# Digital Twin-Driven Stability Optimization Framework for Large Underground Caverns
## 2 Digital Twin Framework for Real-Time Monitoring and Optimization of Underground Caverns
### Deformation Trigger Levels  
The digital twin framework implements a four-tier trigger system for deformation monitoring based on industry standards of monitoring and control in tunnel construction published by the International Tunnelling and Underground Space Association (ITA) and European Standard [50; 51]. The trigger levels for the construction of the underground cavern are defined under a system of Green, Amber, and Red Trigger Levels for movements during and after construction, as shown in Figure 5. The convergence trigger level in underground construction is based on predicted effects obtained from the appropriate detailed design calculations and relate to crown and wall movements and stresses and strains within support systems.  
The Clear Condition corresponds to deformation remaining below 40% of the maximum predicted value. At this level, behavior is considered stable and consistent with numerical simulations, and routine construction can proceed with monthly monitoring cycles. Construction activities proceed as planned, taking into account the design, the agreed-upon method of working, and any limitations imposed in the design documentation. Regular pre-planned reviews of excavation progress and monitoring shall be undertaken.  
The Green Trigger Level is defined as a threshold of 40-60% of the maximum predicted deformation. Upon breaching this level, construction may continue but with biweekly monitoring to track any deviations more closely. Additionally, the adequacy of the support system should be verified against DT model checks to ensure consistency with projected structural behavior. A review of measurements and trends should also be undertaken to confirm stability.  
The Amber Trigger Level is a threshold that aligns with a DT-based range of 60-80% of maximum predicted deformation. It serves as a limit upon which action must be taken to ensure that the allowable value is not exceeded. In this stage, daily monitoring is initiated along with a FEM reanalysis to understand structural responses better. A thorough inspection of the affected vicinity should be conducted at the earliest feasible opportunity to identify any signs of unexpected structural behavior. This assessment should include a comprehensive review to evaluate potential risks, determine the presence of imminent danger, and delineate the extent of the hazardous zone [11]. Additionally, a detailed  
Figure 5: Description of trigger levels.  
examination of installed support measures, such as shotcrete, anchors, and bolts, should be carried out to assess their integrity and effectiveness in maintaining stability. This also includes a global analysis and the evaluation of monitoring data to evaluate the status, trends, and causes of movements and assess if further movement is likely to occur.  
The Red Trigger Level is typically triggered when deformation exceeds 80% of the predicted maximum, indicating a critical risk of structural compromise. It serves as the final threshold, above which construction activities must be suspended to safeguard both personnel and structural integrity. Emergency protocols must be executed, which may include shotcrete reinforcement, structural support installations, or site evacuation. On breaching this level--or if monitoring data suggest that the Red Trigger Level is likely to be reached--the structure must be immediately inspected, and emergency response and contingency plans should be instigated without delay. Additionally, a comprehensive risk assessment must be conducted to address any further movement or adverse trends resulting from ongoing construction activity.