chapter 6 soil compaction€¦ · stabilize or improve the properties of the soil the third...
TRANSCRIPT
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CHAPTER 6
SOIL COMPACTION
Omitted Sections
6.6, 6.7, 6.8
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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.
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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.
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SOIL COMPACTION
Increases unit weight
Increases shear strength
Increases bearing capacity
Increases stability of slopes of embankments
Decreases settlement of structures
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SOIL COMPACTION
Air
Water
Solid
Air
Water
Solid
Compaction
reduced
You remember well-graded
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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
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SOIL COMPACTION
Soil Solid
Soil Solid
water
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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:
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• 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
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Standard Proctor Test
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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
Standard Proctor Test
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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.
Standard Proctor Test
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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%.
Standard Proctor Test
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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).
Standard Proctor Test
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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
Standard Proctor Test
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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.
Standard Proctor Test
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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
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FACTORS AFFECTING COMPACTION
Effect of Soil type and gradation
Fine grain soilneeds more waterto reach optimum.
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FACTORS AFFECTING COMPACTION
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
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FACTORS AFFECTING COMPACTION
•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.
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FACTORS AFFECTING COMPACTION
• 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
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EXAMPLE 6.1
TEXT IN SI UNITS
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EXAMPLE 6.1
TEXT IN SI UNITS
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EXAMPLE 6.1
TEXT IN SI UNITS
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EXAMPLE
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EXAMPLE
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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)
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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.
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Modified Proctor Test
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)
Standard Proctor Test
Modified Proctor Test
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Modified Proctor Test
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
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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
Standard Proctor Test
Modified Proctor Test
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.
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SOIL COMPACTION
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FIELD COMPACTION
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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.
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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
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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
Contact pressure 600 – 700 kN/m2
Combination of pressure and kneading
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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
Contact pressure 1400 – 7000 kN/m2
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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
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FIELD COMPACTION
Handheld vibratory plates can be used for effective compaction of
granular soils over a limited area.
Handheld vibratory
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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:
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FIELD COMPACTION
Compaction of Silty Clay
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FIELD COMPACTION
Vibratory Compaction of Sand
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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.
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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)
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FIELD COMPACTION
Dividing by
R
1 R
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FIELD COMPACTION
Solve for R
Approximate formula for granular soils
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EXAMPLE 6.8
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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.
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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
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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
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Field Density Test
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Field Density Test
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Proctor test
Sand Cone Test
EXAMPLE 6.9
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EXAMPLE 6.9
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EXAMPLE 6.9
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Proctor test
Sand Cone Test
EXAMPLE 6.10
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Proctor test
Sand Cone Test
EXAMPLE 6.10
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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.
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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
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Special Compaction Techniques
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Special Compaction Techniques
Vibroflotation
A technique for in situ
densification of thick layers of
loose granular soil deposits.
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Special Compaction Techniques
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Special Compaction Techniques
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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
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Special Compaction Techniques
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.
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Special Compaction Techniques
Typical patterns of Vibroflot probe spacings
for a column foundation
Compaction over
a large area
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EXAMPLE 6.11
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Special Compaction Techniques
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
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Special Compaction Techniques
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.
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THE END