clayey

soils.

Compaction Curves Encountered in Soils

Typical

•Some curves have more

than one peak others have no

peak.


• The standard Proctor mold and

hammer were used to obtain these

compaction curves.

• For all cases the number of layers

was equal to 3.

For the standard Proctor test

2. Effect of Compaction Effort

Compaction effort maxdg

.optw

Standard Proctor

EXAMPLE 6.1

TEXT IN SI UNITS

EXAMPLE 6.1

TEXT IN SI UNITS

EXAMPLE 6.1

TEXT IN SI UNITS

EXAMPLE

EXAMPLE

If you are checking the field compaction of a layer of soil and the compaction curve

for the soil is shown in Figure 1. From the specifications, the dry density of the

compacted soil should be at least 95% of the maximum value and within ± 1% of

the optimum water content. When you did the sand cone test, the volume of the soil

excavated was 1153 cm3. It weighed 2209 grams wet and 1875 grams dry.

a. What is the compacted dry density?

b. What is the field water content?

c. What is the relative compaction?

d. Does the test meet the specifications (explain)?

e. If it does not meet, what should be done to

improve the compaction so that it will meet the

specifications?

f. What is the degree of saturation of the field

sample?

g. If the sample were saturated at constant

density, what would be the water content?

Example (2nd

Midterm Exam Fall 40-41)

Modified Proctor Test

Modified Proctor Test (ASTM D-1557, AASHTO T-180)

With the development of heavy rollers (also requirements of heavy

aircrafts and trucks) and their use in field compaction, the standard

Proctor test was modified for better representation of the field

conditions. This is sometimes referred to as modified Proctor test.

•Mold 1/30 ft3 in volume (same as for standard test)

•5 layers

•25 blows (same as for standard test)

•10 lb hammer

•18 inch drop

Developed in WWII by U.S. Army Corps of Engineers to better

represent the compaction required for airfield to support heavy

aircraft.


Layer 1

Layer 2Layer 3

Layer 4

Layer 5

Drop = 457.2 mm(18 in)

Drop = 304.8 mm(12 in)

hammer = 2.5 kg (5.5 lb)

hammer = 4.54 kg (10 lb)




Modified

Proctor

Test

Standard

Proctor

Test

944 cm3944 cm3Volume of mold

53# of layers

4.54 kg2.5 kgMass of hammer

45.7 cm30.5 cmDrop of hammer

2525# of hammer blows

Because it increases compactive effort, the modified Proctor test results in

an increase of the maximum dry unit weight of the soil, and this is

accompanied by decrease in the optimum moisture content.

333 / 600/ 5.592lb/ft-ft 12375)30/1(

)1(5.5)3)(25(mmkNmmkNE

Compaction Energy for Unit Volume of Soil



333 / 2700/ 3.2693lb/ft-ft 56250)30/1(

)5.1(10)5)(25(mmkNmmkNE

Note: In the field, compactive effort is the number of passes of the roller of

a certain type and weight on a given volume of soil.

SOIL COMPACTION

FIELD COMPACTION

FIELD COMPACTION

The most common types are:

1. Smooth-wheel rollers (smooth-drum rollers)

2. Pneumatic rubber-tired rollers

3. Sheepsfoot rollers

4. Vibratory rollers

Most of the compaction in the field is done by means of ROLLERS.

FIELD COMPACTION

1. Smooth-wheel rollers (smooth-drum roller)

Proof rolling subgrades

Finishing operation of fills with sandy &clayey soils

Provide 100% coverage

Contact pressure 310 – 380 kN/m2

Not suitable for producing high g for thicker layers

FIELD COMPACTION

2.Pneumatic rubber-tired rollers

Heavily loaded with several rows of tires

Tires are closely spaced 4 -6 in a row

Provide 70-80% coverage


Combination of pressure and kneading

FIELD COMPACTION

3.Sheepsfoot rollers

Drums with a large number of projections

Area of each projection 25 – 85 cm2

Most effective in compacting clayey soils


FIELD COMPACTION

4.Vibratory rollers

Efficient in compacting granular soils

Vibrators can be attached to smooth-wheel, pneumatic rubber-tired,

or sheepsfoot rollers to provide vibratory effects to the soil.

Figure 6.20 Principles of vibratory rollers

FIELD COMPACTION

Handheld vibratory plates can be used for effective compaction of

granular soils over a limited area.

Handheld vibratory

FACTORS AFFECTINGFIELD COMPACTION

Soil type

Moisture content

Thickness of lift

Intensity of pressure

Area over which the pressure is applied

No. of roller passes

There are several factors that must be considered to achieve the

desired unit weight of compaction in the field:

FIELD COMPACTION

Compaction of Silty Clay

FIELD COMPACTION

Vibratory Compaction of Sand

FIELD COMPACTION

Relationship between dry unit weight

and number of passes

Relationship between dry unit weight,

number of passes, and depth.

