Two 6.4m diameter Robbins single shield TBMs have completed the 550m twin tunnels underneath Dulles International Airport, Virginia, US reported the manufacturer in November.

The tunnels are part of a new train system at the airport on which a number of different excavation techniques have been used. These include cut and cover, TBM, NATM and roadheader. The Robbins TBMs were used to construct tunnels as part of Phase 1 of the project. When all phases are completed, the system will form a 5km underground circle linking several airport concourses. The US$1.2bn Airport Train System (ATS) aims to eliminate the current transportation system that uses rubber tyred vehicles, which add to airport congestion.

Each TBM bored two drives, 460m and 100m long respectively. The geology was mudstone, sandstone and siltstone with UCS values ranging from 32MPa to 48MPa. A precast segmental lining was used to support the tunnel. Both TBMs had to enter sharp turns with radii of 125m as they approached the main airport terminal.

The machines were refurbished, updated and modified single shield TBMs prepared specifically for the Phase 1 tunnels. This included a new back-up system, thrust controls, segment erector and modifications to the cutterhead.

The contractor JV of Atkinson/Clark/Shea selected Robbins to supply the TBMs. The owner had stipulated the use of single shield TBMs due to the short tunnel lengths, tunnel lining requirements and the need to provide immediate grouting at the tail shield to limit settlement.

Approximately 1,100m of ATS tunnel is being excavated by NATM, while 2,300m of tunnel is being constructed by cut and cover in areas close to existing facilities as well as in areas where above ground excavation will not disrupt airport activities.