## 3 Parameter Calibration
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(E_{1}\) (MPa) & \(E_{2}\) (MPa) & \(\
u_{12}\) (-) & \(\
u_{23}\) (-) & \(G_{12}\) & \(f_{\mathrm{cu}}^{(1)}\) & \(m_{0}^{*}\) (-) & \(e\) (-) \\
45,000 & 39,000 & 0.254 & 0.165 & 17,000 & 63.91 & 19.98 & 0.51 \\ \hline \(m_{g1}^{*}\) (-) & \(f_{\mathrm{cy}}^{(1)}\) & \(A_{\mathrm{h}}\) (-) & \(B_{\mathrm{h}}\) (-) & \(C_{\mathrm{h}}\) (-) & \(D_{\mathrm{h}}\) (-) & \(G_{\mathrm{h}}\) (-) & \(H_{\mathrm{h}}\) (-) \\
6.77 & 25.5 & 9.5\(\times 10^{-3}\) & 1.0\(\times 10^{-3}\) & 8.817 & 1.0\(\times 10^{-6}\) & 0.0 & 1.529 \\ \hline \(\gamma\) (-) & \(\delta\) (-) & \(A_{\mathrm{s}}\) (-) & \(B_{\mathrm{s}}\) (-) & \(\varepsilon_{\mathrm{f}}^{(1)}\) (-) & \(l\) (mm) & GSI (-) & D (-) \\
1.529 & 1.201 & 20 & 1 & 3.4\(\times 10^{-4}\) & 2.5 & 100 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material parameters of the TI-RDP model for intact _Innsbruck quartz-phyllite_. Figure 4: Illustration of the geometry of a specimen together with the orientation of the foliation planes \(\beta\) and the boundary conditions (prescribed top displacement \(u_{3}\); confining pressure \(\sigma_{0}\)). Figure 5 shows the computed maximum deviatoric stress \(\sigma_{\text{dev}}=\sigma_{33}-\sigma_{0}\) in terms of the confining pressure \(\sigma_{0}\) and the inclination angle \(\beta\) of the foliation planes, compared to the respective peak values of the experimental data. From Figure 5 follows, for a particular confining pressure, a strong direction dependence of the maximum deviatoric stress. Figure 6 shows the computed deviatoric stress-strain curves of the triaxial compression tests together with the corresponding results from the experiments by [PERSON] [51]. Despite of the large scatter of the experimental results, for the considered confining pressures of \(\sigma_{0}=12.5\) MPa, \(25\) MPa and \(37.5\) MPa and inclination angles of the foliation planes \(\beta=0^{\circ}\), \(60^{\circ}\) and \(90^{\circ}\), the numerically predicted strength exhibits good agreement with the corresponding mean values of the experimentally determined strengths. Furthermore, in the top row of Figure 7, the computed distribution of the displacement magnitude is shown for an advanced stage of the simulation corresponding to a vertical top displacement of \(u_{3}=-1\) mm, already indicating the predicted failure mode for the confining pressure of \(12.5\) MPa and the respective inclination angles \(\beta\). In the bottom row of Figure 7, the corresponding observed failure mechanisms in the experiments are depicted. A comparison of the observed and computed failure modes confirms good agreement for all three inclination angles of the foliation planes. As noted from Figure 7, the experimentally obtained crack does not necessarily pass through the center of the specimen as predicted by the numerical simulations. This discrepancy in the prediction of the location of the crack is traced back to the location of the introduced artificial imperfection in the numerical model. However, investigations of [PERSON] and [PERSON] [53] concluded that the location of imperfections, e.g., weak elements, does not influence the resulting global stress-strain curves. Thus, it is assumed that the calibration of the material parameters is not affected by the eccentric crack paths. Figure 5: Computed maximum deviatoric stress \(\sigma_{\text{dev}}\) in terms of the inclination angle of the foliation planes \(\beta\) and the confining pressure \(\sigma_{0}\) together with the experimental data, reprinted with permission from Ref. [51], 2014, Blümel. Figure 6: Computed deviatoric stress-strain curves of the triaxial compression tests for confining pressures of \(\sigma_{0}=12.5\) MPa (**top**), 25 MPa (**middle**) and 37.5 MPa (**bottom**).