## LESSON 12. Tri-Axial Test

12.1. Various Types of Tri-axial Tests Fig. 12.1. Stages of Tri-axial tests. Fig. 12.2. Tri-axial tests test setup

The usual sizes of the samples are: 76 mm (length) x 38 mm (diameter) or 100 mm (length) x 50 mm (diameter). Thus, the length/diameter ratio of the cylindrical sample is 2. This test is suitable for both sand and clay. Depending on whether drainage is allowed or not during all round cell pressure and shearing, three types of triaxial tests are conducted:

(i) Unconsolidated Undrained (UU) test

(ii) Consolidated Undrained (CU) test

(iii) Consolidated Drained (CD) test

Unconsolidated Undrained (UU) test

In this type of test, pore pressure is developed during shearing. However, the pore water pressure is not measured. Thus, effective stress value is unknown. The parameters are determined in terms of total stress (cu). It is a very quick test. The determined parameters are used for the analysis under undrained condition such as short term stability.

Consolidated Undrained (CU) test

In this type of test, pore pressure is developed during shearing and it is also measured. Thus, effective stress value is known. The parameters are determined in terms of effective stress (c' and Ø' ). It is faster than CD test, but slower than the UU tests. This test is preferred to determine c' and Ø'.

Consolidated Drained (CD) test

In this type of test, no excess pore pressure is developed during the whole test. Shearing is done very slowly to avoid build-up the excess pore water pressure. The parameters are determined in terms of effective stress (c' and Ø'). It is a very slow test (can take few days). The determined parameters are used for the analysis under fully drained condition such as long term stability.

In general, during the test, deviator stress (σ1 - σ3), all round pressure (σ3) and pore water pressure is measured (for example in CU test). From the test data, (σ1)f value at failure is determined for different cell pressure (σ3). After subtracting the pore water pressure, (σ1)'f  and σ'3  are determined. Mohr-Circles are drawn for different (σ1)'f  and σ'3 values. Failure envelop is drawn by drawing a common tangential line for all the circles (as shown in Figure 12.3). From that line, strength parameters c' and Ø are determined. Figure 12.4 shows typical deviator stress-axial strain and volume change-axial strain plot of soils obtained from CD trai-axial tests. Fig. 12.3. Shear stress-normal stress plot. Fig.12.4. Deviator stress-axial strain and volume change-axial strain plot of

soils obtained from CD t Tri-axial tests.

References

Ranjan, G. and Rao, A.S.R. (2000). Basic and Applied Soil Mechanics. New Age International Publisher, New Delhi, India

PPT of Professor N. Sivakugan, JCU, Australia