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Oedometer tests are designed to simulate the one-dimensional deformation and drainage conditions that soils experience in the field. The soil sample in an oedometer test is typically a circular disc of diameter-to-height ratio of about 3:1. The sample is held in a rigid confining ring, which prevents lateral displacement of the soil sample, but allows the sample to swell or compress vertically in response to changes in applied load. Known vertical stresses are applied to the top and bottom faces of the sample, typically using free weights and a lever arm. The applied vertical stress is varied and the change of the thickness of the sample is measured.
For samples that are saturated with water, porous stones are placed on the top and bottom of the sample to allow drainage in the vertical direction, and the entire sample is submerged in water to prevent drying. Saturated soil samples exhibit the phenomenon of consolidation, whereby the soil's volume changes gradually to give a delayed response to the change in applied confining stresses. This typically takes minutes or hours to complete in an oedometer and the change of sample thickness with time is recorded, providing measurements of the coefficient of consolidation and the permeability of the soil.
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CE 326 Mod 10.5a Consolidation test
Consolidometer Test for Calculation of Void Ratio | Soil Mechanics
CONSOLIDATION TEST SOIL MECHANICS 2013
Transcription
Hello this is Professor Kitch.
Welcome to this webcast which is the first
of two on section 10.5. This webcast covers
the consolidation lab test
After this webcast, you should be able to
describe the lab consolidation test
Explain why we generally use a semi-log plot
of strain versus the log of effective stress
as the stress-strain curve for soils, rather
than the arithmetic stress-strain curve used
for nearly all other materials.
Once you have the strain versus log-sigma-prime
curve you should be able to identify the recompression
curve, virgin curve and rebound curve
You need to be able to derive the relationship
between strain and change in void ratio and
explain why we can plot consolidation data
as either strain or void ratio versus the
log of effective stress
You should be able to define the preconsolidation
stress and explain why we always see one in
our laboratory data
And finally, you should be able to perform
Casagrade method to estimate the preconsolidation
stress from lab data
We discussed the basic consolidation process
in the previous section. Specifically we discussed
how a change in total stress immediately generates
an excess pore pressure and that as that excess
pore pressure dissipates, the soil consolidates
and the stress is transferred from pore pressure
to the soil skeleton in the form of effective
stress.
We�re now starting sections that will allow
you to compute the magnitude of consolidation
settlement. In understanding the magnitude
of consolidation settlement, it helps to start
off by looking at the 1 dimensional consolidation
test.
In the field, if we have a compressible layer,
shown here with a thickness of H-naught.
And that layer is covered by a wide areal
fill
we�ll have one dimensional consolidation
and experience a consolidation settlement
of delta-c
To understand what�s happening in the field
we can take a sample of soil from location
A, transport it to the lab, and put a specimen
from that sample in a device that allows for
only one dimensional consolidation.
We can then place a load, P, on the specimen
and it will consolidate one dimensionally.
If we measure what happens in the lab, we
should be able to use that data to predict
the field settlement.
This test is called the one dimensional consolidation
test, or the oedometer test.
Three are two main types of consolidometers,
the floating ring and the fixed ring. The
only one we�ll discuss here is the floating
ring consolidometer,
which is the most common type.
In the center is a short cylindrical soil
specimen about the size and shape of a hockey
puck.
It�s usually about three quarters to one
inch thick and 3 to 4 inches in diameter
The specimen is contained in a confining ring
which is made of metal, is very stiff compared
to the specimen, and therefore prevents any
lateral strain, thereby ensuring that we have
one dimensional consolidation.
There are porous stones placed on the top
and the bottom of the specimen to allow drainage
of pore water out of the specimen.
With porous stone on the top and bottom, the
water can drain out from the center of the
specimen in both directions.
A loading cap is place on top to distribute
applied forces over the entire area of the
specimen
And a dial gage or digital device is used
to measure the vertical displacement.
We place a load, P, on the top of the specimen
and the loading cap distributes the load evenly
over the specimen
The final effective stress that is applied,
is then P over A minus the pore pressure,
u.
Since the specimen is so thin the average
head of pressure is an inch or less
so we can assume the pore pressure is zero
and the final effective stress is just equal
to P over A
When the load P is placed on the cap, the
specimen starts to consolidate and we measure
the vertical displacement, delta-z, versus
time
When the sample is finished consolidating,
we can measure the final vertical displacement,
delta-zf
We can then compute the vertical strain as
the change in height over the initial height
or delta-z over H-naught
We can then incrementally increase the vertical
stress several times, measuring the vertical
deflection with each load.
From the data measured from each load increment
we can create a table of vertical effective
stress versus vertical strain.
