In Washington DC, the underground pedestrian tunnel connecting the main terminal with concourse B at Dulles Airport opened at the end of 2004.
The 12.2m wide tunnel was designed by Dr. G. Sauer Corporation as a subconsultant for EarthTech (formerly Kaiser Engineers) with responsibility for the NATM design of the 235m long tunnel that had only 4.6m of cover. The remit also included designing the waterproofing and monitoring the cut-and-cover structures at both ends of the tunnel.
The NATM mined approach was chosen by the client, Metropolitan Washington Airports Authority following a cost comparison carried out by the Dr. G. Sauer Corporation on mined NATM versus cut-and-cover construction. The NATM method had the benefit of less disruption since the tunnel passes under two taxilanes.
Excavation was expected to be handled solely by roadheaders or rippers, but competent rock was detected in the form of moderately jointed siltstone. The contractual method of working was amended to allow pre-splitting with explosives, following which the roadheader was able to extract the loosened rock.
