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## STRENGTH AND DENSITY OF SOILS

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### 2.Effect of soil type Different soils having various maximum densities and optimum moisture contents according to specific gravity of soil particles themselves, ... – PowerPoint PPT presentation

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Title: STRENGTH AND DENSITY OF SOILS

1
STRENGTH AND DENSITY OF SOILS
2
• Soil compaction
• Compaction is the reduction in voids content due
to air being forced out of the soil or dissolved
in the soil water by mechanical means

3
• The purposes of compaction are
• 1.Increasing the soil shear strength
• 2.Decreasing the tendency of subgrade to settle
under repetition wheel loads
• 3.Reducing the soil permeability
• 4.Increasing the resistance to frost heave
• .5.Decreasing the tendency to volume change.

4
• The dry density of the soil is determined by
means of the following equation
• ?d (?m / (1wc))
• where
• ?m wet density pct or gm/cm3
• wc moisture content

5
• Zero air voids curve is shown in figure 10, it
represent the theoretical density what this soil
will attain if all the voids were filled with
water. The values of this curve is calculated
from the following formula
• ?d(Z.A.V.) Gs.?w/ 1(Gs.wc/100)
• where
• Gs specific gravity of solids

6
Factors Affecting Compaction
• 1.Effect of moisture content
• The behavior of the soil mass under dynamic
compaction having three stages.
• In the first stage, when the moisture content is
less than the optimum value, the soil does not
contain sufficient moisture to flow readily under
the blows of the hammer.

7
• When the moisture content is increased, the soil
flow more readily under the "lubricating" effect
of the additional water (second stage), and the
soil particles move closer together, thus lead to
increasing the density. This effect is continued
until the optimum moisture content is reached.
• More increase in moisture content tends to
overfill the voids but does not decrease the air
content. As a consequence, the soil particles are
forced apart and the unit weight decreases.

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9
• 2.Effect of soil type
• Different soils having various maximum densities
and optimum moisture contents according to
specific gravity of soil particles themselves,
Particle size distribution and grain shape of
particles. Figure 14 shows the moisture-density
curves obtained by compacting a variety of soils
by standard AASHO compaction.

10
• 3.Effect of compactive effort
• In laboratory, dry density depends upon method of
compaction or number of blows per each side of
specimen. Also, in the field compaction affected
by number of rollers. The amount of compactive
effort expended changes both maximum density and
optimum moisture content .

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12
-Determination of Soil Density
• Laboratory tests
• From the most famous laboratory test methods are
standard AASHO- and modified AASHO compaction
tests.

13
• a-Standard AASHO compaction test (standard
proctor test)
• This test provides for dynamic compacting using
25 blows of a 5.5 Ib hammer with free fall of 12
in. on each of 3 layers of soil in a cylindrical
mold.

14
• b-Modified AASHO compaction test (modified
proctor test)
• This method is similar to standard method, except
the free fall distance of the hammer is 18 in.
instead 12 in., the weight of the hammer is 10 Ib
and the soil is compacted in 5 layers. The
compactive effort is 56300 ft-Ib/ft3, while by
standard method was 12400 ft-Ib/ft3.

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16
-The relation between standard and modified
compaction
17
• -Field Compaction Procedures
• Compaction greatly increases the supporting power
of the subgrade. Since the subgrade is not
sufficiently compacted during construction,
additional consolidation may occurs under traffic
with resulting settlement and possible failure.

18
1-Compaction criteria
• a-Cohesive subgrade Minimum 95 of AASHO
(T180,Method D) density for the top 12 in. and
minimum 90 for all fill areas below the top 12
in.
• b-Cohesionless subgrades Minimum 100 of
AASHO,density for the top 12 in. and minimum 95
below this for all fill areas.
• c-Bases, subbases and improved subgrade, minimum
100 of AASHO density.

19
• Soil, in the field, is compacted by applying
energy in one of the following three ways
• 1.pressure, by using rollers
• 2.impact, by using hammers
• 3.vibration,by using vibrators.

20
Types of field compaction equipments
• a-Smooth wheel rollers
• This type of rollers include the three-wheel type
(3-18 ton), tandem rollers (1-14 ton) and
three-axle tandem rollers (12-18 ton).
• This type is effective to compact fine grained
subgrade soils, granular base and subbase, and
also macadam and other coarse aggregate base
courses.

21
b-Pneumatic-tired rollers
• These compactors are either of the single-wheel
or multiple wheel types. The ability of
compaction is dependent on the tire inflation
pressure and the contact area between the tire
and the ground. The gross weight of
pneumatic-tired compactors is a secondary
parameter.
• This type is suitable for compacting fine grained
subgrade soils, granular base, subbase and

22
c-Sheep foot rollers
• This type of compactors consist of hollow
cylindrical steel drum. Compaction performance
depends on the foot pressure and the coverage of
ground obtained per pass. These in turn depend
upon the gross weight of the rollers, the area of
each foot, the number of feet on contact with the
ground and the total number of feet per drum.

23
d-Vibrators
• Vibrators consist of a vibrating unit of either
the out-of balance weight type of a hydraulic
type mounted on screwed, plate or roller.
Densities resulting from vibration often exceed
100 of modified AASHO density with case. The
vibrator will compact the soil from the surface
downward.

24
Determination of soil density in the field
• R.C (Dry density of soil in field / Dry
density of soil in laboratory)x100
• 1- Sand-Replacement Method
• A hole about 4 in. in diameter is excavated with
suitable tools to the depth of layer being
tested, and the weight of soil removed is
determined and a moisture content is calculated
after drying a sample .

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26
• 2-Core -Cutter Method
• The dolly being placed over the cutter and the
apparatus is rammed into the soil . The cutter
containing the soil is then dug out of the
ground. The weight and volume of the soil is
determined, and also the moisture content is
evaluated.
• This method is convenient and quick. The cutting
edge is easily damaged. It is suitable for soft,
cohesive soil, not suitable for stony soils.

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28
• 3-Volumeno-meter Method
• A lamp of soil approximately 18 in3 in volume cut
from the compacted soil. The lump is trimmed
clear of loose material, weighted, coated with
thin film of paraffin max and re-weighed. The
volume of soil and paraffin is found from the
volume of displaced water when it is immersed in
the volumeno-meter. Where the volume of paraffin
wax is known, then the volume of soil can be
calculated. A sample of soil is cut from the
specimen and its moisture content determined.
This method can only be used on cohesive soils.

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30
-Factors Affecting the Soil Compaction in the
Field
• The degree of compaction or density, which be
obtained for a soil during construction process
depends upon moisture content at compaction
type of soil method and type of compaction and
compactive energy.

31
• The choice of compaction equipment should
logically depend primarily on the type of soil,
the natural moisture content and the density
specification.
• 1- Granular materials
• Granular materials without fines are compacted
efficiently by vibration, sheep-foot rollers can
also be used. Pneumatic rollers and
smooth-wheeled rollers will give satisfactory
results if the granular material contains a small
amount of fines.

32
• Effect of Number of Passes
• Heavy rollers will a chieve adequate compaction
with fewer numbers of passes than lighten rollers.