Proctor Compaction Test

Introduction:

Standard Proctor Compaction Test - (ASTM D 698-78 (standard) and D 1557-78 (re-approved 1990))

In 1933, R. R. Proctor proposed a test to determine the maximum dry density and optimum moisture content of a soil under a given compaction effort. Slightly modified version of that test is referred to as Proctor compaction test.

Modified Proctor compaction test

With the development of heavy rollers and their use in field compaction, the standard Proctor test was modified to better represent the compaction effort used in field compaction of soils. For conducting the modified Proctor compaction test, the same mold is used as in the case of standard Proctor test. However, the soil is compacted in five layers by a hammer the weighs 4.54kg. The drop of the hammer is 457.2 mm. Similar to the standard Proctor compaction test, the number of hammer blows per each layer is limited to 25. The rest of the procedure of the modified Proctor compaction test is similar to that of the standard Proctor compaction test.

Objectives:

Objectives of this test are to determine the optimum water content and the maximum dry density by standard Proctor compaction test.

Apparatus:

The following apparatus are required (Figure 1).

Figure 1 - Apparatus used in standard Proctor compaction test.

1. Mold There shall be a cylindrical mold of diameter 101.6 mm and a height of 116.4mm. The total volume of the mold is 944 cm3.The mold shall be fitted with a detachable base plate and a removable extension approximately 50 mm high. (Figure 2)

2. A metal hammer There shall be a metal hammer having a 50 mm diameter circular face, and weighing 2.49 kg. The hammer shall be equipped with a suitable arrangement for controlling the height of drop to 305mm. (Alternatively there can be a hammer of 2.5 kg weight with a drop 300 mm). (Figure 3)

Figure 2 Mold

Figure 3 Metal hammer

3. Balances A balance readable and accurate to 1 g (with a capacity 20 kg) and a balance readable and accurate to 0.01 g.

4. Mixing tools Miscellaneous tools such as mixing pan, spoon, trowel, spatula etc.

5. Metal tray A large metal tray (600 mm X 500 mm and 80 mm deep).

6. Straightedge A steel straightedge, 300 mm long, 25 mm wide, and 3 mm thick with one beveled edge.

7. Sample extruder (Optional) An apparatus (such as a jack) for extruding specimen from the mould.

8. An oven Thermostatically controlled oven to provide temperature 105 110 C0.

9. Cans Cans to take samples for moisture content determination.

Procedure:

1. Obtain about 3 kg of air-dried soil in the mixing pan and brake all the lumps.

2. Add suitable amount of water. (See Note 01 & 02)

3. Determine the weight of the empty mold with the base plate and the collar (M1) to the nearest 1g.

4. Fix the collar and the base plate.

5. Compact the moist soil in to the mold in three layers of approximately equal mass. (Each layer shall be compacted by 25 blows). Blows must be distributed uniformly over the surface of each layer so that the hammer always falls freely. The amount of soil must be sufficient to fill the mold, leaving not more than 6 mm to be struck off when the extension is removed. (See Note 03))

6. Detach the collar carefully without disturbing the compacted soil inside the mold and using a straightedge trim the excess soil leaving the mold.

7. Obtain the weight of mold with base plate and the moist soil (M2).

8. Extrude the sample and break it to collect the sample for water content determination preferably at least two specimens one near the top and other near the bottom.

9. Weigh an empty can, (M3) and weigh again with the moist soil obtained from the extruded sample in step 8 (M4).

10. Keep this can in the oven for water content determination.

11. Repeat step 4 to 10. During this process weight M2 increases for some time with the increase in moisture and decreases thereafter. Conduct at least two trials after the weight starts to reduce. (See Note 04)

12. After 24 hours get the weight of oven dried sample (M5).

Note 01 -

The amount of water to be added with air dried soil at the commencement of the test will vary with the type of soil under test. In general, with sandy and gravely soil a moisture content of 4% to 6% would be suitable, while with cohesive soils a moisture content of about 8% to 10% below the plastic limit of the soil, would be usually be suitable.

Note 02 It is important that the water is mixed thoroughly and adequately with the soil, since inadequate mixing gives rise to variable test results. This is particularly Important with cohesive soil when adding a substantial quantity of water to the air dried soil.

With clays of high plasticity, or where hand mixing is employed, it may be difficult to distribute the water uniformly through the air dried soil by mixing alone, and it may be necessary to store the mixed sample in a sealed container for a minimum period of about 16 hours before continuing with the test.

Note 03 It is necessary to control the total volume of the soil compacted; since it has been found that if the amount of soil struck off after removing the extension is too great, the test results will be inaccurate.

Note 04 The water added for each stage of the test should be such that a range of moisture contents is obtained which includes the optimum moisture content. In general, increments of 1% to 2% are suitable for sandy and gravely soils and of 2% to 4% for cohesive soils. To increase the accuracy of the test it is often advisable to reduce the increments of water in the region of the optimum moisture content.

Observation:

The test is repeated, using the same soil after adding some water until the wet density of the soil in the mold is lower than for the previous moisture content.

(Observations are given in Annex-1.)

Calculations and result:

The bulk density, in kg/m3 of each compacted specimen shall be computed from the equation;

= M2 M1

V

Where;

M1 is the weight of the mold and base, in kg

M2 is the weight of mold, base and soil, in kg

V is the volume of the mold in m3

Moisture content can be obtained from the equation;

w = M4 M5

M5 M3

Where;

w is the moisture content of the soil as a fraction

M3 is the weight of an empty can

M4 is the weight of the moist soil obtained from the extruded sample with container

M5 is the weight of oven dried sample

Dry density can be obtained using the following relationship;

dry = wet

1 + w

100

Where;

wet is the wet density

The dry densities dry, obtained in a series of determinations is plotted against the corresponding moisture content, w. A smooth curve is drawn through the resulting points and the position of the maximum on this curve is determined. Thus the maximum dry density and the corresponding water content is obtained from the graph (Annex-1).

