Triaxial tests
Triaxial tests are performed to calculate the shear strength of soil, both as total and effective stress, and determine the angle of friction (Φ) and its cohesion (C). Furthermore, the shear failure plane is not forced, like with the DS-test, and the stress distribution of the plane is more or less uniform. More so, the triaxial tests can be conducted on any soil type, under either drained, undrained, consolidated or unconsolidated conditions. Usually the triaxial tests are performed under isotropic conditions, meaning that the tests are consolidated under an all-round hydrostatic pressure and that for instance the earth pressure coefficient (K0) is not taken into account. When other consolidation conditions, e.g. based on K0, are required the tests are performed under anisotropic conditions.
The condolidated triaxial tests may be performed under normal consolidated (NC) conditions or under over-consolidated (OC) conditions. In case of the former the anisotropic consolidation pressure is equal to the effective or maximum overburden pressure. In case of the latter, the anisotropic consolidation pressure is commonly based on the in-situ effective stress. Furthermore, the K0 during over consolidated triaxial tests is depending on the over-consolidation ratio (OCR), that is the ratio of maximum past effective consolidation stress and the present effective overburden pressure.
The basic setup for any triaxial test is identical notwithstanding the requested conditions or soil type.
A sample (undisturbed or disturbed) is prepared to height to diameter ratio of 1:2; undisturbed samples are simply trimmed to size, whilst disturbed samples are either remoulded (cohesive soils) or re-constituted (cohesionless soils). Next the prepared sample is encased in a rubber skin and placed in a Perspex loading cell. The cell gets pressurised using water depending on the requested conditions and pressures. During the test each pressure changes, both inside and outside the specimen, are recorded until the sample shears. All recorded values are subsequently used to determine numerous parameters, e.g. shear strength and friction angles.
Geolab Wiertsema has twenty-three triaxial devices set up in a conditioned part of the laboratory in Tolbert, The Netherlands.
The unconsolidated and undrained triaxial test (UU) is the most basic triaxial test and is quite similar to a unconfined compression test (UCS), with the addition of a confining fluid pressure in a loading cell. This tests determines the undrained shear strength (cu) of the soil, e.g. to asses (short term) soil stability. The specimen is put under an all-round pressure, after which a deviator stress is generated between the vertical stress (or axial strain) and horizontal stress until shearing occurs. During this test no saturation or consolidation is performed on the sample and drainage of the specimen is not allowed. The responses of the soil during the shear stage, e.g. changes in deviator stress against axial strain, are monitored constantly until the sample has sheared and/or a specified failure criterion is reached. To calculate the test results correction on the deviator-stress are applied conform “Reduction of axial resistance due to membrane and side drains in the triaxial test” by De Greeuw et al. 2001.
Isotropic consolidated and undrained triaxial tests (CIU) are similar to the UU-test, yet during which the specimen is firstly saturated and consolidated prior to the test; the prepared specimen is saturated and then consolidated to a predefined stress, commonly the in-situ effective stress. In case of soils with a (very) low permeability, the consolidation stage may take up to a week . - The principle of the CIU test is that the specimen is, as much as possible, returned to its in-situ state from before sampling occurred. - After saturation and consolidation the test is conducted like during the UU-test: an axial strain (vertical stress) is applied, generating a deviator stress, until shearing occurs and/or a predefined failure criterion is reached.
During the CIU test the pore pressure is constantly measured as well and then used to calculate the effective stress. Based on this, the internal angle of friction (Φ) and the cohesion (C) is derived. In order to carry out the calculations corrections on the deviator-stress and a membrane correction are applied conform “Reduction of axial resistance due to membrane and side drains in the triaxial test” by De Greeuw et al. 2001.
Isotropic consolidated and drained triaxial tests (CID) are similar to the CIU-test, yet the specimen is allowed to drain any excess fluid. The shearing rate is set low enough so excess pore water pressure arises. Similar to the CIU test the specimen is saturated and then consolidated to a predefined stress, commonly the in-situ effective stress. After saturation and consolidation the test is conducted like during the UU-test: an axial strain (vertical stress) is applied, generating a deviator stress, until shearing occurs and/or a predefined failure criterion is reached. This type of triaxial test is conducted under drained conditions, that is, the specimen is allowed to drain excess water. This means that no excess pore pressure can arise. Commonly this test is performed on (high) permeable (coarse-grained) soils.
In order to carry out the calculations corrections on the deviator-stress and a membrane correction are applied conform “Reduction of axial resistance due to membrane and side drains in the triaxial test” by De Greeuw et al. 2001.
Isotropic and anisotropic consolidated, undrained triaxial tests (CIAU) constitute of twice consolidated specimens, which are firstly consolidated under isotropic conditions followed by a second consolidation under anisotropic conditions. During the anisotropic consolidation an additional vertical stress is applied under constant load, with a specific rate to prevent untimely shearing. The additional vertical stress is based on the earth pressure coefficient (K0).
After the anisotropic consolidation results in a minimal volume change of the specimen, the shearing phase is begun until shearing occurs.
With the two consecutive consolidation stage the specimen is brought back to its original in-situ state, or very close to it, resulting in a more accurate simulation of the soil conditions.
In order to carry out the calculations corrections on the deviator-stress and a membrane correction are applied conform “Reduction of axial resistance due to membrane and side drains in the triaxial test” by De Greeuw et al. 2001.
