Triaxial strength testing

This test is similar to USC test but triaxial strength testing is carried out with range of different confining stress. Specimen is prepared with similar conditions that are discussed above in introduction. After that sample is mounted in rubber sleeve and then in triaxial cell designed by Hoek et al (1968) (figure 7). Further this system is placed to testing machine (figure 8). Hoek cell can take into itself core with height/diameter ratios of 2-2.5:1 [1]. There is an oil-filled space between steel walls and rubber sleeve in the cell. This oil is generated confining pressure by means of pump that capable to create a stress up to 70 MN/ . The stiffness of the testing machine is approximately 4900 kN/mm. Because of this additional deformation of rock that generates with stored energy in the machine is almost absent.

Figure 7. Triaxial cell

Figure 8. Triaxial testing machine

There are two types of techniques in that test. One consists of standard single point, other technique named multi-failure state test. In the single point technique the sample is pressured by confining stress at the selected level that further is maintained. After that axial load is applied to specimen and value of the load is the same as constant confining pressure. Then axial load is increased to sample failure at maximum strength.

In the multi-failure test it can possible to obtain several stress points of the failure from one core specimen. The testing procedure includes the serial use of the axial load and confining stress until the latter reaches a pre-selected level [1].

The results of single point technique are represented in the figures 9, 10. And relationship between axial load and axial displacement from multi-failure state test is represented in figure 11.

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