Specimen Calculation

Consider sample 1 (Trial number 1)

Mass of the compacted soil inside the mold = 5.618 4.074 = 1.544 kg

Volume of the mould

= 944 cm3

Bulk density of the soil

= 1.544 / (944 x 10-6)

= 1635.59 kg/m3

Moisture content sample 1 (Trial number 1)

Moisture content

= Mass of water/ mass of dry soil

= (117.74 112.14) / (112.14 9.64)

= 0.0546

Dry Density

=1635.59 / (1 + 0.0546)

= 1550.91 kg/m3

Discussion:

1) What is the theory behind the relationship between soil compaction and moisture content?

To investigate the relationship between the moisture content and the soil compaction, soil is compacted in the mould at different water contents by applying equal amount of compaction energy (Standard Proctor energy). If the dry unit weight of the soil is obtained at different water contents, the plot of the dry unit weight vs, moisture content will take a shape similar to the one shown below:

At low moisture contents, the compaction becomes less effective and as a result the dry unit weight is low. When water is added to the soil during compaction, it acts as a softening agent on the soil particles. The soil particles slip over each other and move into a densely packed position. As a result the dry unit weight after compaction first increases as the moisture content increases. However, beyond a certain moisture content, any increase in the moisture content tends to reduce the dry unit weight. This is because the water takes up the spaces that would have been occupied by the solid soil particles. The moisture content, at which the maximum dry unit weight is attained, is generally referred to as the optimum moisture content. Water contents less than optimum are referred to as dry of optimum and those more than optimum are referred to as wet of optimum. The testing procedure used to obtain the maximum dry unit weight and the optimum moisture content is referred to as Proctor compaction test.

In clay soils the theory behind the relationship between soil particles is different.

Clay particles have a flaky shape and they are electrically charged and carry a net negative charge in them. Generally the faces of these clay particles are negatively charged, whereas, the ends are positively charged. Clay particles usually form two types of structures, namely, flocculated structure or dispersed structure. If the charge on the clay particles are not neutralized by the bi-polar water molecules or other cations, the positively charged edges will be attracted to the negatively charged faces of the clay particles to form an open card-house type of an arrangement, as shown in Figure 4, termed as flocculated structure. On the other hand, when the electrical charges carried by the clay particles are somewhat neutralized by bi-polar water molecules or cations, the clay particles are surrounded by a thicker layer of absorbed water. Under such conditions, the clay particles can form a dispersed structure with near parallel orientation as shown in Figure 5.

Figure 4 - Flocculated structure.

Figure 5 - Dispersed structure.

The structure of the compacted fine grained soils with water content can be illustrated as shown below.

Consider a soil at low moisture content with a water content of wB as shown in the above Figure. The small quantity of water present in the soil is not sufficient to form a thick absorbed water layer surrounding the clay particles to neutralize the electrical charge of the clay particles. Therefore, when the soil is compacted, the clay particles are pushed close to each other and the negatively charged faces of the clay particles attract the positively charged edges to form a flocculated structure.

When little bit more water is added to the soil prior to compaction, say wC, in the above Figure, a thicker adsorbed water layer than at B will be formed surrounding the clay particles. The particle faces are negatively charged but not as much as they were at wB. Once subjected to compaction stresses, most particles get aligned in a face-to-end configuration but some get pushed into face-to-face arrangement as well since the face-to-end contacts are not as strong as at wB. The resulting structure is still flocculated but neither is it quite as open as at wB nor are the particle contacts quite as strong as at wB. The dry unit weight that is obtained at wC is as such greater than at wB.

This trend will continue with addition of more and more water resulting higher and higher dry unit weight with structures which are less and less flocculated. However, after the optimum moisture content, the adsorbed water layer surrounding the clay particles are well developed and most of the electrical charges on clay particles are neutralized. During the compaction process these well developed adsorbed water layers surrounding the particles interfere with each other and prevent particles from coming close to each other to assume a dense packing. The energy supplied by the compaction process gets dissipated in futile bumping of one adsorbed layer with another. The compacted soil so produced has a low dry unit weight and form a dispersed structure with parallel alignment of particles.

2) Discuss the purpose of introducing the Modified Proctor Compaction test for resent road construction.

With the development of heavy rollers and their use in field compaction, the standard Proctor test was modified to better represent the compaction effort used in field compaction of soils.

If the compactive effort is increased using a heavier hammer falling over a larger height compacting the soil in the mold with five number of layers, such compaction test procedure is referred to as the Modified Proctor Compaction Test. The moisture content vs dry unit weight relationship generally observed for standard and modified Proctor compaction testing of the same soil type is shown in the following Figure.

It is evident from the above Figure that the maximum dry unit weight is increased and the optimum moisture content is reduced when the compactive effort is increased. It is also noticed that an increased in the compactive effort produces a very sizable increase in dry unit weight for soil when it is compacted at water contents drier than optimum water content (C to D in the above Figure). On the other hand, when soil is compacted wet of the optimum water content, an increase in the compactive effort produces only a small increase in the dry unit weight (E to F in the above Figure).

By considering the above facts the Modified Proctor Compaction test is better to used for resent road construction and therefore it was introduced.