The Triaxial Test

00:10:30
https://www.youtube.com/watch?v=_vOwcilCrnA

Zusammenfassung

TLDRThe video explains the limitations of the sharebox test in soil stress analysis, particularly its assumption that normal stress equals effective stress, which is problematic for low permeability materials like clay. It introduces the triaxial test as a more reliable alternative, detailing its setup with a rubber membrane and water pressure cells to control stresses. The video emphasizes the importance of monitoring pore water pressure and how effective stress is calculated. It discusses drained and undrained tests, their implications for soil behavior, and how to derive shear strength parameters from the results, including the Mohr-Coulomb failure envelope.

Mitbringsel

  • 🔍 The sharebox test has limitations in low permeability materials.
  • 💧 Pore water pressure must dissipate for accurate effective stress measurement.
  • 🧪 The triaxial test provides a more reliable method for soil stress analysis.
  • 📏 Effective stress is calculated by subtracting pore water pressure from normal stresses.
  • 📊 Drained and undrained tests reveal different soil behaviors under stress.
  • 📈 The Mohr-Coulomb failure envelope helps determine shear strength parameters.
  • ⚖️ Axial strain measures vertical displacement in triaxial tests.
  • 🔄 Undrained shear strength remains constant regardless of starting conditions.
  • 📉 Increased pore water pressure can lead to material failure.
  • 🧱 Consolidated tests allow for material settling before loading.

Zeitleiste

  • 00:00:00 - 00:05:00

    The sharebox test has limitations as it assumes that the normal stress applied to the lid is the effective stress in the sample. This assumption holds true if the pore water pressure generated during the test dissipates quickly, which is feasible for high-permeability materials like sand. However, for low-permeability materials like clay, the test requires slow shearing to allow pore pressure dissipation, leading to uncertainty about whether effective stress is being measured. To address these limitations, the triaxial test was developed, which allows for better control and measurement of pore water pressure and effective stress during soil testing.

  • 00:05:00 - 00:10:30

    In a triaxial test, a soil sample is encased in a non-permeable membrane, and axial and confining stresses are applied. The test allows for monitoring of pore water pressure, enabling the calculation of effective stress. The results from multiple tests can be used to create a Mohr-Coulomb failure envelope, which provides insights into shear strength parameters like cohesion and friction angle. The triaxial test can be conducted as drained or undrained, affecting the behavior of the soil under different loading conditions, with undrained tests showing consistent maximum shear stress regardless of initial conditions.

Mind Map

Video-Fragen und Antworten

  • What is the main limitation of the sharebox test?

    The sharebox test assumes that normal stress equals effective stress, which may not be true for low permeability materials.

  • How does pore water pressure affect the sharebox test?

    Pore water pressure must dissipate quickly for the test to be valid; otherwise, effective stress cannot be accurately determined.

  • What is a triaxial test?

    A triaxial test is a method to analyze soil stress using a cylindrical soil sample within a membrane, allowing for controlled stress application.

  • What are the two types of triaxial tests?

    The two types are drained tests, where water can flow out, and undrained tests, where pore water pressure builds up.

  • How is effective stress calculated in a triaxial test?

    Effective stress is calculated by subtracting pore water pressure from the normal stresses.

  • What is the significance of the Mohr-Coulomb failure envelope?

    It helps determine the shear strength parameters of soil, including cohesion and angle of friction.

  • What happens to the stress conditions if pore water pressure increases?

    If pore water pressure increases, both principal stresses shift, potentially leading to material failure.

  • What is the difference between consolidated and unconsolidated tests?

    Consolidated tests allow the material to settle before loading, while unconsolidated tests do not.

  • What does axial strain measure in a triaxial test?

    Axial strain measures the vertical displacement of the soil sample relative to its initial height.

  • What is the undrained shear strength?

    It is the maximum shear stress reached in undrained tests, regardless of starting conditions.

