Underwater road tunnels have become critical infrastructure in Norway and, more recently, in the Faroe Islands. Initially excavated to bypass fjords, they later developed to replace ferry services across longer marine straits. The 24 km-long Suðuroy Tunnel will be the longest underwater fixed link in the archipelago and among the longest worldwide. Due to its exceptional length and prolonged poorly lit driving conditions, special safety measures are planned, including two large equal-design caverns functioning as U-turn roundabouts for emergency vehicle evacuation. This study examined the feasibility of excavating these underground widenings within the geological and geotechnical context of the site, primarily composed of sub-horizontal basaltic lava flow cores interbedded with lava breccias and volcaniclastic sandstones. To address knowledge gaps in local lithostratigraphy, rock strength, deformability, and in-situ stress conditions, geological and geomechanical characterisation was carried out using desk studies and field mapping. Preliminary cavern geometries were defined for numerical modelling, with two design solutions emerging: one with a central pillar and one full-span cavern without internal supports.

Continuum-based simulations explored various cavern sizes. Within basaltic flow core units, the stress regime was mostly moderate, with maximum principal stress less than half the uniaxial compressive strength. Tensile stresses reached 80–85% of the rock’s tensile strength, and displacements remained under 4.5 mm, suggesting cavern excavation feasibility. The ratio of horizontal to vertical in-situ stress had a greater influence on stability than rock mass parameters such as Young’s modulus, Poisson’s ratio, or compressive strength. Caverns in both planned positions along the alignment showed similar stress and displacement responses and comparable geomechanical behaviour across the two sites. Simulations involving lava breccia and volcaniclastic sandstone indicated higher instability risks: breccia exhibited potential for spalling and damage accumulation, while sandstone approached the onset of failure. Based on modelling results, two optimal cavern configurations were proposed: (i) a 20-m outer radius with a 4-m central pillar, and (ii) an 18-m radius full-span excavation without support.

This study advances understanding of large-scale underground widenings designed for road roundabouts. It also highlights the need for more site-specific data on stress fields and rock mass strength for the current project, as the models primarily relied on desk-based estimates. Complementary approaches, such as combining continuum modelling with discrete element methods and incorporating structural reinforcement, would provide further insight into brittle failure mechanisms and help evaluate the full feasibility and improvement of such cavern excavations.