his master’s thesis investigates the design and structural behavior of Steel-Concrete-Steel (SCS) composite immersed tunnels, addressing challenges posed by traditional reinforced concrete designs. These challenges - limitations in weight, constructability, and long-term durability, particularly in deep-water environments - are exacerbated by the increasing demand for larger capacity tunnels and the need for efficient, sustainable construction methods. This research explores the impact of construction-induced irregularities, such as interface gaps and shear connector deformation, on the load-carrying capacity and overall performance of SCS tunnels. The study focuses on how irregularities produced during different construction phases affect both the local and global structural behavior of the composite cross-section. The research approach involves four distinct stages. First, a thorough examination of existing literature provides a foundation of knowledge regarding immersed composite tunnels, encompassing material properties, construction stages, and manufacturing processes. Second, a detailed SCS composite immersed tunnel design is developed, incorporating findings from a reinforced concrete tunnel case study to inform geometry and material selection. Third, numerical analysis using DIANA software is conducted. This includes 2D finite element modeling (FEM) of an SCS composite beam and a 2D FEM of the complete SCS cross-section to investigate the interaction between steel plates and concrete core under various interface conditions (strong bond versus slip) and imperfections (gaps, reduced shear connector effectiveness). Both linear elastic and nonlinear material behavior are considered. The findings highlight the critical role of horizontal interface connections and the nonlinear behavior of concrete in determining the overall load-carrying capacity and stability of SCS tunnels. The primary load bearing mechanism identified is the compressive strut within the concrete core; however, the study reveals that the structure’s failure is consistently initiated by the failure of this compressive strut, due to crushing under load. This failure mode underscores the limitations of traditional composite theory design methods and highlights the importance of the strut - and - tie model for more accurate design approaches. Construction imperfections significantly impact structural performance: strong interface connections are crucial for effective load transfer, while webs (vertical steel plates) enhance stability. The research concludes with recommendations to improve design practices, emphasizing the importance of strong horizontal connections and careful construction to maintain compressive strut integrity and avoid premature failure.