The present work investigates the existing underpass of Avenida Pérez Galdós in Valencia, a central urban infrastructure built between the late 1960s and early 1970s within one of the city’s most important traffic corridors. Despite its strategic role, very limited information is available regarding its original geometry and structural configuration. The absence of original design drawings, structural calculations and as-built documentation makes the assessment of its current behaviour highly uncertain. The objective of this work is to reconstruct and interpret the geomechanical behaviour of the underpass by integrating the technical information currently available with the data collected during recent investigation campaigns.