## 2 Brief Introduction to Triaxial Tests  
The procedures of triaxial tests are described in [2; 3]. According to the ISRM [2], there are three types of triaxial tests--namely Type I, Type II, and Type III--as shown in Figure 1, and their typical results are shown in Figure 2. A very brief description of the different test types is as follows:  
* Type I: An individual rock specimen is tested with a single pre-set confinement. It is required to perform at least three tests with three different confinements in order to obtain the peak strength envelope. This means that at least three samples are required to fulfil the test and construct the peak strength envelope. Typical results of one test are presented in Figures 1a and 2a;
* Type II: Three different confinement levels are applied in the same sample. At each confinement level, the sample is tested until it tangents its peak strength at that confinement level, and then the confinement is changed to the next level for the test to continue. With the three different confinement levels in one test, it is possible to construct the peak strength envelope for the sample from only one test. Typical results of one test are presented in Figures 1b and 2b;
* Type III: The test is carried out with the initial confinement until the sample tangents its peak strength. The linear portion of the axial stress versus axial strain curve has a slope of \(V\) (\(V=E\) is the Young's modulus of the sample). After the initial step, the confinement and the axial stress are simultaneously increased in such a way that the obtained axial stress versus axial strain curve has the same slope as \(V\). Typical results of one test are presented in Figures 1c and 2c.  
A more detailed description of the test types can be found in [2].  
According to the ISRM [2], with test Type I (\"individual test\"), individual points on the failure (peak strength) envelope are obtained from several tests (Figure 1a), while with test Type II (\"multiple failure state test\" as in Figure 1b) and test Type III (\"continuous failure state test\" as in Figure 1c), the strength envelope is produced with a single test employing a stepwise or continuous procedure as described above.  
From a rock mechanics point of view, the UCS of a rock sample can be estimated based on the results of the triaxial tests as follows:  
* Fitting a curve based on the results from triaxial tests. Depending on the behaviour of the rock, the fitting curve can be linear or non-linear;
* Plotting the strength envelope in a graph where the vertical axis represents the major principal stress (normally the axial stress during the triaxial test) and the horizontal axis represents minor principal stress (normally the confinement during the triaxial test);
* Extending the strength envelope until it intersects the vertical axis (at this point, the minor principal stress is zero). This intersection point can be interpreted as the UCS of the sample.  
In this paper, test Type II has been selected to be applied because (a) different confinement levels can be applied to the same rock sample so it does not require a large quantity of samples as for test Type I, and (b) it is easier to perform than test Type III when it comes to controlling the confinement that applies to the rock sample.  
Rock samples used in this study are solely granite. The reason for choosing granite for the study is that (a) granite is a relatively homogenous rock, (b) a general database of granite is available internationally for comparison, and (c) the granite that is tested (Iddefjord granite) is a reference rock type at our laboratory.