Triaxial shear test

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]

Triaxial versus True triaxial test

Although the name triaxial test suggests that the stresses would be different in three directions, this is not true in the test as is usually done. In this test with oil or water as confining medium, the confining pressures are equal in all directions (i.e. in terms of principal stresses: for a compression test: σ1 ≠ σ2 = σ3 and for tensile: σ1 = σ2 ≠ σ3). Only in a true triaxial test the stresses in all three directions can be different (i.e. σ1 ≠ σ2 ≠ σ3).

Test Execution

Loose granular materials

For loose granular materials like sand or gravel, the material 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 water (mostly water but may be any other 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.

Cohesive (non-loose) materials

The test for cohesive (non-loose) materials (e.g. clay, rock) is similar to the test for loose granular materials.But takes more time as compared to loose materials(ed. sand).

Triaxial test on rock

For rock testing the sleeve may be a thin metal sheeting rather than latex. Triaxial testing on rock is fairly seldom done because the high forces and pressures required to break a rock sample imply very costly and cumbersome testing equipment available at few laboratories in the world.

Fluid and gas pore pressure measurement

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.

Basic concept

The principle behind a triaxial shear test is that the stress applied in the vertical direction (along the axis of the cylindrical sample) can be different from the stresses applied in the horizontal directions perpendicular to the sides of the cylinder, i.e. the confining pressure. In a homogeneous and isotropic material this produces a non-hydrostatic stress state, with shear stress that may lead to failure of the sample in shear. In homogeneous and anisotropic samples (e.g. bedded or jointed samples) failure may occur due to bending moments and, hence, failure may be tensile. Also combinations of bending and shear failure may happen in inhomogeneous and anisotropic material.

A solid is defined as a material that can support shear stress without moving. However, every solid has an upper limit to how much shear stress it can support. The triaxial test is designed to measure that limit. The stress on the platens is increased until the material in the cylinder fails and forms sliding regions within itself, known as shear bands. A motion where a material is deformed under shear stress is known as shearing. 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 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.

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.

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 (see above) 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 results in drainage. The rate of axial deformation is kept constant, i.e. is strain 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.

Unconsolidated Undrained (UU)

In an unconsolidated undrained test the sample is not allowed to drain. The sample is compressed at a constant rate (strain-controlled).

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.

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 3-540-29249-7.
  5. ↑ Goodman, R.E. (1989). Introduction to Rock Mechanics. Wiley; 2 edition. p. 576. ISBN 978-0-471-81200-5.
  6. ↑ ASTM WK3821 (2011). New Test Method for Consolidated Drained Triaxial Compression Test for Soils (under development). http://www.astm.org'' (ASTM International, West Conshohocken, PA, 2003).
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ BS 1377-1 (1990). Methods of test for soils for civil engineering purposes. General requirements and sample preparation. BSI. ISBN 0-580-17692-4.
  10. ↑ 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.
  11. ↑ 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.

See also