Proctor Compaction testing, an overview The proctor compaction test provides the basis
for quality control testing of compacted fills.
The original test method, commonly known as the standard proctor test was developed in
1933 for construction processes in use at that time.
This test has largely been replaced by a revised method called the modified proctor test.
The modified test uses nearly five times the energy to compact the soil as compared to
the standard Proctor test.
This accounts for the increased capacity of modern construction equipment.
We will be conducting the modified Proctor test on our soil samples.
The test is used to determine the relationship between moisture content and dry unit weight.
Most often you compact the soil in the laboratory to obtain the maximum dry unit weight, then
require the contractor to achieve at some specified percentage of this value in the
field.
The higher dry unit weight values indicate higher quality fill.
Note there is a certain moisture content, known as the optimum moisture content that
produces the maximum dry unit weight.
The proctor compaction test results consist of approximately five data points on a dry
unit weight vs. moisture content plot.
These data points are obtained by preparing samples at five different moisture contents,
compacting each sample according to the test procedures, and determining dry unit weight
for each.
Ideally one point should be near optimum moisture content and two points each should be above
and below optimum.
We will attempt to obtain the first data point near the optimum moisture content.
The other data points will be taken at intervals of approximately 2% change in moisture content.
That is, the five samples should be at optimum minus 4%, optimum minus 2%, optimum, optimum
plus 2%, and optimum plus 4%.
In a dry soil, we achieve better compaction by first adding water to raise its moisture
content to near optimum.
This water provides lubrication, softens clay bonds, and reduces surface tension forces
with the soil.
However, if the soil is too wet, then there is little or no air left in the voids, and
it thus becomes very difficult or impossible to compact.
Your data should show this characteristic increase of dry density up to optimum moisture
content and decrease of dry density after optimum moisture content.
In addition to plotting the dry unit weight versus compaction moisture content, you will
also plot corresponding to 100% saturation(zero air voids curve ) and a curve corresponding
to 80% saturation.
These two curves are used as a quality check on your data.
Since it is impossible for a soil to be compacted beyond 100% saturation, and points lying to
the right of the 100% saturation curve must be in error.
In addition, most soils achieve optimum moisture content at about 80% saturation, and the right
limb of the proctor curve should be between the 80% and 100% saturation curves and approximately
parallel to the 100% curve.
Prepare the sample by spreading it out in one of the large trays and sun drying.
Break down the clumps of soil with a mallet or other tool, and pass the sample through
the #4 sieve.
Discard any gravel.
You will need five pounds of soil for each compaction test.
According to ASTM standards you may not reuse a soil sample that has previously been used
in a compaction test.
Therefore you will need 25 lbs of soil for the compaction test.
Before compacting the first sample, you should attempt to identify the moisture content corresponding
to the optimum condition.
When the soil is at optimum moisture content it should be possible to compact a sample
of the soil into a ball the size of a golf ball using your hand.
When this golf ball sized sample is squeezed between your thumb and forefinger it should
break apart in a brittle fashion.
If the sample cannot be formed into a compact ball it is dry of optimum.
If it can be formed into a compact ball but the ball deforms when squeezed rather that
breaking apart it is wet optimum.
The goal is to have data points distributed from well dry of optimum to wet of optimum
with one or two points close to optimum.
Take a handful of soil and add water using a spray bottle until it is near optimum using
the previously stated description.
Refill your spray bottle with water, weigh it and record the weight of the full bottle.
Weigh out 5 lbs of soil in a bowl.
Mix water into the soil using the spray bottle until the sample appears to be at optimum
moisture content.
Weigh the water bottle and determine how much water was added to the sample.
Obtain the combined weight of the proctor mold and its base.
Assemble the proctor mold onto its base and attach the collar.
Remember, do not include the collar in the assembly weight.
Now it's time to compact soil into the mold.
We want to compact the soil in 5 equal layers with each layer receiving 25 blows of the
hammer.
Place some of the prepared soil into a cylindrical steel mold until it is about 1/3 full.
Compact the soil by applying 25 blows from the modified proctor hammer.
Be sure to distribute these blows across the top of the sample.
Place approximately the same quantity of soil into the mold to form the second layer and
compact it in the same fashion.
Continue this process until five layers have been placed and compacted.
Remember that you want to have 5 layers of compaction.
You may need to adjust the quantity of soil in the layers as you compact to ensure the
mold is neither over or under filled after the last layer is compacted.
When you have completed compacting the last layer, the compacted soil should be slightly
above the joint between the mold and the collar.
Carefully remove the collar with a twisting motion
Then use the steel striking tool to trim the excess soil from the top of the mold.
The top of the soil should now be flush with the top of the mold.
Determine the combined weight of the proctor mold with its base and the soil compacted
inside the mold.
Remove the soil from the mold using the hydraulic sample extractor.
Be sure you have received instructions on proper use of the sample extractor before
attempting to use this device!
Break open the soil and obtain a representative sample from the center of the specimen for
a moisture content test.
After you have determined the moisture content you will be able to compute the dry unit weight.
The entire process is repeated using a new sample of soil at a different moisture content.
Determine the approximate moisture content based on the amount of water you added to
your initial test.
Continue testing new samples and new moisture contents until you have sufficient data points
to plot a curve of dry density versus compaction moisture content.
Engineers evaluate properties such as, shear strength, compressibility, or permeability,
using compaction.
Also, foundation soils are often compacted to improve their engineering properties.