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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.

8
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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 .

11
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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.

15
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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
    improved subgrade courses.

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 .

25
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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.

27
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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.

29
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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.
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