CE 327L: Atterberg Limits testing, an overview
The Atterberg limits are measurements of a soil's ability to absorb and hold water
and are used to define plasticity of soil.
There are two Atterberg limits for engineering usage: the liquid limit and the plastic limit,
each with its own standardized test.
These tests are important in the classification of fine grained soils.
In particular the liquid limit and plastic limit are used to distinguish silts from clays
and to classify the plasticity of silts and clays.
Atterberg limits tests quantify the relationship between the consistency of a soil and its
moisture content.
ASTM D4318 defines consistency as the relative ease with which a soil can be deformed.
Thus, a soil with a soft consistency can be easily deformed and molded without cracking,
while a soil with a stiff consistency cannot be deformed without cracking or breaking apart.
Consider a sample of a clayey or silty soil that is relatively dry with a moisture content
of say 10%.
At this moisture content the soil will be stiff and act as a semisolid material that
can be deformed but will crack if you try to mold it into different shapes.
As water is added to this soil sample, increasing its moisture content, it will become progressively
softer.
At some point it will no longer act as a semisolid material, but will act as a plastic, material
that can be deformed and molded without cracking.
If additional water is added further increasing the moisture content of the soil, the soil
will eventually act as viscous liquid that will ooze and flow.
The Atterberg limits are water content measurements that defined the boundaries between the three
soil states of semisolid, plastic, and liquid
When a soil has a moisture content at its plastic limit, it is at the boundary between
semisolid and plastic behavior.
If water is removed from the soil, it will behave like a semisolid that cracks when it
is deformed.
If water is added it will act like a plastic material that will be moldable and deform
without cracking.
When a soil has a moisture content at its liquid limit, it is at the boundary between
plastic and liquid behavior.
If water is removed it will act like a plastic material that will be moldable and deform
without cracking.
If water is added it will act like a viscous liquid that can ooze and flow.
The purpose of the Atterberg limit tests is to determine these moisture contents in a
standardized way.
For example if the tests indicate the soil is at is liquid limit when it has a moisture
content of 61%, then its liquid limit (LL) is 61.
Likewise, if the tests determine it is at the plastic limit when its moisture content
is 22%, then the plastic limit (PL) is 22.
There are two different tests, one to determine each of the Atterberg limits.
Each test has its own equipment and procedures.
But before discussing the equipment, we must first describe how to prepare samples for
testing.
You will need 150 to 300g of soil for these tests.
These tests are performed on only the fine grained portion of a soil.
If the soil contains significant amounts of sand or gravel we must pass the soil through
a #40 sieve to remove the larger sand and gravel particles before performing the Atterberg
limits tests.
However, in sieving the soil it is important that we don't remove silt and clay lumps
with sand particles.
Therefore sample preparation is needed.
The soil must be dry enough that you can sieve it, but not so dry that you cannot break up
the clay and silt lumps.
Check the moisture of removed soil, if needed dry this soil by laying it out in the sun
or placing it in the microwave oven until it can be pulverized by a mortar and pestle.
Once the sample is properly dried use a mortar and pestle to breakup slit and clay lumps
until they can pass through the #40 sieve.
Use just enough force to break up the silt and clay lumps without breaking up sand and
gravel piece.
The objective is to separate the larger sand and gravel from the clay and silt.
If the soil being tested contains little or no material coarser than the #40 sieve, you
can skip the pulverizing a sieving procedures.
The plastic limit test procedure involves taking a sample of soil slightly wetter than
the plastic limit and carefully rolling the soil sample into a thread 1/8 of an inch in
diameter as shown.
So long as the soil can be rolled into a 1/8 inch diameter rod without falling apart the soil
is wetter than the plastic limit.
As the soil is manipulated and rolled out it gradually loses water.
When it reaches the point where it crumbles or breaks apart as it is rolled into a 1/8 inch
diameter rod, it is at the plastic limit.
To perform the test, select a sample of soil about the size of a small marble.
You should be able to roll the soil into a ball without it cracking but it should not
be sticky.
Roll the ball of soil into a rod on the frosted side of the glass plate.
Continue rolling it relatively quickly until it is 1/8 of an inch in diameter.
