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Triaxial shear test

From Wikipedia, the free encyclopedia

Triaxial apparatus with sample attached ready for testing.

A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil (e.g., sand, clay) and rock, and other granular materials or powders. There are several variations on the test.[1][2][3][4]

In a triaxial shear test, stress is applied to a sample of the material being tested in a way which results in stresses along one axis being different from the stresses in perpendicular directions. This is typically achieved by placing the sample between two parallel platens which apply stress in one (usually vertical) direction, and applying fluid pressure to the specimen to apply stress in the perpendicular directions. (Testing apparatus which allows application of different levels of stress in each of three orthogonal directions are discussed below, under "True Triaxial test".)

The application of different compressive stresses in the test apparatus causes shear stress to develop in the sample; the loads can be increased and deflections monitored until failure of the sample. During the test, the surrounding fluid is pressurized, and the stress on the platens is increased until the material in the cylinder fails and forms sliding regions within itself, known as shear bands. The geometry of the shearing in a triaxial test typically causes the sample to become shorter while bulging out along the sides. The stress on the platen is then reduced and the water pressure pushes the sides back in, causing the sample to grow taller again. This cycle is usually repeated several times while collecting stress and strain data about the sample. During the test the pore pressures of fluids (e.g., water, oil) or gasses in the sample may be measured using Bishop's pore pressure apparatus.

From the triaxial test data, it is possible to extract fundamental material parameters about the sample, including its angle of shearing resistance, apparent cohesion, and dilatancy angle. These parameters are then used in computer models to predict how the material will behave in a larger-scale engineering application. An example would be to predict the stability of the soil on a slope, whether the slope will collapse or whether the soil will support the shear stresses of the slope and remain in place. Triaxial tests are used along with other tests to make such engineering predictions.

During the shearing, a granular material will typically have a net gain or loss of volume. If it had originally been in a dense state, then it typically gains volume, a characteristic known as Reynolds' dilatancy. If it had originally been in a very loose state, then contraction may occur before the shearing begins or in conjunction with the shearing.

Sometimes, testing of cohesive samples is done with no confining pressure, in an unconfined compression test. This requires much simpler and less expensive apparatus and sample preparation, though the applicability is limited to samples that the sides won't crumble when exposed, and the confining stress being lower than the in-situ stress gives results which may be overly conservative. The compression test performed for concrete strength testing is essentially the same test, on apparatus designed for the larger samples and higher loads typical of concrete testing.

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Transcription

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.

Test execution

For soil samples, the specimen is contained in a cylindrical latex sleeve with a flat, circular metal plate or platen closing off the top and bottom ends. This cylinder is placed into a bath of a hydraulic fluid to provide pressure along the sides of the cylinder. The top platen can then be mechanically driven up or down along the axis of the cylinder to squeeze the material. The distance that the upper platen travels is measured as a function of the force required to move it, as the pressure of the surrounding water is carefully controlled. The net change in volume of the material can also be measured by how much water moves in or out of the surrounding bath, but is typically measured - when the sample is saturated with water - by measuring the amount of water that flows into or out of the sample's pores.

Rock

For testing of high-strength rock, the sleeve may be a thin metal sheeting rather than latex. Triaxial testing on strong rock is fairly seldom done because the high forces and pressures required to break a rock sample require costly and cumbersome testing equipment.

Effective stress

The effective stress on the sample can be measured by using a porous surface on one platen, and measuring the pressure of the fluid (usually water) during the test, then calculating the effective stress from the total stress and pore pressure.

Triaxial test to determine the shear strength of a discontinuity

The triaxial test can be used to determine the shear strength of a discontinuity. A homogeneous and isotropic sample fails due to shear stresses in the sample. If a sample with a discontinuity is orientated such that the discontinuity is about parallel to the plane in which maximum shear stress will be developed during the test, the sample will fail due to shear displacement along the discontinuity, and hence, the shear strength of a discontinuity can be calculated.[5]

Types of triaxial tests

There are several variations of the triaxial test:

Consolidated drained (CD)

In a 'consolidated drained' test the sample is consolidated and sheared in compression slowly to allow pore pressures built up by the shearing to dissipate. The rate of axial deformation is kept constant, i.e., strain is controlled. The idea is that the test allows the sample and the pore pressures to fully consolidate (i.e., adjust) to the surrounding stresses. The test may take a long time to allow the sample to adjust, in particular low permeability samples need a long time to drain and adjust strain to stress levels.

