Since 2006, China has invested nearly USD $200 billion in rail infrastructure – a plan that promises to be one of the largest national railway expansions since that undertaken by the U.S. in the 19th century. Guangzhou’s metro expansion is part of the Pearl River Delta Inter-City Rapid Rail Project and China’s first ever inter-city rail link. The 32.2 km (20 mi) Guang-Fo line running between Guangzhou and Foshan was awarded in 12 separate lots. The owner, Guangzhou Metro Company, chose to utilize 16 different TBMs.
Lot 12, running between Jushu, Xilang, and Hedong stations, was awarded in 2007 to the China Communication Construction Corp., 2nd Navigation Engineering Bureau Ltd. (CCCC). The contractor, CCCC, selected two 6.3m (20.5 ft) diameter Robbins EPBMs for the parallel 2.6 km (1.6 mi) long rail tunnels. The two cutterheads began turning in January and February 2009, after being launched from the cut and cover site of Jushu station in southern Guangzhou.
The geology on the metro’s Lot 12 consisted of a complex layered profile, ranging from highly weathered to slightly weathered granite, coarse sand, and silt at pressures up to 4 bar. Around 70% of the tunneling was through a mixed face, with the alignment above the spring line in soft soils and the bottom half of the tunnel in rock of at least 50 MPa (7,250 psi) UCS. The remaining 30% consisted of flowing sand with high water content.
Both Robbins EPBMs were designed with spoke-type cutterheads including large opening ratios of 37%, which allowed for smooth flow of muck into the mixing chamber. Both 432 mm (17 in) hard rock single disc cutters and carbide bits were used to combat the mixed ground conditions expected.
Four independent foam injection points on the cutterhead were used to further consolidate the muck flow. Foam was used on the Guangzhou tunnels because it was less costly and it also reduced the required cutterhead torque. Muck was removed using an 800 mm (31.5 inches) diameter shaft-type screw conveyor due to the fact that no large boulders were predicted. Active articulation was chosen for this project, mainly because much of the transit twists beneath the city, with curve radii as small as 200 m (656 ft).
The two Robbins EPBs were launched in December 2008 and January 2009 respectively. After only seven months of boring, the two machines achieved more than 16 project records including a best month of 377 m (1,235 ft) – higher than any of the 16 TBMs that have worked on the Guang-Fo Metro Project.
Surface settlement was a major concern as the tunnels run beneath rivers, research sites, roadways, and vulnerable building foundations. Back-fill grout was used to fill the gap between the 300 mm (12 in) thick, pre-cast concrete segment rings and the surrounding soil. Some of the high-risk areas included the 80 m (262 ft) wide, 4 m (13 ft) deep Huadi River between Jushu and Xilang stations, and also the Pearl River Fisheries Research Institute, which has numerous sensitive ponds used for research into high-yield fish farming.
Both machines finished a month ahead of schedule, and operated around 95% availability. As of August 2009, only 66 disc cutters had been changed on the first machine and 46 on the second, while no carbide bits had been changed on either of the TBMs. The first machine completed its initial breakthrough into the Xilang station on August 15, 2009 and its final breakthrough in September. The second machine broke through into the Xilang station in September and made its final breakthrough in October 2009.