Geological Survey of Iran Scientific Quarterly Journal of Geosciences 1023-7429 36 2 2026 06 22 Stability analysis and support system design for the lower headrace tunnel bifurcation at the Rudbar pumped storage power plant Stability analysis and support system design for the lower headrace tunnel bifurcation at the Rudbar pumped storage power plant 123 148 243525 10.22071/gsj.2026.537222.2213 FA Masoud Heidari Department of Engineering Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, Iran Mohammad Hossein Jowkar Department of Engineering Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, Iran Mashallah Khamehchiyan Department of Engineering Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, Iran Mohsen Sharifi Boroujerdi Mahab Ghods Consulting Engineers Company, Tehran, Iran Journal Article 2025 07 26 Pumped storage power plants play a crucial role in balancing electricity supply and demand during peak periods. Given the structural complexity of underground facilities and the potential risks of design oversights, this study evaluates the structural stability of the lower headrace tunnel bifurcation at the Rudbar Pumped Storage Power Plant in Lorestan, Iran. A multi-method approach was employed, integrating empirical classification systems (Q, RMR, RMi, GSI, and Protodiakonov coefficient) with 3D numerical modeling using FLAC3D software. Initial results indicated that the underground opening is marginally stable with a low factor of safety (FOS), necessitating the installation of a support system. Upon implementing the designed support system comprising 10 cm of reinforced shotcrete (steel grid) and 2 meter long fully grouted anchors in a 2×2 m pattern the internal forces were analyzed. The efficiency of the system was verified using capacity diagrams and Sakurai’s hazard level criteria, ensuring an FOS of over 1.5. Furthermore, the Ground Reaction Curve (GRC) and Longitudinal Deformation Profile (LDP) confirmed the elastoplastic behavior of the rock mass. Based on these analyses, an optimal excavation step of 2 meters (full-face) is recommended for ensuring the long-term stability of the tunnel bifurcation. Pumped storage power plants play a crucial role in balancing electricity supply and demand during peak periods. Given the structural complexity of underground facilities and the potential risks of design oversights, this study evaluates the structural stability of the lower headrace tunnel bifurcation at the Rudbar Pumped Storage Power Plant in Lorestan, Iran. A multi-method approach was employed, integrating empirical classification systems (Q, RMR, RMi, GSI, and Protodiakonov coefficient) with 3D numerical modeling using FLAC3D software. Initial results indicated that the underground opening is marginally stable with a low factor of safety (FOS), necessitating the installation of a support system. Upon implementing the designed support system comprising 10 cm of reinforced shotcrete (steel grid) and 2 meter long fully grouted anchors in a 2×2 m pattern the internal forces were analyzed. The efficiency of the system was verified using capacity diagrams and Sakurai’s hazard level criteria, ensuring an FOS of over 1.5. Furthermore, the Ground Reaction Curve (GRC) and Longitudinal Deformation Profile (LDP) confirmed the elastoplastic behavior of the rock mass. Based on these analyses, an optimal excavation step of 2 meters (full-face) is recommended for ensuring the long-term stability of the tunnel bifurcation. http://www.gsjournal.ir/article_243525_907a1f3d7f1401c79c072507cc3d9919.pdf