Two 6.4-m (21-ft) diameter Robbins Main Beam TBMs were launched in winter 2009-2010 beneath Chongqing, China–a mega-city of more than 35 million people.  The two machines are excavating twin 12 km (7.5 mi) long tunnels for Line 6 of the Chongqing Metro, and are the first hard rock TBMs to be used in the city.  

Each machine is boring under sections of very low cover ranging from 10 to 60 m (33 to 197 ft) in sandstone from 12 to 50 MPa (1,700 to 7,300 psi) UCS.  The geology requires a ground support program of rock bolts, ring beams every 750 mm (2.5 ft), and wire mesh in the L1 area, as well as shotcrete in the L2 area.  

The ground support is particularly difficult given the tunnel alignment. “Installing ring beams throughout the tunnel and rock bolting while negotiating tight vertical and horizontal curves at good production rates is a challenge not to be underestimated,” said Steve Chorley, Robbins International Field Service Manager.   Several curves with radii as small as 400 m (1,300 ft) are present along the tunnel alignment.  
Specially designed back-up systems are also being used to deal with any water inflows during excavation.  The back-ups are raised 1.2 m (3.9 ft) above the invert on a steel structure, allowing room for double-track muck cars to remove muck and transport ground support materials behind the machine.   Any groundwater flows under the machine through the tunnel invert.   

Both High Performance (HP) TBMs started up from a common portal after shop assembly in Chongqing. The first machine was moved through the start chamber on a specially designed walking cradle, and began cutting rock on December 11.  The second machine was launched a month later.  As of late February 2010, the two machines had excavated 800 m and 420 m (2,600 and 1,400 ft), respectively, starting under about 20 m (65 ft) of cover.   

During tunneling, the Robbins walking cradle system allows the TBMs to be moved forward through eight cut and cover station sites.  The system minimizes stoppage time associated with attaching transportation dollies or other walking equipment, and allows the machines to instead move directly onto the cradle once entering the station.  

The tunnels are the first new lines to be excavated for the city’s metro system, which is planned to include a network of nine rail routes and approximately 30 km (18 mi) of tunnels excavated by both TBM and drill and blast. The routes will be both above and below ground to accommodate the hilly terrain of the city.  Currently, Chongqing relies on buses for the majority of its public transportation.

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