Hello this is Professor Kitch.
Welcome to this webcast on section 12.9.
This is the second of two webcasts covering laboratory shear strength tests.
This presentation is on the triaxial shear test.
When you have finished this presentation you should be able to describe the boundary conditions
for the unconfined compression test Sketch the setup for a triaxial test and show
the applied loads and stresses As well as draw a free body diagram of a soil
specimen during testing And finally, given data from a triaxial test,
you should be able to plot the appropriate Mohr-circles and determine the strength parameters
phi and c This is a picture of a typical triaxial test
The specimen being tested is located in the triaxial cell and enclosed in a flexible membrane.
The triaxial cell is placed in a load frame.
The purpose of the load frame is to apply a vertical load to the specimen.
This load is measure by a load cell located near the top of the load frame.
Tubing connects the cell to pressure and volume control modules which are used to regulate
and measure the pressure within the cell as well as control the drainage of pore fluid
out of the specimen.
A computer data acquisition system controls the test and records the data.
Now let's take a closer look at the triaxial cell itself
This figure illustrates the triaxial cell and explains how it applies stresses to the
soil and how it controls drainage conditions.
The specimen sits on a pedestal inside the cell with a loading cap on top.
It is surrounded by an impermeable latex membrane that separates the soil and its pore fluid
from the fluid that fills the cell.
There are two pressure gages attached to the cell.
One measures the pressure of the fluid within the cell and one measures the pore pressure
within the specimen.
There is a valve which controls drainage within the specimen itself.
The valve can either be closed, thereby preventing drainage, or open, thereby allowing drainage.
A loading piston on top of the cell allows an axial force to be applied to the soil specimen
A dial gage attached to the top of the specimen lets us to measure vertical deformation.
There are two distinct phases of any triaxial test.
The first phase is consolidation during which the cell pressure is increased.
This provides a uniform confining stress all around the specimen equal to the minor principal
stress, sigma-3.
During this phase the soil may be allowed to consolidate depending on the type of test
being performed.
The second phase of the test is the shear phase.
During this phase a load is applied to the piston at the top of the cell.
This load increases the stress at the top of the specimen.
Since there are no shear stresses on either the top or the sides of the specimen, therefore
these are principal plans.
The major principle stress, sigma-1, is applied to the top of the specimen and the cell pressure
provides the minor principal stress, sigma-3, to the sides of the specimen.
The vertical stress is gradually increased until the specimen fails.
This is a relatively simple schematic of a triaxial cell.
Actual cells are more complicated and have additional drainage lines.
However this schematic covers the essential components of a triaxial cell.
In actual lab testing there is a phase that precedes the consolidation phase.
This preceding phase is the saturation phase.
It's critical during testing that the soil be 100% saturated and the details of getting
a specimen to 100% saturation are an important part of testing.
However, for purposes of our discussion we will simply assume the specimen is at 100%
saturation before consolidation.
There are several different types of triaxial tests.
The different tests are distinguished by the drainage conditions applied during the consolidation
and shear phases of the test.
In Phase 1 pressure is added to the water surrounding the specimen to providing a confining
and consolidation stress.
During the consolidation, the drainage valve may either be open allowing drainage or closed
creating an undrained condition.
If the valve is closed we call the test unconsolidated and use the letter U to label this phase.
Since the specimen is saturated at the start of the consolidation phase, there can be no
change in the volume of the specimen when the valve is closed and excess pore pressures
will be generated during this phase.
If the valve is open the specimen is free to drain and we call this phase consolidated
and use the letter C to label it.
In this case the soil will be allowed change volume and all excess pore pressures will
be dissipated by the end of the consolidation phase
During phase 2, the shear phase, again the drainage valve may either be opened or closed.
If the valve is open we call the phase drained and use the letter D as a label.
In a drained test the assumption is that there are no excess pore pressures generated during
shearing.
To ensure this is the case, we must run the test very slowly to ensure any pore pressures
that might be generated during shearing have time to dissipate.
This is particularly important for clay soils, which have a low hydraulic conductivity.
If we close the drainage valve during the shear phase, we have an undrained test represented
by the symbol U.
