## 2 Overview of Unloading Experiments  
As mentioned earlier, the fact that unloading stress paths occur around excavations is common knowledge for all rock mechanical engineers. The most illustrative example is the discing phenomenon that occurs when cores are drilled in deep boreholes [19]. The discing or macro-fracturing of the rock is due to the relaxation of stress, and it is an extreme form of damage due to stress relaxation. [PERSON] et al. [20; 21] aimed to quantify core damage. Therefore, they manufactured artificial or synthetic rock specimens under a triaxial stress state that was followed by the unloading of one or more stress components. They compared the behaviour of synthetic rocks which were kept under simulated reservoir conditions with the simulated coring process by unloading, i.e., the simulation of the release of stress during core drilling. They observed that the latter could result in a stiffness that was two or three times less than the traditional stiffness [20]. For the unloading due to the coring, the UCS values also were systematically lower. It is clear that damage was induced during unloading, whether or not macro-fractures were observed. In [13; 22], cubic specimens of crinoidal limestone were tested in a true triaxial or poly-axial cell, and loading and unloading schemes were applied. The crinoidal limestone is composed mainly of calcite. Two stress paths were followed. First, the conventional loading was applied. The specimens were loaded until a pre-defined isotropic stress state was reached, followed by an increase in two stress components until failure occurred, and the minor principal stress remained constant. Second, for the unloading scheme, the minor principal stress was decreased from the pre-defined isotropic stress state until failure occurred. A small difference was applied between the major and intermediate principal stress, allowing a two-dimensional approach for describing the fracture patterns. It also facilitated the numerical simulations of the experiments. For some of the specimens, the loading and unloading schemes were stopped in a controlled way prior to the macro failure. Figure 2a shows an example for the unloading scheme of a micro-photograph of a petrographic thin section. Some approximate vertical micro-fractures were induced that corresponded to some of the cleavage planes of the calcite crystals, i.e., approximately perpendicular to the minor principal stress. Figure 2b shows an example of a failed specimen (again for the unloading scheme). Relatively wide fractured zones were induced, and the widths of these zones generally ranged between 0.05 and 0.2 mm. Their orientation corresponded to the overall angle of 60\({}^{\circ}\) to 70\({}^{\circ}\). In addition to the wide macro-fractures, some thin fractures were present, e.g., activated cleavage planes. Their orientation was for the failed specimen not only perpendicular to the minor principal stress, as was observed when the unloading scheme was stopped in a controlled way prior to the macro-failure. No clear difference was observed between the loading and unloading schemes in the macro-fracture pattern. Two-dimensional numerical simulations also were conducted, using the boundary element code DIGS (Discontinuity Interaction and Growth Simulation). This code allows for explicit fracture modelling [23]. The cracks are modelled as displacement discontinuities. As for the laboratory experiments, no significant effect was observed on the macro-fracturing pattern between the loading and unloading schemes. A global shear type of failure was observed in both schemes. However, the simulations showed that more tensile cracks occurred during the unloading scheme [13; 22]. In the literature, there is a large amount of test results of a single loading-unloading cycle (e.g., [24]) or of multiple loading-unloading cycles (e.g., [25; 26; 27]). Such research is interesting and worthwhile, but its aim is different from the aim of the stress paths presented in this paper.