## 6 Discussion: Consequences of Results Obtained  
At the end of this paper, one should reflect on the consequences of the above results. It is well known that rock evolves from one stress state to another stress state under in situ conditions, e.g., around an excavation. The initial stress state is always under compression. Close to the excavation, where the probability of failure normally is the largest, at least one stress component decreases. In laboratory experiments, one starts from a free specimen, i.e., a specimen that is under zero stress. For all compression tests (e.g., uniaxial, triaxial, and true triaxial), one externally increases the compressive stress(es). Or, in other words, one loads the specimen until failure occurs. And these data are used to determine the failure envelope and other characteristics. So, in laboratory experiments for a basic characterisation of rock, the stress paths are different from the in situ stress paths. As illustrated by the discrete simulations for a RVE model, the followed stress paths had a non-negligeable effect on the failure envelope or strength. The reason for this was the different micro-fracturing over the entire period of the stress paths. The question remains as to how this difference affects the calculations and design applications. First, if one would assume the rock to be a perfect continuum, i.e., without any micro-fissures and no occurrence of such features during the stress variation, although we know that this is a theoretical (and incorrect) model, we still apply it frequently. If we conduct linear elastic or perfect elasto-plastic calculations for such a continuum model, the results presented above indicate a systematic error. Let us consider a simple set-up, i.e., the in situ stresses are isotropic, a circular opening is excavated or drilled, and no support is present. In this case, the major principal stress, i.e., the tangential stress, is twice the initial stress, and the minor principal stress, i.e., the radial stress, is equal to zero. For the medium-to-strong black box rock (Figure 5), the conclusion is that, for in situ stresses smaller than 39.7 MPa (half the uniaxial strength of 79.4 MPa; Table 1 (a)), no failure would occur around the circular opening, i.e., when the rock is characterised by a classic loading experiment. However, if one were to characterise the rock by a combination of simultaneous loading and unloading, i.e., stress path S1(loa),S3(unl), the conclusion would be that the surrounding rock has already failed at an in situ stress of about 15 MPa. The intersect of the failure envelope S1(loa),S3(unl) with the vertical axis was situated at about 30 MPa (Figure 5). The difference in failure prediction was large, i.e., an isotropic, in situ stress of about 40 MPa vs. 15 MPa. Of course, this information must be compared to other uncertainties that are present in rock mechanical studies. However, the difference with other uncertainties is that the effect of the stress path seems to be a systematic overestimation of the strength in classic laboratory experiments, while the other uncertainties are linked to spatial variations and the heterogeneous nature of the rock material. Sometimes the latter results in larger values, and sometimes it results in smaller values. In other words, they are situated around an average. And, of course, one could criticise the choice of a continuum model and linear elastic or perfect elasto-plastic calculations, but that is not the point in this discussion. For various reasons, especially the simplicity of the calculations, continuum and elastic models are still used extensively. However, more and more discontinuum models are being used. They show that incorporating flaws or weaknesses in the model has an important effect on when fractures are induced around a circular opening, both at the laboratory scale (e.g., [38; 48]) and in situ (e.g., [58]).