Robbins EPB Will Cut Through Highly Permeable Ground

Chengdu, China’s 11 million inhabitants will soon have a reduced commute time thanks to an extensive new metro system. Five lines totaling 126 km (78 mi) will be constructed using multiple TBMs. Robbins signed a contract with China Railway Construction Corp, Ltd. (CRCC) in January 2009 for a 6.26 m (20.5 ft) diameter EPB and back-up system on Line 2, Lot 18 of the Chengdu Metro. A commissioning ceremony was held in Chengdu on Oct. 14, 2009.

Geology will include a matrix of weathered rocks found nowhere else in China, consisting of highly permeable pebbles, sand and clay. Subsidence will be intensively monitored — crews will utilize probe drilling and ground consolidation if settlement is detected.

A mixed ground cutterhead, mounted with 17-in. diameter disc cutters and carbide bits, will be utilized to effectively excavate the variable geology. Cobbles averaging from 20 to 80 mm (0.8 to 3.1 in.) in diameter are predicted, with diameters of as much as 120 mm (4.7 in.) possible. Muck will be removed using an 800 mm (31 in.) diameter shaft-type screw conveyor and battery-operated rolling stock.

Assembly of the TBM was completed in a manufacturing facility in Chengdu. The machine was due for an autumn 2009 launch, and will bore two 1.35 km (0.8 mi) long sections in a suburban area of the city. The tunnel alignment will allow the machine to pass 25 m (82 ft) below residential buildings, and includes several curves with a minimum 400 m (1,300 ft) radius. Active articulation will be utilized to prevent ring deformation in curved portions of the tunnel.