## 1 Introduction  
One of the most significant challenges in rock mechanics is understanding the mechanisms involved in the behaviour and failure of rock, both in situ and in laboratory experiments. Hence, numerous research projects focus on conducting laboratory experiments and simulating them. One of the key issues in conducting such investigations is to get a better understanding of the variability and uncertainty of rock characteristics, e.g., moduli, uniaxial compressive strength (UCS), and Brazilian tensile strength (BTS). Often a ratio of two is observed between the maximum and minimum measured values for a specific rock and for a specific test. This large variation often is linked to the heterogeneous nature of rock and the presence of micro-defects, pores, voids, flaws, layering, anisotropy, etc. (e.g., [1, 2, 3, 4, 5]). The strength of rock is a non-intrinsic property, which means that its value is influenced (1) by the geometric characteristics of the specimen (e.g., its volume, shape, and orientation), (2) by environmental aspects (e.g., humidity and temperature), (3) by the loading rate (e.g., conventional loading rates, dynamic loading, and creep), and (4) by the test set-up. The three most common tests are the Brazilian tensile tests, uniaxial or unconfined compression tests, and conventional triaxial tests. All three types of tests are conducted on cylindrical specimens. Based on the combination of these tests, the failure envelopes are determined, and the latter are used as input for design projects. Moreand more, true triaxial or poly-axial test equipment has become available to test cubic or rectangular specimens [1; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The main advantage of these true triaxial tests is that three different major principal stress components can be applied to investigate the effect of the intermediate principal stress [9; 15; 16]. The tests also allow the variation of the three principal components independent of each other as a function of time. Nearly all such experiments are started from an isotropic stress state, followed by an increase in at least one of the principal stress components (Figure 1c, red and blue arrows). The initial isotropic stress state is either a fully zero-stress state or a non-zero, isotropic stress state. The latter stress state is created by increasing equally the three principal stresses starting from a state of zero stress. No damage should be induced during the initial equal stress increase. From a practical point of view, such loading experiments are entirely logical. Cored or cut specimens are, by definition, not loaded. However, as is well known, this procedure is the opposite of what happens in situ. The rock is in equilibrium under a state of triaxial stress, and at least one stress component decreases due to the excavation, resulting in damage (e.g., micro-fracturing) and, possibly, even in the failure of the rock. The simplest example to illustrate the actual in situ stress path is by considering an in situ isotropic stress state and the excavation of a circular opening (i.e., a borehole or a tunnel). Before the excavation, the three principal stress components are equal (Figure 1a). After excavation, the radial stress, which is a principal stress component, is decreased to zero or to the supporting pressure, i.e., the pressure of the drilling fluid for a borehole or the pressure that supports the tunnel (Figure 1b). In the latter application, first, the radial stress drops to zero, after which it is increased due to the installation of active support or due to further deformation if passive support is installed. However, in both cases, the final radial stress around a tunnel is small in comparison to the initial in situ stress. The tangential principal stress component increases during excavation. Without any support, it is doubled in comparison to the initial in situ stress. When support pressure is used, its value must be subtracted from the tangential stress.