There will be no volume change in the specimen in this case
and consequently shear induced excess pore pressures will be generated.
We commonly measure the pore pressures generated during shear.
This allows us to compute the effective stress within the specimen.
We use a two letter designation to identify the different types of triaxial tests.
The first letter specifies the drainage conditions during the consolidation phase and the second
letter specifies the drainage conditions during the shear phase.
The UU test is an unconsolidated-undrained test.
In this test the drainage valve is always closed.
This is a total stress test and does not usually entail measuring the pore pressures generated.
It is also called a Q or quick test since we do not have to wait for consolidation nor
do we have to wait for drainage during the shear phase.
The CD test is a consolidated-drained test.
In this test the drainage valve is always open.
This is an effective stress test since no excess pore pressures are allowed to accumulate.
It is also called an S or slow test since we have to wait for consolidation to be completed
during phase 1 and we have to shear the specimen slowly during phase 2 so that no excess pore
pressures are generated.
The CU test is a consolidated-undrained test.
In this test the drainage valve is open during consolidation, but closed during shear.
It is also called an R test presumably because it's faster than the S test but slower than
the Q test.
During shear we normally measure the pore pressures generated.
This allows us to compute both the total and effective stresses during shear.
Finally, we sometimes perform unconfined compression tests on cohesive soils.
Strictly speaking this test is not a triaxial test.
It is not performed in a triaxial cell.
It is a simple compression test without any consolidation or any confining pressure.
It is similar to an unconfined compression test on a concrete cylinder.
Since this test is not performed in a triaxial cell, we have no control over the drainage
conditions.
In this test we shear the soil very quickly and assume there is no drainage.
This test is equivalent to a UU test with sigma-3 equal to zero.
In the next part of this webcast, we will exam the stress paths followed in each of
the four tests we just discussed.
In each case we will present a schematic of the specimen on the left side showing the
boundary conditions and applied stresses.
On the right we will draw the Mohr-circles for the test, plot the failure envelop and
determine the shear strength parameter, phi and c.
In the unconfined compression test, a vertical stress is applied to the soil without any
lateral confining stress.
That is the minor principal stress, sigma-3, is equal to zero.
Therefore the Mohr-circles for this test are all tangent to the tau axis.
The vertical stress is then increased until the soil fails.
At failure the maximum principle stress is denoted sigma-1f or simple called sigma-1
at failure This is a total stress test-we have no measure
of the pore pressures and cannot determine the effective stresses in the soil
The peak shear stress at failure is a measure of the undrained shear strength s-u.
There is no failure envelop for the unconfined compression test since we have only one Mohr
circle in this test.
The UU test is another total stress test.
Like the unconfined compression test we do not measure the pore pressure and do not compute
effective stresses.
The purpose of the UU test is to determine the undrained shear strength s-u.
The drainage valve is closed throughout the UU test.
Initially there is no stress on the specimen and the Mohr-circles appears as a point at
the origin of the Mohr-Coulomb diagram.
As we apply the cell pressure during the consolidation phase
the Mohr-circle remains a point because sigma-1 and sigma-3 are both equal to the cell pressure.
However, the Mohr-circle move to the right along the sigma access as the cell pressure
increases.
Once the consolidation phase is complete, the shear phase starts by increasing the vertical
stress applied to the specimen.
The minor principal stress, sigma-3, does not change as we shear the specimen.
We continue to increase the vertical stress until the specimen fails.
Again the maximum principal stress at failure is sigma-1f
and the soil has an undrained shear strength s-u.
Because the drainage valve was closed during the consolidation phase, no consolidation
occurred and the soil did not gain any strength during this phase.
Therefore the undrained shear strength under a confining stress is no greater than the
undrained shear strength measured by an unconfined compression test of the same soil as shown.
Similarly if we conducted another test on an identical soil specimen
but raised sigma-3 during consolidation to the point shown, the soil still would not
gain any strength because no consolidation is allowed.
This third test would also have the same undrained strength as the two previous tests.
If we plot the total stress failure envelope for this test it will have zero slope and
intersect the tau axis at s-u.