Once we have our table of strain versus effective
stress, we can plot a stress-strain curve
for our soil.
You�re probably used to seeing such data
plotted as stress versus-strain, but in geotechnical
engineering we generally plot the data as
strain versus stress. We also generally plot
stress increasing downward because when soils
compress they move down.
As you can see from the curve shown, soil
stress-strain behavior is highly non-linear.
The portion of the curve from A to B to C
is the initial loading curve for the soil.
Notice that the amount of strain from zero
to 200
is more than that from 200 to 400
which is more than that from 400 to 600.
This indicates that the soil is always getting
stiffer as it consolidates. This makes since.
As the soil consolidates, the soil particles
get more and more tightly packed and as a
result, the soil skeleton gets stiffer and
stiffer.
Interestingly, if we plot the strain versus
the log of effective stress, the stress-strain
curve becomes much simpler as shown on the
right. In this semi-log space, the curve is
very close to being composed of three straight
lines
We call the portion from the beginning of
the plot to the first brake, from A to B,
the recompression curve.
The next portion of the curve from a bit past
B to C is called the virgin curve.
and the final portion, which occurs during
unloading of the specimen, is called the rebound
curve.
It may not be readily apparent, but we can
also plot the curves as void ratio as a function
of effective stress rather than strain versus
effective stress. Notice that the void ratio
decreases as the soil consolidates�which
makes sense since the void volume decreases
as the soil skeleton compresses.
The deformation of the soil during consolidation,
can be expressed either as strain or a change
in void ratio.
Let�s use our phase diagram to determine
the relationship between void ratio change
and strain.
On the left we have our initial conditions.
For convenience, we�ll assume the volume
of the voids is 1 unit.
Recalling that the void ratio is the equal
to the volume of voids divided by the volume
of solids
When the volume of solids is equal to 1, the
volume of the voids will simply be equal to
the void ratio, e. In this case it will be
e-naught, the initial void ratio.
Now, after loading and consolidation, there
will be a change in volume of the soils, and
all of the change will be in the void space,
and therefore the change in volume will be
equal to delta-e.
If our soil is in one dimensional compression,
then the change in void ratio must come only
from vertical compression
Therefore, we can write that the vertical
strain, epsilon-z
will be equal to the change in length of our
soil element over the original length or delta-l
over l-naught
If we look at the initial phase diagram on
the left, we�ll see that the original length
of the element is 1 plus e-naught.
And from the phase diagram on the right, we
see that the change in length is delta-e
Therefore the vertical strain epsilon-z is
equal to delta-e over one plus e-naught. That�s
why we can plot our consolidation curve either
as either strain versus log-sigma-prime, or
void ratio versus log-sigma-prime. Epsilon-z
and delta-e are directly proportional to one
another and the proportionality constant is
1 over 1 plus e-naught.
So let�s look at how the stress-strain curve
for soils gets its characteristic shape in
semi-log space
To do this let�s go back a few hundred thousand
year, just a few minutes in geologic time,
and consider a point A just below the bottom
of a Pleistocene ocean during a depositional
period.
At this point the soil is under some very
small effective stress shown as point 1 on
our semi-log stress-strain curve.
As the deposition continues the effective
stress increases, the soil at point A consolidates
and undergoes strain. On our strain versus
log effective stress plot it travels alone
a line from point 1 to point 2.
If the deposition continued, the stress path
of the soil would continue down the dotted
line shown. However, it doesn�t. The deposition
at this location stops, and we enter the Holocene
epoch.
Neanderthals go extinct, homo sapiens get
bigger brains, the stone age gives way to
the bronze age then the iron age.
Finally, the industrial revolution happens
along, we start spewing billions of tons of
CO2 and other pollutants into the atmosphere.
A English teacher, a history teacher, and
writer, open a coffee house in Seattle featuring
a two tailed mermaid in their logo.
Three decades later barista turned Starbucks
CEO Howard Schultz, shows up at the empty
lot above point A and decides to build yet
another Starbucks because the nearest one
is over 200 feet away.
Schultz hires your geotech firm to do a site
investigation.
Having read about the Kansai airport and knowing
settlement might be an issue, you make a boring
at the site
and take an undisturbed sample back to the
lab.
In the process of taking the sample, the soil
is unloaded and it follows the stress path
form point 2 to point 3. Notice that the soil
does not travel back along the original compression
line from point 1 to point 2.
This is because soil is not elastic and undergoes
unrecoverable plastic strain during consolidation.
So in unloading it�s stiffer than in loading.
We now take a specimen from our field sample
and place it in a consolidometer
and perform a 1-D consolidation test.
