The University of Toledo Soil Mechanics Laboratory

1Soil Moisture-Density Relationship – Standard and Modified Proctor Tests Introduction

For earthwork construction it is important to compact soils to a dense state so that the soils will attain satisfactory engineering properties. It is also desirable to know the optimum soil conditions for compacting a given soil. According to compaction theory, when samples of a soil are compacted at different water contents using the same compactive energy, there is optimum water content at which the soil will reach a maximum dry density. The optimum water content and dry density depend on the soil composition and the amount of compactive energy used. The moisture-density relationship of a soil is a graph of dry density versus water content, for a given compactive effort. The data points obtained from compacting several samples at different water contents form a smooth curve, called the compaction curve, which is used to obtain the optimum water content and maximum dry density. The two standardized tests in use today, the standard and modified Proctor tests, differ only by the amount of compactive energy. In practice, the standard or modified Proctor test is conducted on a soil and specifications are written stating: 1) that the soil moisture content during compaction should be within a specified percent of the optimum water content; and 2) that the dry density of the compacted soil should be 90 to 100 percent of the maximum dry density.

Apparatus

1. No. 4 sieve for Procedure A or 3/8-in sieve for Procedure B 2. Mixing tools including bowl, spoon and spray bottle 3. Mold assembly including a base, a mold with a 4.0-inch diameter and volume of 1/30

cubic foot (944 cm3) and a collar 4. Manual rammer as follows:

a) Standard Proctor (ASTM D 698) – free fall distance of 12.0 inches and mass of 5.5 lbm (2.5 kg)

b) Modified Proctor (ASTM D 1557) – free fall distance of 18.0 inches and mass of 10.0 lbm (5.54 kg.

5. Rubber mallet 6. Straightedge tool 7. Sample extruder including a jack and frame 8. Platform balance 9. Water content containers and drying oven

1 ASTM D 698 – 91 (Reapproved 1998); ASTM D 1557 – 91 (Reapproved 1998)

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Procedure A. Preparation (one week before test)

1) Pass approximately 8 lbs. of air-dried soil through a #4 (Procedure A). It will be necessary to pulverize the soil. This can be done using a Proctor mold and rammer.

2) Using the spray bottle, add approximately 250 ml of water to the soil and mix thoroughly in a pan until the soil is uniform in color. This will increase the water content of the soil by approximately 7%. Transfer the soil to a plastic bag and close the bag.

B. Laboratory

1) Determine and record the weight of the mold.

2) Assemble and secure the mold to the base and the collar to the top of the mold.

3) Compact the soil in the mold according to the standard or modified Proctor procedures as follows as directed by the instuctor:

a) Standard Proctor (ASTM D698) – Standard rammer using 3 layers and 25 blows

per layer; b) Modified Proctor (ASTM D1557) – Modified rammer using 5 layers and 25

blows per layer.

Soil should be mixed thoroughly in the mixing pan and placed loosely in the mold and lightly tamped before compaction. After compacting each of the layers below the top layer and before placing soil into the mold, use a sharp object to loosen the soil on the surface of the soil and around the edge of the compaction mold. Observe the compaction behavior of the soil in order to estimate the amount of loose soil that should be placed in the mold so that the top layer will extend at least ¼-inch but not more than ¾-inch into the collar after compaction. The compacted layers should be approximately equal in thickness.

4) When compacting the soil, place the mold on the concrete floor of the laboratory and

stand adjacent to the mold. Hold the sleeve of the mold in one hand slightly above the surface of the soil. Use the other hand to apply the blows while holding the sleeve of the rammer in a nearly vertical position. Move the sleeve of the rammer around the surface of the soil after each blow. With a little practice the blows can be applied at a rapid rate of approximately 25 blows/min.

5) After compaction, place the mold on the counter and carefully remove the collar from

the mold. It is important to prevent the soil from breaking off below the surface of the mold. This can be accomplished by tapping on the collar with a rubber mallet and by pushing down on the surface of the soil with the thumbs while pulling up on outside of the collar.

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6) Carefully trim the soil level with the top and the bottom of the mold using the

straightedge. Care must be taken so that the soil does not break off below the top of the mold. Best results are achieved by initially trimming the soil from around the edges of the mold and gradually working toward the center. Fill any holes on the surface of the mold using the trimmed soil by pressing down with the wide edge of the straightedge and then scrapping the surface of the soil again.

