The excavations of portal sections are always critical steps during the tunnel construction process, as previously excavated portal slopes can be influenced by ongoing tunnel excavations. Possible deformations that may appear in tunnels during excavations can affect the stability of portal slopes, causing the slopes to deteriorate. In addition, potential failures that may occur at portal slopes may adversely affect the tunnel support systems. Therefore, there is an obvious direct relationship between the stability of the tunnel and that of its portal slopes. In this study, the effects of portal slope failure at the entrance portal of the Geminbeli Tunnel (within the scope of the Zara-Geminbeli-Susehri State Highway Project in Turkey) and the appropriate solution methods for this failure are explained and discussed. For the purpose of this study, the geological and geotechnical data of the failure zone are interpreted, and the necessary numerical analyses are performed to understand the performance of the proposed engineering measures. Additionally, the performances of the recommended remedial measures and redesigned support systems are discussed.