Consolidated undrained (CU)

In a 'consolidated undrained' test the sample is not allowed to drain. The shear characteristics are measured under undrained conditions and the sample is assumed to be fully saturated. Measuring the pore pressures in the sample (sometimes called CUpp) allows approximating the consolidated-drained strength. Shear speed is often calculated based on the rate of consolidation under a specific confining pressure (whilst saturated). Confining pressures can vary anywhere from 1 psi to 100 psi or greater, sometimes requiring special load cells capable of handling higher pressures.

Unconsolidated undrained

In an 'unconsolidated undrained' test the loads are applied quickly, and the sample is not allowed to consolidate during the test. The sample is compressed at a constant rate (strain-controlled).

True triaxial test

Triaxial testing systems have been developed to allow independent control of the stress in three perpendicular directions. This allows investigation of stress paths not capable of being generated in axisymmetric triaxial test machines, which can be useful in studies of cemented sands and anisotropic soils. The test cell is cubical, and there are six separate plates applying pressure to the specimen, with LVDTs reading movement of each plate.[6] Pressure in the third direction can be applied using hydrostatic pressure in the test chamber, requiring only 4 stress application assemblies. The apparatus is significantly more complex than for axisymmetric triaxial tests, and is therefore less commonly used.

Free end condition in triaxial testing

The Danish triaxial in action

Triaxial tests of classical construction had been criticized for their nonuniform stress and strain field imposed within the specimen during larger deformation amplitudes.[7] The highly localized discontinuity within a shear zone is caused by combination of rough end plates and specimen height.

To test specimens during larger deformation amplitude, "new" [8] and "improved"[9] version of the triaxial apparatus were made. Both the "new" and the "improved" triaxial follow the same principle - sample height is reduced down to one diameter height and friction with the end plates is canceled.

The classical apparatus uses rough end plates - the whole surface of the piston head is made up of rough, porous filter. In upgraded apparatuses the tough end plates are replaced with smooth, polished glass, with a small filter at the center. This configuration allows a specimen to slide / expand horizontally while sliding along the polished glass. Thus, the contact zone between sample and the end plates does not buildup unnecessary shear friction, and a linear / isotropic stress field within the specimen is sustained.

Due to extremely uniform, near isotropic stress field - isotropic yielding takes place. During isotropic yielding volumetric (dilatational) strain is isotopically distributed within the specimen, this improves measurement of volumetric response during CD tests and pore water pressure during CU loading. Also, isotropic yielding makes the specimen expand radially in uniform manner, as it is compressed axially. The walls of a cylindrical specimen remain straight and vertical even during large strain amplitudes (50% strain amplitude was documented by Vardoulakis (1980), using "improved" triaxial, on non saturated sand). This is in contrast with classical setup, where the specimen forms a bugle in the center, while keeping a constant radius at the contact with the end plates.

Post-liquefaction testing. The fine sand specimen was liquefied during consolidates undrained (CU) cycles and recovered with consolidated drained (CD) cycles many times. The wrinkles formed due to the volume change imposed by iterating between CU liquefaction and draining. In a liquefied state the sample becomes soft enough to imprint thin latex. During CD cycles - stiff enough to preserve the imprinted pattern.

The "new" apparatus has been upgraded to "the Danish triaxial" by L.B.Ibsen.[10] The Danish triaxial can be used for testing all soil types. It provides improved measurements of volumetric response - as during isotropic yielding, volumetric strain is distributed isotopically within the specimen. Isotropic volume change is especially important for CU testing, as cavitation of pore water sets the limit of undrained sand strength.[11] Measurement precision is improved by taking measurements near the specimen. The load cell is submerged and in direct contact with the upped pressure head of the specimen. Deformation transducers are attached directly to the piston heads as well. Control of the apparatus is highly automated, thus cyclic loading can be applied with great efficiency and precision.

The combination of high automation, improved sample durability and large deformation compatibility expands the scope of triaxial testing. The Danish triaxial can yield CD and CU sand specimens into plasticity without forming a shear rupture or bulging. A sample can be tested for yielding multiple times in a single, continuous loading sequence. Samples can even be liquefied to a large strain amplitude, then crushed to CU failure. CU tests can be allowed to transition into CD state, and cyclic tested in CD mode to observe post liquefaction recovery of stiffness and strength.[12] This allows to control the specimens to a very high degree, and observe sand response patterns which are not accessible using classical triaxial testing methods.

