Triaxial test is used for determination of shear characteristics of all types of soil under different drainage conditions. In this test, a cylindrical specimen usually of length to diameter ratio of 2 (i.e. 76×38 mm and 50×100 mm) is stressed under conditions of axial symmetry.
This test is carried out in two stages :
1st Stage – Consolidation Stage
2nd Stage – Shear Stage
In 1st stage the specimen is subjected to an all round confining pressure (σc) on the sides and at the top and bottom. This stage is known as consolidation stage.
In 2nd stage the specimen is subjected to additional axial stress known as deviator stress (σd) is applied on the top of the specimen through a ram. When the axial stress is increased the shear stresses develop on inclined planes due to compressive stresses.
The total stress in the axial direction at the time of shearing is, σ1 = σc + σd.
Minor Principle Stress = σc = σ3
Major Principle Stress = σ1 = σc + σd
Note :
- The vertical sides of the specimen are principal planes, as there are no shear stresses on the sides.
- Because of axial symmetry, the intermediate principle stress (σ2) is also equal to the confining pressure (σc).
Measurements made in Triaxial Test :
- During Triaxial Test, the axial strain is determined by measuring the change in length of the specimen, using either a dial gauge or a displacement transducer.
- The Pore Water Pressure can be measured by a pore pressure measuring apparatus connected to the pressure line of the triaxial cell after closing the valve in the drainage line.
- If the development of pore water pressure is not to be allowed (as in drained test), the change in volume of specimen can be determined by allowing the flow of water from soil through the drainage line (keeping pore water line closed) into a burette attached to this line. The volume of water collected in burette is measured.
Change in Volume of specimen = Total Volume of specimen – Volume of water collected in burette
Properties of Triaxial Test :
- Pore water pressure can be measured accurately.
- Volume changes are measured.
- There is no rotation of principal stresses during test.
- The failure plane is not forced.
- The specimen can fail on any weak plane or can simply bulge.
- The stress distribution on the failure plane is fairly uniform.
- Drainage conditions can be controlled, whatever may be the type of soil.
Drainage Conditions in Triaxial Test :
| Type of Test | Stage – I (Application of Cell Pressure) | Stage – II (Application of Additional Axial Stress) | Type of Analysis | Test Duration |
| Unconsolidated Undrained (UU) | Unconsolidated (drainage closed) | Undrained (drainage closed) | Total Shear Stress Parameters i.e. c & φ | About 15 min |
| Consolidated Undrained (CU) | Consolidated (drainage allowed) | Undrained (drainage closed) | Total & Effective Shear Stress Parameters i.e. c & φ and c’ & φ’ respectively | 24 hrs for Stage – I and 2 hrs for Stage – II |
| Consolidated Drained (CD) | Consolidated (drainage allowed) | Drained (drainage allowed) | Effective Shear Stress Parameters i.e. c’ & φ’ | 10 – 15 days |
Merits and Demerits of Triaxial Test :
Merits
- There is complete control on drainage conditions irrespective of type of soil.
- Pore pressure and Volumetric changes can be measured.
- Stress distribution on failure plane is uniform.
- The specimen is free to fail on weakest plane.
- The state of stress at all intermediate stages upto failure is known.
- The test is suitable for accurate research work.
Demerits
- The apparatus is elaborative, costly and bulky.
- The drained test takes a longer period as compared with that in a direct shear test.
- The strain condition in the specimen are not uniform due to frictional restraint produced by the loading cap and the pedestal disc.
- It is not possible to determine the cross-sectional area of the specimen accurately at large strains.
- The test simulates only axis-symmetrical problems.
- The consolidation of the specimen in the test is isotropic whereas in the field the consolidation is generally anisotropic.
