This study presents the design, analysis, and implementation of a tunnel portal extension at the Bolu Tunnel, located on the Istanbul side of the Trans-European Motorway (TEM), in response to recurring mudflows and landslides in the Asarsuyu region. The project aimed to improve traffic safety and infrastructure resilience by constructing buried multiplate corrugated steel structures as a rapid and modular solution. Two tunnel extensions were constructed: Structure #2 (90.15 m) and Structure #1 (66.85 m). They both incorporated backfill heights up to 5 m to resist potential extreme mudflow scenarios. A finite element model was developed using Plaxis 2D to simulate soil-structure interaction under various load cases, including dead load, live traffic load (KGM45), and surcharge from accumulated mud. The soil layers were modeled based on geotechnical investigations, utilizing a Hardening Soil model for fill and embankment materials and Mohr-Coulomb parameters for natural rock. Steel structures were modeled as elastic plates, with interface elements introduced to improve realism at soil-structure boundaries. Finite element analysis results revealed maximum displacements of 0.068 m under critical loading conditions, and the structural capacity utilization remained within safe limits: 50% for plates and 33% for bolts. Section checks based on AASHTO LRFD 2020 confirmed structural adequacy against wall resistance, global buckling, and combined load effects. Field construction was completed efficiently—Structure #2 in 14 days and Structure #1 in 10 days—without any safety incidents. Post-installation, both structures were backfilled with engineered material. The findings demonstrate that the corrugated steel extension design offers a fast, cost-effective, and structurally sound solution for mitigating geohazards such as mudflows in high-risk mountainous regions, ensuring the long-term safety and serviceability of critical highway infrastructure.