Test standards

The list is not complete; only the main standards are included. For a more extensive listing, please refer to the websites of ASTM International (USA), British Standards (UK), International Organization for Standardization (ISO), or local organisations for standards.

  • ASTM D7181-11: Standard Test Method for Consolidated Drained Triaxial Compression Test for Soils[13]
  • ASTM D4767-11 (2011): Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils[14]
  • ASTM D2850-03a (2007): Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils[15]
  • BS 1377-8:1990 Part 8: Shear strength tests (effective stress)Triaxial Compression Test[16]
  • ISO/TS 17892-8:2004 Geotechnical investigation and testing—Laboratory testing of soil—Part 8: Unconsolidated undrained triaxial test[17]
  • ISO/TS 17892-9:2004 Geotechnical investigation and testing—Laboratory testing of soil—Part 9: Consolidated triaxial compression tests on water-saturated soils[18]

References

  1. ^ Bardet, J.-P. (1997). Experimental Soil Mechanics. Prentice Hall. ISBN 978-0-13-374935-9.
  2. ^ Head, K.H. (1998). Effective Stress Tests, Volume 3, Manual of Soil Laboratory Testing (2nd ed.). John Wiley & Sons. ISBN 978-0-471-97795-7.
  3. ^ Holtz, R.D.; Kovacs, W.D. (1981). An Introduction to Geotechnical Engineering. Prentice-Hall, Inc. ISBN 0-13-484394-0.
  4. ^ Price, D.G. (2009). De Freitas, M.H. (ed.). Engineering Geology: Principles and Practice. Springer. p. 450. ISBN 978-3-540-29249-4.
  5. ^ Goodman, R.E. (1989). Introduction to Rock Mechanics. Wiley; 2 edition. p. 576. ISBN 978-0-471-81200-5.
  6. ^ Reddy, K.R.; Saxena, S.K.; Budiman, J.S. (June 1992). "Development of A True Triaxial Testing Apparatus" (PDF). Geotechnical Testing Journal. ASTM. 15 (2): 89–105. doi:10.1520/GTJ10231J.
  7. ^ ROWE, P W, Barden, L, "IMPORTANCE OF FREE ENDS IN TRIAXIAL TESTING" Journal of Soil Mechanics & Foundations, Volume: 90
  8. ^ "New Oedometer and New Triaxial Apparatus for Firm Soil" Archived 2017-06-07 at the Wayback Machine
  9. ^ Vardoulakis, I. (1979). "Bifurcation analysis of the triaxial test on sand samples". Acta Mechanica. 32 (1–3): 35–54. doi:10.1007/BF01176132. S2CID 124243347.
  10. ^ Ibsen, L.B. (1994). "The stable state in cyclic triaxial testing on sand". Soil Dynamics and Earthquake Engineering. 13: 63–72. doi:10.1016/0267-7261(94)90042-6.
  11. ^ vbn.aau.dk[full citation needed]
  12. ^ onepetro.org[full citation needed]
  13. ^ ASTM D7181 (2011). Standard Test Method for Consolidated Drained Triaxial Compression Test for Soils). ASTM International, West Conshohocken, PA, 2003.{{cite book}}: CS1 maint: numeric names: authors list (link)
  14. ^ ASTM D4767-11 (2011). Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils. ASTM International, West Conshohocken, PA, 2003. doi:10.1520/D4767-11.{{cite book}}: CS1 maint: numeric names: authors list (link)
  15. ^ ASTM D2850 - 03a (2007). Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils. ASTM International, West Conshohocken, PA, 2003. doi:10.1520/D2850-03AR07.{{cite book}}: CS1 maint: numeric names: authors list (link)
  16. ^ BS 1377-1 (1990). Methods of test for soils for civil engineering purposes. General requirements and sample preparation. BSI. ISBN 0-580-17692-4.{{cite book}}: CS1 maint: numeric names: authors list (link)
  17. ^ ISO/TS 17892-8:2004 (2007). Geotechnical investigation and testing - Laboratory testing of soil - Part 8: Unconsolidated undrained triaxial test. International Organization for Standardization. p. 24.{{cite book}}: CS1 maint: numeric names: authors list (link)
  18. ^ ISO/TS 17892-9:2004 (2007). Geotechnical investigation and testing -- Laboratory testing of soil -- Part 9: Consolidated triaxial compression tests on water-saturated soils. International Organization for Standardization. p. 30.{{cite book}}: CS1 maint: numeric names: authors list (link)

See also

This page was last edited on 25 December 2022, at 17:47
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