Design methodology for permanent complex structures: Secondary lining design of junctions

Mott MacDonald was appointed to undertake the design package C121 for the design of all Sprayed Concrete Lining (SCL) tunnels for the Crossrail project. All types of junctions’ design is one of the most complex SCL structures into this design package.

Tunnel sections have been designed in 2D and 3D for both primary linings and secondary linings. These designs have been carried out using a numerical modelling software (FLAC) for the temporary case, and a FEM structural software (STAAD), for the permanent case, once a calibration process between both software have been done. However, junctions present more complexity than a single tunnel section and a plain strain analysis is not enough to assess their structural behaviour. The 3D modelling methodology is the most reliable method to accurately assess the impact of a child (smaller diameter) tunnel opening in a parent tunnel (larger diameter).