## 5 Engineering Application  
Auxiliary workings for the deepening of Shaft II, including the hoisting machine chamber and the technological shaft inset, were excavated between 2023 and 2024. Both the hoisting machine chamber and the technological shaft inset were excavated using the pilot drift expansion method, as depicted in numerical models.  
The overall construction sequence of the pilot drift expansion method is as follows: First, a small pilot drift is excavated and temporarily supported. In the next phase, the main excavation is expanded to its final dimensions (in Polish coal mining a drill and blast method is typically used) and supported with primary support. Finally, after reaching an equilibrium state between the primary support and an adjacent rock mass, secondary support is installed to ensure the long-term stability of the structure.  
This method is particularly advantageous in weak or medium-quality rock masses, as it allows for controlled excavation while minimizing the risk of instability. By first creating a smaller pilot tunnel, stress redistribution within the rock mass is facilitated, reducing excessive deformations and ensuring safer working conditions. The pilot drift expansion method is widely used in underground mining and civil engineering projects, especially for large-section chambers and tunnels requiring careful ground control [41; 54]. This approach enhances excavation safety and stability, making it a preferred technique in challenging geological conditions.  
Figure 8: Support capacity plots of the secondary support of the technological shaft inset ((**a**)—M-N plot for shotcrete; (**b**)—Q-N plot for shotcrete; (**c**)—M-N plot for steel sets; (**d**)—Q-N plot for steel sets).  
Geotechnical monitoring of the considered excavations was carried out through visual inspections, which included regular observations of the primary and secondary support, sliding in the yielding joints, as well as the state of the excavation face and adjacent rock mass, particularly block formation and joint propagation. Throughout the entire excavation period up to the present date, no critical conditions that could lead to ground failure or potential excavation collapse have been observed. Overall, the primary and secondary support systems, validated using analytical and numerical design methods, have proven effective in maintaining excavation stability. The results confirm the reliability of the adopted design methodology and highlight the effectiveness of the applied support solutions under the given geological conditions.  
The actual view of the hoisting machine chamber, supported with primary and secondary support, is shown in Figure 9.  
Direct validation of the finite element model using in situ monitoring data (e.g., convergence measurements) was not feasible due to practical constraints. Both numerical models discussed in this study were developed in accordance with established engineering practices and calibrated against the analytical solutions presented in Section 4, which indirectly supports their predictive capability. However, the model's assumption of isotropic and homogeneous material behavior may not fully capture local geological variations or anisotropic effects. In the absence of direct field measurements, certain aspects of stress distribution and rock mass displacement remain unverified. Future studies should incorporate detailed monitoring data to enhance the validation and refinement of numerical predictions.