Soil compaction : Compaction is the reduction in voids content due to air being forced out of the...

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

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.

The dry density of the soil is determined by means of the following equation:

d = (m / (1+wc))

where m = wet density pct or gm/cm3 wc = moisture content

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

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.

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.

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.

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 .

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

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.

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.

-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.

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.

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.

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.

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.

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.

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.

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 .

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.

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.

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.

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.

Effect of Number of Passes

Heavy rollers will a chieve adequate compaction with fewer numbers of passes than lighten rollers.