7) Determine the weight of the soil and mold as accurately as possible.

8) Use the extruder to remove the soil sample from the mold. Slice or break the compacted

soil in half by slicing axially through the sample from the outside through the center. Obtain a water content sample from the cut face of each half of the specimen. Obtain the water content of the samples as per previous labs.

9) After compacting the soil, increase the water content of the soil by adding approximately

75 ml. of water and mixing thoroughly.

10) Repeat steps 2 through 9 so as to obtain five data points with five water contents using either the standard and modified Proctor procedures.

Note: It is not permissible reuse soil after it is compacted in a Proctor mold. This procedure is followed in a teaching laboratory in order to reduce the amount of soil required for the testing.

Calculations

The density of the soil can be computed using either the English or the Standard

International (SI) metric system. For the English system, the density or unit weight is given in units of lbf/ft3. For the SI metric system, the density is given in kN/m3. If the soil mass is determined, then the unit weight, γ, (kN/m3) is computed using Newton’s second law of motion (F = M · a) and the following conversions and calculations.

(1) sec/

sec/ 81.9 )()( 22

mkgNmkggkgMNW

⋅×⋅=⋅=

(2) 101000944

)( 3

36

33 mcm

NkN

cmN

VW

mkN

××==γ

(3) 6.366 )( 33 ×=mkN

ftlbfγ

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The dry density or dry unit weight is then obtained by

(4) w) 1( +

=ρρd

(5) w) 1( +

=γγ d

where w is the water content expressed as a fraction.

For any given water content, the associated dry density for zero air voids (degree of saturation equal to 100%) is computed. This is a useful verification of the compaction curve since, in theory, it is not possible to have a higher dry density. Dry densities greater than the dry densities on the 100% saturation curve may be an indication of error or an incorrect value of specific gravity. The equations for the dry density and unit weight at 100% saturation are

(6)

s

w

wd

wρρ

ρρ

+=

(7) wG 1 s+

= wsd

G γγ

Supplementary Calculations

The following computations can be completed using phase relationships that are derived from the defined relationships.

(8) 1- d

wsGeγγ

=

(9) 1 e

en+

=

(10) e

wGS s=

Results

Complete the following tables and figure using either the English or SI system of units. Compute the dry unit weight and water content for each point and show the results in Table 1. Compute the dry density at 100% saturation (zero air voids curve) for five assumed water contents using Equation 6 or 7, respectively and show the results in Table 1. Show the curves for density (dry unit weight) vs. water content and dry unit weight at 100% saturation vs. water content from Table 1 on Figure 1. Determine the optimum moisture content and maximum dry unit weight. Compute the void ratio, porosity and degree of saturation for each sample and show the results in Table 2.

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Conclusions

Estimate the optimum water contents and maximum dry unit weight that would be obtained if the other procedure, standard or modified, had been used.

Did the results come out as expected? Explain. What is the range of water contents for compacting the soil in the field if the specified dry

unit weight is 99% of the modified Proctor maximum dry unit weight? How do void ratio, porosity and degree of saturation vary with dry density?

Table 1 – Proctor Test Calculations

Date

Data Point No.

Wt. Soil + Mold

Wt. Mold

Wt. Soil

Wet Density

Dry Density

Water Content Sample Top Bot. Top Bot. Top Bot. Top Bot. Top Bot.

Tare No.

Tare + Wet Soil (g)

Tare + Dry Soil (g)

Mass of Tare (g)

Mass of Dry Soil (g)

Mass of Water (g)

Water Content %

Average Water ContentDry Density at S=100%Void RatioPorosity (%)Degree of Saturation (%)

Point - 1 Point - 2 Point - 3 Point - 5Point - 4

Volume of Mold

Specific Gravity (Assumed)

Proctor Compaction Test Group Soil Description

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Sample Calculations:

100

110

120

130

0 5 10 15 20 25Water Content (%)

Dry

Uni

t Wei

ght (

lb/ft

3 )

15.716.2

16.717.2

17.718.2

18.719.2

19.720.2

Dry

Uni

t Wei

ght (

kN/m

3 )

Figure 1 – Proctor Compaction Curve with Zero Air Voids Curve (Gs = 2.7)

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Picture 1, 2 and 3 – Proct

Picture 1, 2 and 3 - Proctor Compaction Apparatus

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