Chengdu’s Metro Line 2 includes 26 stations and 17.6 km (10.9 mi) of tunnels between Longquandong and Shiniu areas of the city. Seven lines totaling 274 km (170 mi) are planned to be operational by 2035, and will service 13.1 million daily passengers.
The contractor, CRCC Bureau 23, selected a Robbins EPB with a mixed ground cutterhead for the potentially variable conditions, as well as the back-up system, soft ground cutting tools and spares. The machine was launched in January 2010 to bore two 1.4 km (0.9 mi) sections of parallel tunnel, with a breakthrough at the midway point into an intermediate station. The tunnel alignment allowed the machine to pass 25 m (82 ft) below residential buildings, and included several curves with a minimum 400 m (1,300 ft) radius.
The tunnels for Lot 18 of Line 2 are located in highly variable, permeable alluvium, stiff sand, and clay, requiring a unique EPB TBM design and careful monitoring for settlement. This complex alluvial geology is unlike that found anywhere else in China. Cobbles averaging from 20 to 80 mm (0.8 to 3.1 in) in diameter were predicted, with diameters of as much as 120 mm (4.7 in) possible.
The mixed ground, spoke type cutterhead was mounted with Tungsten carbide knife-edge bits and seven 17-inch (432 mm) diameter disc cutters around the gauge. A foam injection system was used to stabilize the running ground, allowing each cubic meter of foam mixture to stabilize about 40 rings of ground. Subsidence was intensively monitored and crews were trained to utilize probe drilling and ground consolidation if settlement was detected. Variable frequency (VFD) drives allowed the cutterhead rotation to be kept low (around 1.5 RPM at maximum) to also minimize surface settlement. High advance rates were instead achieved using increased cutterhead torque, which results in a faster rate of penetration. One-liquid type back-filling grout was used to fill the gap between segment lining which consisted of 300 mm (12 in) thick reinforced concrete segments set in a 5+1 arrangement.
In June 2010, the machine had broken through into the intermediate station, approximately 1,397 m (4,583 ft) into the 2.7 km (1.6 mi) long tunnel. Following scheduled maintenance, the machine was relaunched to bore the remaining section of the tunnel. Cutter wear was very minimal, with only three cutters changed since the start of boring.
By the time of tunnel completion in December 2010, the machine had achieved a project landmark of 129 m (423 ft) in one week, and 459.5 m (1,507 ft) in one month – higher rates than at least 4 other machines working on Line 2 in similar geology.