Pontoon-type submerged floating tunnels and floating bridges: Current research trends, engineering challenges, and future perspectives

Pontoon-type submerged floating tunnels (PSFTs) and floating bridges (FBs) are emerging alternatives to conventional crossings, particularly for deep fjords, wide spans, and soft seabeds. This review synthesises recent advances in buoyancy-based structures, covering structural concepts, historical development, and dynamic response under environmental, traffic, and accidental loads. Key challenges include high-fidelity modelling of environmental loading, e.g., nonlinear, viscous, and coupled wind-wave-current interactions and structure-specific behaviours (submerged hydrodynamics for PSFTs and aerodynamic stability for FBs). Experimental limitations persist, notably scale effects, replication of multi-hazard scenarios, and facility constraints. We highlight the need for integrated numerical-experimental approaches, robust multi-hazard assessment frameworks, improved scaling laws, and dedicated test infrastructure. Recommendations emphasise interdisciplinary collaboration, large-scale validation campaigns, and standardised design guidance to improve safety, reliability, and constructability. The review aims to provide engineers and researchers with a consolidated reference on the state of the art and to inform future innovation in cross-water transportation.