During the initial part of the test, the stress
path for the soil closely follows the rebound
curve from point 3 to 4. But when it gets
near the original consolidation curve at point
4, the stress path turns down and follows
the original depositional consolidation curve
or virgin curve to point 5
The consolidation test continues with unloading
during which the soil travels along the rebound
curve from point 5 to 6.
However, realize that the only information
you will have is the laboratory strain versus
log effective stress curve shown here.
Due to plastic strain during consolidation,
it will have a recompression curve,
a virgin curve,
and rebound curve.
And the point where the recompression and
virgin curves meet is the preconsolidation
stress, sigma-c-prime.
If we have a high quality specimen our testing
is done very carefully, we�ll get a strain
versus log effective stress plot with distinct
breaks between different segments of the curve.
And it will be very easy to identify the recompression
curve, the virgin curve,
and find the preconsolidation stress.
However, we don�t always have a high quality
sample. As samples get more disturbed due
to poor sampling techniques or poor handling,
the lab test specimen quality is lower and
the strain versus log effective stress plots
tend to soften and don�t show a clear break
between recompression and virgin loading.
This can make it difficult or impossible to
determine the preconsolidation stress accurately.
If the specimen is of relatively good quality
and tested properly, but doesn�t show a
distinct preconsolidation point, as shown
here,
We can use the method developed by Casagrande
to estimate the preconsolidation stress.
The first step in the Casagrande method is
to locate the point of maximum curvature on
the strain versus log effective stress plot.
From this point you then draw a horizontal
line
and a line tangent to the lab data curve
you then determine the angle between these
two lines
and draw a third line bisecting that angle.
Next you go to the lower end of the virgin
curve and draw a line tangent to the low end
of the curve.
Now locate the point where this extension
of the virgin curve intersects the bisector
line.
That point is the location of the preconsolidation
stress.
This method is based on Cassagrande�s considerable
experience, but there�s no theory behind
it. It is however, a consistent method and
if followed carefully, will ensure some consistency
in estimating the preconsolidation stress.
However if the specimen is highly disturbed
such as the one show in red, there is now
way of accurately estimating the preconsolidation
stress.
Schmertmann extended Cassagrande�s method
into a procedure to reconstruct an estimate
of the field compression curve from a moderately
disturbed laboratory curve. This figure shows
a typical lab data curve and the corresponding
reconstructed field curve using Schmermann�s
method. The procedures are outlined on pages
439 & 440 of your text.
It is important to note that the reconstructed
field curve lies outside of the measured lab
data and therefore constitutes an extrapolation
of the lab data. Extrapolation is always a
dicey endeavor and the accuracy of curves
reconstructed by this method is questionable.
The best approach is to get high quality samples
and use good lab techniques so you get a lab
curve that clearly show the preconsolidation
stress and require little correction.
So let�s summarize
Soils have nonlinear stress-strain curves
and their stiffness increases during consolidation.
We generally plot these curves as strain versus
the log of effective stress or void ratio
versus the log of effective stress.
Plotting in this semi-log space makes the
curves more linear
On unloading soil always exhibits plastic
strain and unloads on a curve stiffer than
the loading curve
We use the laboratory consolidation test to
determine our stress-strain curve
Lab tests will always show both a recompression
and virgin curve, because the field sample
is unloaded during the sampling process
The point where the recompression and virgin
curves meet is the preconsolidation stress
and it represents the greatest effective stress
the soil has ever experienced.
If we have high quality tests with high quality
specimens, we can easily draw tangents to
the recompression and virgin curves and locate
the preconsolidation stress.
When the lab data are moderately disturbed,
you should use Cassagrande�s method as a
consistent way to estimate the preconsolidation
stress.
And finally, we can use Schmertmann�s method
to reconstruct field compression curves for
lab tests on disturbed samples. However, it�s
better to get a high quality sample and generate
a high quality lab curve.
This should not be confused with the similar-looking but unrelated word "odometer", derived from Ancient Greek ὁδός (hodos, "path") which refers to a device to measure the distance travelled by a vehicle.[2]
History
Consolidation experiments were first carried out in 1910 by Frontard. A thin sample (2in thick by 14in in diameter) was cut and placed in a metal container with a perforated base. This sample was then loaded through a piston incrementally, allowing equilibrium to be reached after each increment. To prevent drying of the clay, the test was done in a room with high humidity.[3]
Karl von Terzaghi started his consolidation research in 1919 at Robert College in Istanbul.[3] Through these experiments, Terzaghi started to develop his theory of consolidation which was eventually published in 1923.
