Rock TBMs attack low cover drives in Chongqing
TBM Recorder Rock TBMs attack low cover drives in Chongqing Mar 2010
Desiree Willis, Technical Writer, The Robbins Company
Two 6.4m (21ft) diameter Robbins main beam TBMs are the first hard rock machines to be used beneath Chongqing, China's mega-city of more than 35 million people. The two machines were launched in winter 2009-2010 to excavate twin 12km (7.5 mile) long tunnels for Line 6 of the city's Metro.

Two 6.4m Robbins rock TBMs ready for launch

Each machine is boring under sections of very low cover ranging from 10m to 60m (30- 200ft) 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 on 750mm (2.5ft) centers, 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 as small as 400m (1,300ft) radius, are to be negotiated along the alignment.
Specially designed back-up systems are being used to deal with any water inflows encountered. The back-ups are raised 1.2m (3.9ft) 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 will flow under the machine through the tunnel invert.

Low cover and tight radius curves distinguish the drives

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 800m and 420m (2,600ft and 1,400ft), respectively, starting under about 20m (65ft) of cover.
During the drives, the Robbins walking cradle system will be used to allow 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 instead allows the machines to move directly onto the cradle once entering the station.
The tunnels are the first of new lines to be excavated for the city's metro system, which is planned to include a network of nine rail routes and approximately 30km (18 miles) of tunnels excavated by both TBM and drill+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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