Soil Compaction Test (Lab Report)

1. COMPACTION TEST
1.1 Objective
To obtain the moisture content-dry density relationship for a soil and hence to determine
its maximum dry density (MDD) and optimum moisture content (OMC).
1.2 Introduction
Soil compaction is an economical method of soil improvement, and it is often used to
make ground suitable for the foundations of roads and buildings. It is also used in the
placing of soil fills and in the construction of earth dams to ensure suitable soil
properties. The compaction is normally achieved through the input of energy into the soil
by impact, kneading, vibration or static means.
The extent of compaction depends on the moisture content of the soil and the compactive
effort used. In a compaction test the object is to determine the optimum moisture content
and maximum dry density achievable with a given compactive effort. A plot of dry
density versus moisture content (Figure 1) indicates that compaction becomes more
efficient up to a certain moisture content, after which the efficiency decreases. The
maximum dry density is obtained at this optimum moisture content. If the compaction
process were completely efficient, it would be possible to expel the air from the voids, in
which case the dry density would correspond to a zero-air voids state (i.e. the sample
would be saturated with water). Since perfect compaction is not possible (except at high
moisture contents) the compaction curve will always fall below the ideal or zero-air voids
curve (Figure 1).
It should be noted that there are a number of standards for compaction tests, each
differing in the amount of energy input into compaction. For a given soil the different
tests will produce different maximum densities and optimum moisture contents (i.e. these
parameters are NOT soil properties). The maximum dry density and optimum moisture
content are only relevant for a specified compaction procedure which should be stated
when presenting the results.
In earthworks it is common to specify a dry density within a certain percentage of the
maximum determined from a specified compaction test. For this to be a sensible
procedure it is important that the compactive effort used in the laboratory is comparable
to that supplied by the field equipment.
1.3 Procedure
Record results of this test in Datasheet No 12 (attached).
1. Measure the mass of the mould (M1).
2. Take about 2.5 kg of the soil provided and make sure that it will pass through a No. 4
sieve. The demonstrator may require one group will carry out the Standard Proctor
Compaction test and the other group to carry out the Modified Proctor Compaction
test.
3. In the Standard Proctor test, the soil sample should be mixed with water and placed in
the standard mould provided in 3 approximately equal layers. Each layer is to be
compacted with 25 blows from a 2.5 kg compaction hammer.
4. In the Modified Proctor test, the soil should be compacted in the mould in
approximately 5 equal layers, each layer compacted with 25 blows from the 4.5 kg
hammer.
5. When the mould is full the soil sample should be trimmed and weighed (M2)
6. Determine the moisture content of the compacted soil. Weigh a glass or plastic
dish/can, then place a small amount of the soil into the dish/can and weigh again.
Record these in Datasheet 12.
7. Microwave the soil sample for about 2 minutes until the moisture is completely dried
out. If the soil is still moist, microwave for another 2 minutes to ensure that the
moisture is completely dry. Weigh the dish/can and dry soil record in Datasheet 12.
Compute the moisture content of the compacted soil.
8. Take at least two moisture content readings and the average of the results.
9. The volume of the mould is 1000 cm3
. Compute the bulk density of the compacted
sample.
10. Using the results in 9 and moisture content in 8, calculate the dry density of the
compacted soil.
11. Calculate the dry density at zero and 5% air voids and complete the results in
Datasheet 12. Assume that the specific gravity of the soil particles, Gs is 2.65.
Note:
12. Repeat steps 2 to 11 for a total of at least 4 levels of moisture content.
13. Plot the dry density versus moisture content for the Standard and Modified Proctor
tests. Also plot the zero and 5% air void lines on the same plot.
1.4 Reporting of Results
1. Complete the results of testing in Datasheet No 12.
2. Plot the dry density versus moisture content for the Standard and Modified Proctor
tests. Also plot the zero and 5% air void lines on the same plot. Assume Gs = 2.65.
3. Determine the maximum dry density (MDD) and optimum moisture content (OMC)
of the soil.
4. From the plots of the air void lines, estimate the amount of air voids at maximum dry
density.
5. Students MUST complete Tasks 1-4 above and present these to the demonstrator
before leaving the lab class.
6. A type-written report is to be submitted within one week of the practical class. The
report should be concise and include the following:
a) Objectives of experiment
b) Completed Datasheet No 12 and the results from 1-4 above
c) A description of the soil
d) A brief comment on the effects of increasing compactive effort on the MDD and
OMC of a soil.