Triaxial Test Procedure

The triaxial test procedure is carried out in a container of cylindrical shape and soil sample having a length to diameter ratio is 2. The general sizes of the cylindrical soil samples is 76 mm x 38 mm and 100 mm x 50 mm. For determination of triaxial test value basically Three types of  principal stresses are applied to the soil samples, out of which two no are applied on water pressure inside the confining cell and there are equal value has been observed. The third no principal stress is basically applied by a loading system, on this loading system ram through the top of the cell and it is different to the other two principal stresses cases. A basic typical triaxial cell has been shown in below mention area.

Now the soil mask sample is placed inside a rubber sheath which is properly sealed to the top cap and bottom portion pedestal closed by rubber O-rings. For this experiment with pore pressure measurement, porous type discs are placed at the bottom portion, and sometimes it may be placed at the top of the specimen. Now Filter paper drains may be provided surrounding the outside of the soil specimen in order to speed up the consolidation process of soil. Now Pore pressure which is generated inside the soil specimen during the triaxial testing process that can be measured by means of pressure transducers parameter.

It has been observed that by experimental process the triaxial compression test consists of two stages:

  1. First stage process: In the first stage, a soil sample is setup in the triaxial cell or testing section and confining pressure is applied on it.
  2. Second stage process: In this type of test procedure, additional axial stress (also called deviator type of stress) is thoroughly applied which induces shear stresses value in the soil sample. The axial stresses value is continuously and gradually increased until the soil sample fails inside the triaxial testing instrument.

During both the stages it has been observed that, the applied stresses, axial strain value, and pore water pressure value or change in sample volume can be measured correctly.

Different Test
There are different type of test variations, and those used mostly in practice or laboratory or institution that are:

  • UU (unconsolidated undrained) test process:

In this test or experiment, cell pressure is applied without allowing any type of drainage. Then keeping cell pressure in constant value, deviator stress is increased to failure without drainage. It must be captain mind that the overall test process has been carried out as per proper technical rules and norms

  • CU (consolidated undrained) test process:

In this test experiment, drainage is allowed during cell pressure application process. Then without allowing further drainage, deviator stress value is gradually increased keeping cell pressure in constant parameter.

  • CD (consolidated drained) test process:

It has been observed that, this is almost similar to the CU(consolidated undrained) test procedure except few portions that is as deviator stress value is increased gradually, drainage is permitted thorough out the test. The rate of loading must be kept in very slow condition which is enough to ensure no excess pore water pressure develops thorough out the specimen.

In the UU(unconsolidated undrained)  test process, if pore water pressure is measured, the test is designated by UU( unconsolidated undrained) test criteria . 
In the CU(consolidated undrained) test process, if pore water pressure is measured correctly  in the second stage, the test is symbolized as CU(consolidated undrained  .

Importance of Triaxial Testing

In The first stage in the laboratory the in-situ condition that soil samples at various depths is subjected to various effective stresses or different type of stresses. And Consolidation process will occur if the pore water pressure which develops upon application of confining pressure is allowed to dissipate of soil samples. Otherwise the effective stress on the soil samples is the confining pressure (or total stress) deducting the pore water pressure which is obviously exists in the soil samples.

During the shearing process of  soil samples specimen, it experiences axial strain, and either volume  can be change or development of pore water pressure occurs systematically. The magnitude of shear stress value which is acting on different planes in the soil sample is different case to case. When at some strain the sample of soil has been fails in experiment, this limiting shear stress on the failure plane is basically known as the shear strength.

The triaxial test process has been many advantages over and access the direct shear test:

  •  The soil samples specimen are subjected to uniform stresses and strains value.
  • Different combinations of confining and axial stresses can be applied of soil specimen.
  •  Drained and undrained tests process can be carried out without any issues.
  • Pore water pressures can be measured in undrained tests process.
  •  The complete stress-strain nature of behaviour can be determined easily.