Use the 1/8 inch diameter brass rod as a guide.
Once you have made the 1/8 inch diameter thread, pick it up.
If remains intact, remold the sample into a ball and roll it out again.
When the soil crumbles or breaks into pieces as your roll it out, it is at the plastic
limit.
When the soil reaches the plastic limit, put it into a pre-weighed sample tin and place
a lid on the tin.
Repeat this procedure with new balls of soil until you accumulated five samples in the
tin.
Repeat the entire procedure over again so that you have two tins of soil at the plastic
limit.
Each tin should have five separate samples.
Once you have acquired your samples at the plastic limit, determine the water content
of the samples using the normal procedures for the water content test.
The liquid limit test requires two special pieces of equipment
the Liquid limit device and a Grooving tool
Before you can conduct the liquid limit test, it is necessary to adjust and calibrate the
equipment.
First check the grooving tool to ensure that it is clean and that the tip is not wider
than 2.1mm.
While checking the grooving tool note that the butt end of the tool has a calibration
tab that is 10mm high and 13 mm wide.
Next check the liquid limit device.
Inspect the device for cleanliness.
Check for proper functionality ensuring that the cup fall freely and does not bind.
Verify that the fall height of the cup is 10mm.
To do this, raise the cup to its highest position then slide the grooving tool under the cup
the calibration tab up.
When the cup is at the proper fall height, you should be able to feel the calibration
tap drag on the bottom of the cup, but you should not be able to see the cup rise.
If you don't feel the tab dragging on the bottom of the cup, the cup is too high.
If the tab pushes the cup up visibly, then the cup is too low.
Adjust the fall height as needed.
Place a portion of your soil in an evaporating dish.
Slowly add water to this soil and mix it using a spatula.
Continue until the soil has a consistency slightly stiffer than that of toothpaste.
Place a sample of the soil in the lower portion of the device cup using the spatula as shown.
Once the soil is smoothed out in the cup, cut a V-shaped groove in the soil using the
grooving tool.
The groove should extend from the back to the front the cup.
A clear brass line should be visible at the bottom of the grove, as shown.
The cup is then repeatedly dropped by rotating the crank counterclockwise.
Use a rate of approximately 2 drops per second.
Count the number of drops required for the groove to close a distance of 13 mm, use the
calibration tap on the grooving tool as a reference.
Record the number of drops on your data sheet under "1st try." Reform the soil sample
in the bottom of the cup, cut a new groove and repeat the test.
Keep repeating the test until you have two consecutive tests where the blow count differs
by 3 blows or less.
Circle the final blow count.
This will be the correct blow count for this test.
Do not average the blow counts.
After the final repetition, obtain a sample from the center of the cup in the location
where the groove closed.
Place the sample in a pre-weighed sample tin and obtain its moisture content using the
normal procedures.
The soil is said to be at liquid limit when exactly 25 drops are required to close the
groove 13mm.
If more than 25 blows were required, the soil is dryer than the liquid limit; if less than
25, it is wetter than the liquid limit.
Depending on the number of blows required to close the groove, add or remove water from
the soil to make it more or less stiff and repeat the test
It is not necessary to hit exactly 25 blows to complete the test properly.
The goal is to obtain three samples with blow counts between 15 and 35: one sample with
a count between 15-25 blows, one between 20-30 blows, and one between 25 and 35 blows.
If the number of blows is less than 15 or greater then 35 the soil is too wet or too dry.
Do not record the data.
Adjust the moisture content of the sample and try again.
Plot your data on a semi-log plot with blow count on a logarithmic horizontal axis and
moisture content on an arithmetic vertical axis.
Connect the points with a best fit straight line.
The moisture content that corresponds to the intersection of this line and the 25 blows
coordinate is the liquid limit.
The plasticity index (PI) is the numerical difference between the liquid limit and the
plastic limit.
It is a measure of how much water a soil can absorb between its plastic and liquid states.
In this example, the plasticity index is equal to 61 minus 22 or 39.
From the Atterberg limit test results you determine the group symbol and group name
using the Unified soil Classification System.
This concludes this presentation.
Please note the password.
You will need it to take your pre-lab quiz.