The Massachusetts Institute of Technology played a key role in early consolidation research. Both Terzaghi and Arthur Casagrande spent time at M.I.T. - Terzaghi from 1925 to 1929 and Casagrande from 1926 to 1932. During that time, the testing methods and apparatuses for consolidation testing were improved.[4] Casagrande's contributions to the technique of oedometer testing includes the "Casagrande method" to estimate the pre-consolidation pressure of a natural soil sample.[5] Research was continued at MIT in the 1940s by Donald Taylor.[6]
Both the British Standards Institute and the ASTM have standardised methods of oedometer testing. ASTM D2435 / D2435M - 11 covers oedometer testing by incremental loading. ASTM D3877, ASTM D4546 and AASHTO T216 also provide related procedures for conducting other similar tests for determination of the consolidation characteristics of soils.[7] BS 1377-5:1990 is the relevant British Standard for oedometer testing; the wider BS 1377 series also provides background information and best-practice advice on sample preparation for various geotechnical investigations.[8] There are also two ISO standards on oedometer testing: ISO 17892-5:2017 on incremental loading oedometer tests;[9] and BS EN ISO 17892-11:2019 covers various methods of soil permeability testing, including oedometer tests on saturated samples.[10]
Equipment
Two disassembled oedometers at the University of Cambridge
An oedometer is fundamentally made out of three components: a "consolidation cell" to hold the soil sample, a mechanism to apply a known pressure over the sample, and an instrument to measure the changes in the sample's thickness.[11]
The equipment required to perform an oedometer test is sometimes called an "oedometer test set". A typical inventory of an oedometer laboratory includes:[12]
1 x Bench
3 x Oedometers
3 x Cells, either 50mm or 63.5mm, or 75mm
3 x Dial gauges, either analogue, or digital
1 x Weight set
The consolidation cell is the part of the oedometer that holds the soil sample during a test. At the centre of the consolidation cell is a sample ring where the soil sample is held. The sample ring is typically shaped like a cookie cutter, with a sharp edge on one side, so the ring can be used to cut out a sample slice of soil from a larger block of natural soil. Two slices of porous stone, which fit snugly into the sample ring, provide water drainage to the soil sample while confining it mechanically. These components all fit in a larger cylinder, which has grooves to ensure alignment of the components, and provides water supply and drainage to external plumbing. A rigid loading cap sits on top of the soil sample to apply compressive loads to the soil.[11][13]
The loading mechanism of the oedometer applies a known compressive load, and therefore a known compressive stress since the diameter is fixed, to the soil sample. Most oedometers achieve this with a lever arm and a set of free weights: the free weights provide a known gravitational load, and the lever arm multiplies and transmits the load to the soil sample.[14]
Testing procedures
Schematic drawing of the incremental loading frame developed by Alan Bishop
There are many oedometer tests that are used to measure consolidation properties. The most common type is the incremental loading (IL) test.[15]
Sample preparation
Tests are carried out on specimens prepared from undisturbed samples. A stiff confining ring with a sharp edge is used to cut a sample of soil directly from a larger block of soil. Excess soil is carefully carved away, leaving a sample with a diameter-to-height ratio of 3 or more. Porous stones are placed on the top and bottom of the sample to provide drainage. A rigid loading cap is then placed on top of the upper porous stone. For saturated soil samples, it is important to submerge the entire sample ring in water to prevent the sample from drying out.[15]
Incremental loading
This assembly is then placed into a loading frame. Weights are placed on the frame, imposing a load on the soil. Compression of the sample is measured over time by a dial indicator. By observing the deflection value over time data, it can be determined when the sample has reached the end of primary consolidation. Another load is then immediately placed on the soil and this process is repeated. After a significant total load has been applied, the load on the sample is decreased incrementally. Using a load increment ratio of 1/2 provides a sufficient number of data points to describe the relationship between void ratio and effective stress for a soil.[15]
Results
Oedometer tests provide engineers with very useful data about the soil being tested.
^ abBjerrum, Laurits; Casagrande, Arthur; Peck, Ralph; Skempton, Alec. (1960). From Theory to Practice in Soil Mechanics. (p44) John Wiley & Sons, Inc.
^Bjerrum, Laurits; Casagrande, Arthur; Peck, Ralph; Skempton, Alec. (1960). From Theory to Practice in Soil Mechanics. (p6-7) John Wiley & Sons, Inc.
^"Coefficient of Earth Pressure at Rest", Geotechnical Correlations for Soils and Rocks, John Wiley & Sons, Inc., 2018-06-01, pp. 73–75, doi:10.1002/9781119482819.ch8, ISBN9781119482819
^Taylor, Donald W. (1942). Research on Consolidation of Clays. Massachusetts Institute of Technology
This page was last edited on 19 January 2021, at 17:22