How do the costs of NATM and Cut & Cover Tunnels compare?

A detailed cost comparison between Cut-and-Cover and NATM construction methods has been carried out for a pedestrian tunnel at Dulles International Airport in Virginia (USA).  This tunnel, which serves as an underground corridor for pedestrians between two of the airport terminals, is about 235 m long.  The NATM option has a cross sectional area of 85 m˛ which is comparable to a typical subway station cross section.  The crown of the tunnel is located 4.7 m, and the bottom of the invert 13.1 m below the ground surface. A significant portion of the tunnel’s crown is located in a mixed face (i.e. soil and rock interface).  Also several existing utility lines traverse the alignment of the tunnel, like fuel, storm sewer and fibre optic lines. The construction time was estimated to be about 50 weeks.

Based on the preliminary engineering the cut-and-cover version was envisioned as an open-cut construction with typical support of excavation consisting of shotcrete with tie-backs, soldier pile and lagging.  The cross section was 10.8 m wide and 5.9 m high with a minimal overburden.

The comparable construction costs (NATM was estimated to be 25% cheaper) and shorter construction time and the substantial advantage of low surface impact by the NATM alternative convinced the owner to proceed with the project as a mined tunnel.