The University of Toledo
Soil Mechanics Laboratory
1
Soil 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.
2.
3.
4.
5.
6.
7.
8.
9.
1
No. 4 sieve for Procedure A or 3/8-in sieve for Procedure B
Mixing tools including bowl, spoon and spray bottle
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
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.
Rubber mallet
Straightedge tool
Sample extruder including a jack and frame
Platform balance
Water content containers and drying oven
ASTM D 698 – 91 (Reapproved 1998); ASTM D 1557 – 91 (Reapproved 1998)
Proctor Tests - 1
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.
Proctor Tests - 2
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.
W ( N ) = M (kg ) ⋅ g = kg ⋅ 9.81 m / sec 2 ×
N
kg ⋅ m / sec 2
(1)
γ (
10 6 cm 3
kN
W
N
kN
)
=
=
×
×
V 944cm 3 1000 N
m3
m3
(2)
γ (
lbf
kN
) = 3 × 6.366
3
ft
m
(3)
Proctor Tests - 3
The dry density or dry unit weight is then obtained by
ρd =
γd =
ρ
(4)
(1 + w)
γ
(5)
(1 + w)
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
ρd =
γd =
ρw
ρ
w+ w
ρs
(6)
Gsγ w
1 + wG s
(7)
Supplementary Calculations
The following computations can be completed using phase relationships that are derived
from the defined relationships.
e = Gs
γw
-1
γd
(8)
n=
e
1+ e
(9)
S=
wG s
e
(10)
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.
Proctor Tests - 4
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
Proctor Compaction Test
Group
Date
Soil Description
Specific Gravity (Assumed)
Volume of Mold
Point - 1
Data Point No.
Point - 2
Point - 3
Point - 4
Point - 5
Wt. Soil + Mold
Wt. Mold
Wt. Soil
Wet Density
Dry Density
Water Content Sample
Top
Bot.
Top
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 Content
Dry Density at S=100%
Void Ratio
Porosity (%)
Degree of Saturation (%)
Proctor Tests - 5
Bot.
Top
Bot.
Top
Bot.
Top
Bot.
Sample Calculations:
20.2
120
18.7
18.2
17.7
110
17.2
16.7
16.2
100
15.7
0
5
10
15
Water Content (%)
20
25
Figure 1 – Proctor Compaction Curve with Zero Air Voids Curve (Gs = 2.7)
Proctor Tests - 6
3
19.2
Dry Unit Weight (kN/m )
19.7
3
Dry Unit Weight (lb/ft )
130
Picture 1, 2 and 3 – Proct
Picture 1, 2 and 3 - Proctor Compaction Apparatus
Proctor Tests - 7