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Untertitel
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Automatisches Blättern:
  • 00:00:03
    so a major limitation of the sharebox
  • 00:00:05
    test is that um it assumes or it has to
  • 00:00:08
    assume that the uh normal stress that we
  • 00:00:11
    apply to the lid of the box is in fact
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    the normal effective stress existing in
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    the sample now that um for that
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    assumption to be true the P water that
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    um is generated during the test is uh
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    quickly dissipated or it's dissipated
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    relatively quickly compared to the rest
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    of the test
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    so what that means is that for um High
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    permeable materials like sand we can
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    have a really rapid sheer boox test um
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    and be comfortable in the knowledge that
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    the PO water pressure is dissipated
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    during the test uh for clay um or low
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    permeable materials um what we have to
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    do in a sharebox test is share the uh
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    the sample really slowly to make sure
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    that the PO pressure is given a chance
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    to dissipate and when even then we're
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    never really quite sure whether the PO
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    water pressure is actually dissipated so
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    we can never really be quite sure that
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    we're talking about the effective stress
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    um the the normal effective stress
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    rather than just the the normal stress
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    so um to overcome some of those
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    limitations uh the the concept of a
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    triaxial test has has been developed um
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    and this is a a cartoon of a triaxial
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    test so the way it works is that we have
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    a cylinder of soil so this is in two
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    dimensions a profile through um the
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    triail we have a cylinder of soil
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    sitting inside a per non-permeable
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    membrane so material like rubber so
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    that's got a sleeve of of of rubber
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    around it um and on top of the the soil
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    we have a uh and that that soil is
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    supported on a um on on a base and on
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    top of that soil we can apply an axial
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    load so we can apply a a
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    stress to the top
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    and that's usually the the net maximum
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    principle stress during the
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    test now the um the minor uh stresses
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    are controlled using this uh water
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    pressure cell so around the outside of
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    this the soil sample we have um a cell
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    of of water um and we can control the
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    pressure within that
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    cell and that gives us our Sigma 3 and
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    sigma 2
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    stresses so what usually happens in a
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    trial test is we'll keep Sigma 3 and
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    sigma 2 uh or the the water pressure
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    constant and increase the uh um the
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    axial stress until we get
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    failure now during the test what we can
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    do is monitor the um the pole water
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    pressure so we'll have a a connecting
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    tube that that connects the the
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    the pole water in the in the the
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    material to a
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    transducin so the key feature about a
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    triaxial test is that we can monitor the
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    PO water pressure during the test um and
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    by taking uh that PO water pressure away
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    from Sigma 1 and sigma 3 we can
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    understand what the effective stress is
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    within the material so Sigma
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    1 uh effective stress is equal to Sigma
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    1 minus the PO water pressure in Sigma 3
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    effective stress is equal to Sigma 3
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    minus the pole water pressure so we take
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    the pole water pressure away from both
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    of the normal
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    stresses another thing that we can
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    measure in the trial test is uh the
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    vertical displacement of the the sample
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    and if we um take the vertical
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    displacement and divide it by the
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    initial um soil uh
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    height uh what we what we derive is
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    What's called the axial strain so Chang
  • 00:04:03
    in h over H note equals axial
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    strain we also um monitor the change in
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    volume of the the water within the cell
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    so if the sample um increased in volume
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    it would push some of this this water
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    out of the cell um we we measure it um
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    out here um and if we take the tangent
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    volume and we divide it by the initial
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    volume what we get is the volumetric
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    strain so these are the things that we
  • 00:04:37
