soil compaction

Priodeep Chowdhury;Lecturer;Dept. of CEE;Uttara University// | Soil Compaction

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COMPACTION

In construction of highway embankments, earth dams and many other engineering structures, loose soils

must be compacted to improve their strength by increasing their unit weight.

Compaction is the densification of soil by removing air voids using mechanical equipment. „ The dense state is achieved through the reduction of the air voids in the soil, with little or no reduction in the

water content. In general, soil densification includes compaction and consolidation.

Compaction is one kind of densification that is realized by rearrangement of soil particles without outflow of water. It is realized by application of mechanic energy. It does not involve fluid flow, but with moisture changing altering. Compaction is the application of mechanical energy to a soil to rearrange the particles

and reduce the void ratio. Consolidation is another kind of densification with fluid flow away. Consolidation is primarily for clayey

soils. Water is squeezed out from its pores under load.

Objectives for Compaction

Increasing the bearing capacity of foundations;

Decreasing the undesirable settlement of structures; Control undesirable volume changes; Reduction in hydraulic conductivity;

Increasing the stability of slopes.

General Compaction Methods

Compaction Effect


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Laboratory Compaction

Origin

The fundamentals of compaction of fine -grained soils are relatively new. R.R. Proctor in the early 1930’s was

building dams for the old Bureau of Waterworks and Supply in Los Angeles, and he developed the principles of compaction in a series of articles in Engineering News-Record. In his honor, the standard laboratory compaction test which he developed is commonly called the proctor test.

Purpose

The purpose of a laboratory compaction test is to determine the proper amount of mixing water amount of mixing water to use when compacting the soil in the field and the resulting degree of denseness which can be expected from compaction at this optimum water.

Impact compaction

The proctor test is an impact compaction. A hammer is dropped several times on a soil sample in a mold. The

mass of the hammer, height of drop, number of drops, number of layers of soil, and the volume of the mold are specified.

Types and Details of Laboratory Compaction Techniques

Two types of compaction tests are routinely performed:

(1) The Standard Proctor Test, and

(2) The Modified Proctor Test.

Each of these tests can be performed in three different methods as outlined in the attached Table 1. In the Standard Proctor Test, the soil is compacted by a 5.5 lb hammer falling a distance of one foot into a

soil filled mold. The mold is filled with three equal layers of soil, and each layer is subjected to 25 drops of

the hammer. The Modified Proctor Test is identical to the Standard Proctor Test except it employs, a 10 lb hammer

falling a distance of 18 inches, and uses five equal layers of soil instead of three. There are two types of compaction molds used for testing. The smaller type is 4 inches in diameter and has a

volume of about 1/30 ft3 (944 cm3), and the larger type is 6 inches in diameter and has a volume of about

1/13.333 ft3 (2123 cm3). If the larger mold is used each soil layer must receive 56 blows instead of 25 (See Table 1)

Table 1 Alternative Proctor Test Methods


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Standard Reference:

ASTM D 698 - Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard

Effort (12,400 ft-lbs/ft3(600 KN-m/m3)) ASTM D 1557 - Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified

Effort (56,000 ft-lbs/ft3(2,700 KN-m/m3))

Laboratoy Compaction Test Summary

Compaction Energy

Compaction Effort is calculated with the following parameters:

1.Mold volume 2. No of Compaction Layer 3. Weight of Hammer 4. Free fall Height 5. No of blows

Compaction Energy

E =no of blows per layer ∗ no of layers ∗ weight of hammer ∗ Free fall height

Volume of mold

So, for Standard Proctor Test :

E =25(no of blows per layer) ∗ 3(no of layers) ∗ 5.5(weight of hammer) ∗ 1(Free fall height)

1/30(Volume of mold)

= 12375 ft-lb/lb3

So, for Modified Proctor Test :

E =25(no of blows per layer) ∗ 5(no of layers) ∗ 10(weight of hammer) ∗ 1.5(Free fall height)

1/30(Volume of mold)

= 56250 ft-lb/lb3

Comparison


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Significance of the Tests

Mechanical compaction is one of the most common and cost effective means of stabilizing soils. An

extremely important task of geotechnical engineers is the performance and analysis of field control tests to

assure that compacted fills are meeting the prescribed design specifications. Design specifications usually state

the required density (as a percentage of the “maximum” density measured in a standard laboratory test), and

the water content. In general, most engineering properties, such as the strength, stiffness, resistance to

shrinkage, and imperviousness of the soil, will improve by increasing the soil density.

The optimum water content is the water content that results in the greatest density for a specified

compactive effort. Compacting at water contents higher than (wet of) the optimum water content results in a

relatively dispersed soil structure (parallel particle orientations) that is weaker, more ductile, less pervious,

softer, more susceptible to shrinking, and less susceptible to swelling than soil compacted dry of optimum to

the same density. The soil compacted lower than (dry of) the optimum water content typically results in a

flocculated soil structure (random particle orientations) that has the opposite characteristics of the soil

compacted wet of the optimum water content to the same density.

Standard Proctor Test: Procedure & Details

In the Proctor test, the soil is compacted in a mold that has a volume of 943.3 cm3.The diameter of the

mold is 101.6 mm. During the laboratory test, the mold is attached to a base plate at the bottom and to an extension at the top. The soil is mixed with varying amounts of water and then compacted in three equal layers

by a hammer that delivers 25 blows to each layer. The hammer weighs 24.4 N (mass 2.5 kg), and has a drop of 304.8 mm. For each test, the moist unit weight of compaction can be calculated as

where W =weight of the compacted soil in the mold

V(m) = volume of the mold (943.3 cm3)

For each test, the moisture content of the compacted soil is determined in the laboratory. With known moisture

content, the dry unit weight γd can be calculated as

(where w (%) =percentage of moisture content.)

