Optimal selection of a metro tunnel excavation method using AHP and TOPSIS: a case study of Tehran metro

Urban tunnel construction often faces challenges due to weak ground conditions or shallow overburden, making effective stabilization of surrounding soil critical. Selecting an appropriate excavation method is essential for controlling ground subsidence and convergence, which are key considerations in tunnel design, especially in densely populated areas. Excavation-induced ground movements can propagate upward, potentially causing surface subsidence and structural damage. Therefore, a systematic decision-making framework is indispensable during the design phase. This study presents a comprehensive model for selecting the optimal excavation method for Phase 4 of Tehran Metro Line 3. The model evaluates a wide range of criteria, including technical, financial, managerial, social, and geo-mechanical factors. Seven candidate alternatives were considered: Open Shield (full face) method, Slurry method, NATM (New Austrian Tunneling Method), fore-poling, twin-tunnel excavation with TBM, EPB (Earth Pressure Balance) Shield method, and Open Trench method. The Analytical Hierarchy Process (AHP) was applied to derive criteria weights, and the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) was employed to rank alternatives. Both methods consistently identified Alternative E as the most suitable excavation method. The proposed framework provides a robust and systematic approach that can be applied to future metro tunneling projects, ensuring informed decision-making under complex technical and environmental constraints.