    monitor in a triaxial test but how do we
  • 00:04:40
    interpret
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    them so what happens in a a drained
  • 00:04:46
    triaxial test is that we usually start
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    the sigma 1 and sigma 3 at around the
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    same um same value the same pressure so
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    we we'd start the test off at the same
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    point on the the y-
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    axis on the x-
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    axis and what we do is we'll increase uh
  • 00:05:06
    so this is Sigma 3 and sigma 1 will
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    start at the same point um in a triaxial
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    test you'd you'd keep Sigma 3 um
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    constant and increase Sigma
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    1 uh up the x-axis until you got to the
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    point of
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    failure so at the point of failure you'd
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    record the axial stress that's Sigma
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    one um effective stress I should say for
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    both of those and what you can do is
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    then draw the well the top half of a
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    more
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    Circle drawing them together through a
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    more Circle and from that you'd be able
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    to get the the maximum share stress U
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    during the
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    test but to uh if you recognize the axis
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    we can can draw a more cool um coolon
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    failure envelope on this axis um but to
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    do that we need to do more than one test
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    we need to do several tests um so what
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    we do is then increase the the
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    conditions at which we'd start the the
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    the second test so we start Sigma 3 at a
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    a larger
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    value um use different
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    color so let's say we'd start the next
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    test here um with Sigma
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    3 um and increase Sigma 1 again until we
  • 00:06:31
    got to the point of failure and let's
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    say that was
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    here Sigma 1 and again we' can connect
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    those up through the top half of a MOS
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    Circle and finally we'll do it again for
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    another um uh another another test so we
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    might start it here and the M material
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    might fail here so
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    so once we've got our three uh test
  • 00:07:02
    results here what we can do is join the
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    uh uh the curves together with a line
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    that touches all three
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    curves and that would be our more coolon
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    failure envelope so if we took the uh
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    the T of the gradient of that line we
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    would find
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    F and The Intercept would be cohesion so
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    we can use the triaxial test like the
  • 00:07:31
    sharebox test to derive sheer strength
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    parameters of angle of friction and
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    cohesion this also helps demonstrate
  • 00:07:40
    what would happen if you increased your
  • 00:07:41
    PO water pressure or the effects of
  • 00:07:43
    changing po water pressure and that if
  • 00:07:45
    you increase your poor water pressure
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    both the sigma 1 and sigma 3 would uh
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    change by the same amount or decrease by
  • 00:07:52
    the the same amount um and the whole
  • 00:07:54
    circle would shift to the left for
  • 00:07:57
    increasing poor water pressure until the
  • 00:07:59
    point where you are so let's say we had
  • 00:08:01
    stress conditions in the soil um that
  • 00:08:04
    created a more Circle um over here and
  • 00:08:08
    it
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    was so our maximum sheer stress would be
  • 00:08:13
    known but what about if we increase po
  • 00:08:15
    water pressure well the whole circle
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    would shift um down uh until it it
  • 00:08:20
    passed the the failure envelope and at
  • 00:08:23
    which point the material would fail and
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    that's why the PO water pressure is
  • 00:08:27
    really under uh important to understand
  • 00:08:30
    stand and as well as the the difference
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    between these two uh principal
  • 00:08:35
    stresses so there's a number of
  • 00:08:37
    different tests that we can do in a
  • 00:08:40
    triaxial test uh the first one I
  • 00:08:42
    mentioned was a a drained uh triaxial
  • 00:08:45
    test and that means that we uh let the
  • 00:08:47
    water flow out of the um the the the the
  • 00:08:51
    material during the test we can also do
  • 00:08:53
    an undrained test uh in which case the
  • 00:08:56
    pole water pressures um uh are the the
  • 00:09:01
    uh we can also do a drain test
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    uh we can also do an undrain test and
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    that's when uh we stop the water flowing
  • 00:09:09
    out of the soil and let the PO water
  • 00:09:11
    pressures um build up um and that's
  • 00:09:14
    really useful for telling uh telling us
  • 00:09:17
    things about the shortterm uh behavior
  • 00:09:19
    of the soil for instance when we're um
  • 00:09:23
    when we're loading the
  • 00:09:25
    material um and what uh the result of an
  • 00:09:29
    UND test would look like um would be
  • 00:09:32
    something like this where no matter
  • 00:09:34
    where you started your test or what
  • 00:09:36
    stress range you did it
  • 00:09:38
    under you would only ever reach the the
  • 00:09:42
    same maximum sheer
  • 00:09:44
    stress so the consequences of doing
  • 00:09:46
    undrain triaxial test
  • 00:09:49
    um would be this so maybe multiple more
  • 00:09:53
    Circles of stress but all reaching the
  • 00:09:55
    same maximum
  • 00:10:00
    uh sheer
  • 00:10:03
    stress um and that's called The
  • 00:10:05
    undrained Shear strength of the
  • 00:10:09
    material and there are also um uh
  • 00:10:11
    variance on this uh between Consolidated
  • 00:10:14
    and unconsolidated and that means
  • 00:10:16
    whether we allow the the the material to
  • 00:10:19
    consolidate before the test um when
  • 00:10:21
    we're we're ramping up the load and I've
  • 00:10:23
    put a a matrix of these different tests
  • 00:10:26
    on um on my website
Tags
  • sharebox test
  • triaxial test
  • effective stress
  • pore water pressure
  • soil mechanics
  • shear strength
  • drained test
  • undrained test
  • Mohr-Coulomb
  • soil behavior