Description

This session is based on a transport infrastructure project in Singapore involving the upgrade of an existing road network to improve traffic capacity and connectivity. The case study focuses on a typical underpass structure that crosses beneath an existing at-grade expressway, with an adjoining depressed road section. The underpass sits below existing ground level, with an excavation depth of up to 2m and a minimum inner vertical clearance of 5.6m, and is designed to account for uplift forces from groundwater pressure. The design combines bottom-up construction for the depressed road with top-down construction for the underpass, retained by an Earth Retaining and Stabilizing System (ERSS) of Secant Bored Pile (SBP) walls that serve as both temporary and permanent structures.

Using MIDAS CIVIL NX, a typical underpass cross-section is modelled as a 2D frame, with the SBP wall represented as pile springs and the surrounding soil as compression-only springs. Beam end releases are applied at the base slab-to-SBP wall connection to represent the moment release arising from drill-in bar constraints at that interface. Load combinations follow the LTA Civil Design Criteria (LTA CDC, Chapter 8) and are imported directly between models using the MCT Command Shell, rather than being recreated manually for each project. For vehicle collision loading on the underpass walls, a plate model with the same pile and soil springs applied is used instead of the frame model, to capture two-dimensional load distribution and local stress concentration around the impact location. Results — including axial force, bending moment, and shear force from the frame elements, and stress distribution from the plate elements — are extracted at planned node locations and compiled into a Word-based design report using the Dynamic Report Generator