Triaxial Shear Test

The triaxial test is a laboratory method used to determine the strength and deformation properties of soil. The test is performed by placing a cylindrical soil sample inside a triaxial cell and applying confining pressure and axial loads to the soil. The resulting axial deformation and pore water pressure are then measured. There are four main types of triaxial tests, each with a different set of conditions for applying confining pressure and axial loads:

1. Consolidated Drained (CD) Triaxial Test: In this test, the soil sample is first consolidated under an isotropic confining pressure and then drained to allow any excess pore water pressure to escape. After this, an axial load is applied to the soil sample and the resulting axial deformation and pore water pressure are measured.

2. Unconsolidated Undrained (UU) Triaxial Test: In this test, the soil sample is kept at a constant water content and is not allowed to drain. An axial confining pressure is applied to the soil sample, and then an axial load is applied. The resulting axial deformation and pore water pressure are then measured.

3. Consolidated Undrained (CU) Triaxial Test: In this test, the soil sample is consolidated under an isotropic confining pressure, but the pore water pressure is not allowed to escape. An axial load is then applied to the soil sample and the resulting axial deformation and pore water pressure are measured.

4. Unconsolidated Drained (UD) Triaxial Test: In this test, the soil sample is not consolidated and is allowed to drain. An axial confining pressure is applied to the soil sample, and then an axial load is applied. The resulting axial deformation and pore water pressure are then measured.

The results of the triaxial test can be used to determine the soil’s strength, compressibility, and deformation characteristics, which are important for the design and evaluation of geotechnical structures like foundations, retaining walls, and slopes.

Unconsolidated undrained test method is the most common and most easiest test taking minimum time of all. The procedure for conducting an Unconsolidated Undrained (UU) Triaxial Test involves several steps, including:

i. Soil sample preparation: A cylindrical soil sample is obtained by cutting it from a larger soil mass and compacting it to the desired density. The sample is placed inside a triaxial cell, which is usually made of metal or plastic.

ii. Soil saturation: The soil sample is saturated with water either by submerging the cell in water or using a vacuum. This is done to ensure that the soil is fully saturated with water.

iii. Confining pressure: An isotropic confining pressure is applied to the soil sample by filling the annular space between the soil sample and the triaxial cell with water. The confining pressure helps to imitate the in-situ stress conditions.

iv. Axial loading: An axial load is applied to the top of the soil sample using a load cell. The axial load is increased incrementally until the maximum desired load is reached or until the soil reaches its failure point.

v. Deformation and pore water pressure measurement: The axial deformation of the soil sample is measured with a displacement transducer and the pore water pressure is measured using piezometers attached to the triaxial cell.

vi. Calculation of soil strength: The results of the test are used to calculate the undrained shear strength (Su) of the soil, which represents the soil’s resistance to shearing. Other strength parameters, such as peak shear strength, can also be derived from the test results.

vii. Data analysis: The results of the UU triaxial test are analyzed to determine the soil’s compressibility, strength, and deformation properties.