Lack of confining pressure

towards the surface

In most cases, about 10 to 15 roller passes

yield the maximum dry unit weight

economically attainable.

SPECIFICATIONS FOR FIELD COMPACTION

Usually it is required for the contractor to achieve a compacted field dry unit weightof say 90 to 95% of the maximum dry unit weight determined in the laboratory byeither the standard or modified Proctor test (Recall previous examples).

Relative compaction, R

For granular soils, specifications can be expressed in terms of relative

density.

where

Applicable if the soil

contains less than 12%

fines (passing No. 200

sieve)

(a)

(b)

From (a) and (b)

FIELD COMPACTION

Dividing by

R

1 R

FIELD COMPACTION

Solve for R

Approximate formula for granular soils

EXAMPLE 6.8

1. Sand cone method

2. Rubber balloon method

3. Nuclear method

Determination of Field Unit Weight of Compaction

Common Methods:

We know that both relative compaction or relative density are both

needed for determination of dry density in the field.

1. Sand cone method (ASTM Designation D-1556)

W1 = weight of the jar, the cone, and the sand

filling the jar

Filling the jar with very uniform dry Ottawa sand

Excavating a small hole in the area where the

soil has been compacted

W2 = weight of the moist soil excavated from

the hole.

W3 = the dry weight of the soil =

= moisture contentRecall

= Ww/Ws

Sand cone method

W4 = combined weight of the jar, the cone,

and the remaining sand filling the jar.

V = the volume of the excavated hole

The cone with the sand-filled jar attached

to it is inverted and placed over the hole.

Wc= weight of sand to fill the cone only

used sand Ottawa oft unit weighdry )( sanddg

The dry unit weight of compaction made in the field is

determined as

V

Wfieldd

3

)(g

W5 = weight of sand to fill both the hole and cone

Sand cone method

Field Density Test

Field Density Test

Proctor test

Sand Cone Test

EXAMPLE 6.9

EXAMPLE 6.9

EXAMPLE 6.9

Proctor test

Sand Cone Test

EXAMPLE 6.10

Proctor test

Sand Cone Test

EXAMPLE 6.10

RUBBER BALLOON METHOD

Determine weight of dry soil

Determine volume of the hole

(can be read directly)

Determine dry unit weight

2. Rubber Balloon Method (ASTM Designation D-2167)

Similar to sand cone method except that the volume of the hole

is determined by introducing into it a rubber balloon filled with

water from a calibrated vessel.

Operates either in drilled holes or on ground surface

Uses radioactive isotope source (Gamma rays)

Measure weight of wet soil per unit volume

It also measure weight of water per unit volume

Determine the dry unit weight of soil

3. Nuclear Method (ASTM D6938 - 15 )

Nuclear density meter (Densometer)

o Measures the weight of wet soil per

unit volume and the weight of water

present in a unit volume of soil.

o The dry unit weight of compacted

soil can be determined by

subtracting the weight of water from

the moist unit weight of soil.

o Dense soil absorbs more radiation

than loose soil.

Nuclear Method

Special Compaction Techniques


Vibroflotation

A technique for in situ

densification of thick layers of

loose granular soil deposits.



The most suitablefor compaction byVibroflotation.

Lower limit of grain-size

distribution for which

compaction by vibroflotation is

effective.

Difficult to compact

The rate of probe

penetration may be slow

and may prove

uneconomical in the long

run.

GSD and compaction by vibrflotation


SN = Suitability No. for rating backfill

where D50, D20, and D10 are the diameters (in mm) through

which, respectively, 50, 20, and 10% of the material passes.

RATING BACKFILL (Brown , 1977)

The smaller the value of SN, the

more desirable the backfill

material.


Typical patterns of Vibroflot probe spacings

for a column foundation

Compaction over

a large area

EXAMPLE 6.11


Dynamic Compaction

Densification of granular soil deposits

Dropping a heavy weight on the ground at regular intervals

Weight of hammer 80-360 kN

Hammer drop 7.5-30.5 m

Degree of compaction depends on:

Weight of hammer

Height of hammer drop

Spacing of locations at which

the hammer is dropped


Blasting

Compaction (up to a relative

density of 80%) up to a depth of

about 18 m over a large area can

easily be achieved.

Usually the explosive charges

are placed at a depth of about

two-thirds of the thickness of

the soil layer desired to be

compacted.

THE END

Documents

CHAPTER 6 SOIL COMPACTION€¦ · Stabilize or improve the properties of the soil The third alternative may be in most cases the most economical alternative. There are different techniques