STRENGTH AND DENSITY OF SOILS

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STRENGTH AND DENSITY OF SOILS
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• 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

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• 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.

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• 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

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• 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

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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.

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• 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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• 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.

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• 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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-Determination of Soil Density

• Laboratory tests
• From the most famous laboratory test methods are
  standard AASHO- and modified AASHO compaction
  tests.

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• 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.

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• 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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-The relation between standard and modified
compaction
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• -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.

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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.

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• 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.

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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.

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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.

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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.

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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.

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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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• 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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• 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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-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.

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• 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.

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