Standard Proctor Test equipment ( Mold & Hammer)


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The values of γd determined from Eq. above can be plotted against the corresponding moisture contents

to obtain the maximum dry unit weight and the optimum moisture content for the soil. Following figure shows such a compaction for a silty clay soil.

For a given moisture content, the theoretical maximum dry unit weight is obtained when there is no air in the void spaces—that is, when the degree of saturation equals 100%. Thus, the maximum dry unit weight at a given

moisture content with zero air voids can be given by

To obtain the variation of γzav with moisture content, use the following procedure:

1. Determine the specific gravity of soil solids.

2. Know the unit weight of water (γw).

3. Assume several values of w, such as 5%, 10%, 15%, and so on.

Standard Proctor compaction test results for a silty clay


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4. Use Eq. to calculate γzav for various values of w.

Analysis of Compaction Curve

The peak point of the compaction curve

The peak point of the compaction curve is the point with the maximum dry density ρd max. Corresponding to the

maximum dry density ρdmax is a water content known as the optimum water content, wopt (also known as the

optimum moisture content, OMC). Note that the maximum dry density is only a maximum for a specific

compactive effort and method of compaction. This does not necessarily reflect the maximum dry density that

can be obtained in the field.

Zero air voids curve

The curve represents the fully saturated condition (S = 100 %). (It cannot be reached by compaction)

Line of optimums Line of optimums

A line drawn through the peak points of several compaction curves at different compactive efforts for the same

soil will be almost parallel to a 100 % S curve, it is called the line of optimums.

Factors affecting Compaction

There are 4 control factors affecting the extent of compaction:

1. Compaction effort;

2. Soil type and gradation;

3. Moisture content; and

4. Dry unit weight (dry density).


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Effect of Compaction Effort

With the development of heavy rollers and their uses in field

compaction, the Standard Proctor Test was modified to better

represent field compaction.

As the compaction effort increases,

the maximum dry unit weight of compaction increase

The optimum moisture content decreases to some extend

The preceding statements are true for all soils. Note, however, that the degree of compaction is no tdirectly proportional to the compaction effort.

Effect of Soil type and gradation

fine grain soil needs more water to reach optimum; and

Coarse grain soil needs less water to reach optimum.

Compaction curves for different soils with the same compact effort ; fine grain soil needs more water to reach optimum and coarse grain soil needs less water to reach optimum.

Effect of Moisture content

Below wopt (dry side of optimum): As the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them

easier to be moved about and reoriented into a denser configuration. At wopt:

The density is at the maximum, and it does not increase any further. Above wopt (wet side of optimum): Water starts to replace soil particles in the mold, and since ρw << ρs the

dry density starts to decrease


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General Information

Each data point on the curve represents a single compaction test, and usually four or five individual

compaction tests are required to completely determine the compaction curve. At least two specimens wet and two specimens dry of optimum, and water contents varying by about

2%. Optimum and water contents varying by about 2%.

Optimum water content is typically slightly less than the plastic limit (ASTM suggestion). Typical

values of maximum Typical values of maximum dry density are around 1.6 to 2.0 1 Mg/m33 with the maximum range from about 1.3 to 2.4 Mg/m3.

Typical optimum water contents are between 10% and 20%, with an outside maximum range of about 5% to 40%. With an outside maximum range of about 5% to

40%. Effect of Compaction on Clay Structure

For a given compactive effort and dry density, the soil

tends to be more flocculated (random) for compaction on the dry side as compared on the wet side.

For a given molding water content, increasing the compactive

effort tends to disperse (parallel, oriented) the soil, especially on the dry side.

Effect of Compaction on Permeability

Increasing the water content results in a decrease in permeability on the dry side of the optimum moisture content

and a slight increase in permeability on the wet side of optimum.

Increasing the compactive effort reduces the permeability since it both increases the dry density, thereby reducing the voids available for flow, and increases the orientation of particles.

Effect of Compaction on Compressibility

At low stresses the sample compacted on the wet side is more compressible than the one compacted on the dry

side.

At the high applied stresses the sample compacted on the dry side is more compressible than the sample compacted on the wet side.


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Effect of Compaction on Swelling

Swelling of compacted clays is greater for those

compacted dry of optimum. They have a relatively

compacted dry of optimum. They have a relatively greater

deficiency of water and therefore have a greater tendency to

adsorb water and thus swell more.

Engineering Properties Summary


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Compaction Vs Consolidation

Compaction, as a phenomenon, is different from the phenomenon of consolidation of soil. The primary differences between the two phenomena may be set out as given in the following Table.


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Problem-1: An earth embankment is compacted at a water content of 18% to a bulk density of 19.2 kN/m3. If the specific gravity of the sand is 2.7, find the void ratio and the degree of saturation of the compacted

embankment. Solution:

Problem-2: The laboratory test data for a standard Proctor test are given in the table. Find the maximum dry unit weight and the optimum moisture content.

Solution:

We can prepare the following table:


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The plot of ρd against w is shown in Figure. From the graph, we observe

Maximum dry density =2020 kg/m3

Optimum moisture content = 13%