This is known as the phi equal zero condition.
Saturated clay soils loaded in undrained conditions fail under phi equal zero conditions.
Now let's consider the consolidated drained or CD test.
In this test the drainage valve remains open during both the consolidation and shear phases
and any excess pore pressures that might be generated are allowed to dissipate so there
is no excess pore pressure.
Therefore the effective stress and the total stress are equal.
This is then an effective stress tests.
During consolidation the Mohr-circle again moves from a point at the origin
to a point at the consolidation stress sigma-3, but in this case sigma-3 is the effective
consolidation stress because there are no pore pressures and the soil will consolidate
and increase in strength.
Sigma-1 is then increased until this soil fails , but in this test the shearing is done
so slowly that no pore pressures are generated.
Therefore the major principal stress at failure is also and effective stress.
If we test a second specimen but consolidate it to a higher effective confining stress,
the soil will gain strength during the consolidation process
When sheared, it will have a larger Mohr circle at failure as shown.
Likewise if we run a third test at yet a higher confining stress , again the soil will consolidate
and have an even larger Mohr-circle at failure.
Using the three failure circles shown, we can plot the failure envelop as the line
tangent to the circles and compute the friction angle, phi-prime, and cohesion, c-prime.
In this case this will be an effective stress failure envelop and phi and c will be effective
strength parameters as noted by the use of the prime symbol.
Let's look at our final test, the CU or consolidated undrained test.
In this test the drainage valve is open while the confining stress is applied
and remains open until the soil it consolidated and all pore pressures are dissipated.
The drainage valve is then closed creating an undrained condition for the shear phase
of the test and we will measure any excess pore pressures that are generated during shear.
The vertical stress is then increased until the soil fails
Since we are measuring pore pressures during the test we know the change in pore pressure
at failure delta-u.
Knowing both the total stresses and pore pressure at failure we can compute the effective stresses
at failure and plot the effective stress Mohr-circle shown here with dashed lines.
This circle is offset from the total stress circle by a value of delta-u.
In the case shown we have a negative excess pore pressure so the effective stress is greater
than the total stress shifting the Mohr circle to the right.
As in previous tests we can test a second specimen at a higher consolidation stress.
When can then shear this specimen until it fails and determine the total stress circle
at failure for this specimen.
Since we also know the pore pressure at failure we can also plot the effective stress circle
at failure again shown with dashed lines.
Finally we can test a third specimen at a higher confining stress and we will obtain
another set of total and effective stress circles at failure.
We now have two sets of failure circles.
The solid circles represent total stresses at failure and the dashed circles represent
effective stresses at failure If we consider just the effective stress circles
we can plot the effective stress failure envelop and determine the strength parameters phi-prime
and c-prime Therefor from a CU test with pore pressure
measurements, we can determine the effective stress strength parameters even though the
shear phase is under undrained conditions .
Now that we have covered the basic triaxial tests, let's discuss some of the advantages
this test has over the direct shear test.
The triaxial testing procedures give us control over drainage conditions during both consolidation
and shear.
Therefore it allows us to measure either the drained strength or the undrained strength
of a soil.
The shear plane is not confined to a fix plane as it is in the direct shear test.
This gives us a more realistic failure surface and a better measure of the strength of a
soil.
The boundary conditions are controlled and known.
This allows us to plot complete Mohr-circles and determine the stresses on the failure
plan.
Finally, we can measure both the axial and volumetric strains during the test which allows
us to measure the stress-strain properties of the soil.
Let's summarize.
The triaxial has the following characteristics.
The test is relatively difficult and time consuming to run
With the exception of the UU test which can be run very quickly
but both the CD and CU test must be run very slowly so that shear induce pore pressures
can dissipate or equilibrate throughout the specimen
The data is more difficult to analyze than direct shear data but at the same time we
get much more information from these tests Finally the strength parameters measured in
the triaxial test are more accurate than those measured in the direct shear test.
You should now review the learning objectives for this presentation.
If you don't feel you have achieved these objectives you should review the presentation
again Example 12.10 in your text presents data from
a CU test.
You should review this example before class.
1.