The Zhengzhou Metro consists of twin underground tunnels as part of a larger project for the 11th Bureau of the China Railway Construction Corp. (CRCC) to create the first metro system in the city. Approximately seven million people live in Zhengzhou so the new metro lines will help relieve congestion and allow Zhengzhou to become a center for rail commerce in China.
The future construction of up to four metro lines is planned for the area, with freight lines between Beijing and Guangzhou (North-South), and Xuzhou and Lanzhou (East-West) intersecting the city. Once complete in 2013, Line 1 of the metro will be 26 km (16 mi) long and include 22 stations. The Zhengzhou Metro Company has invested Yuan 10.2 billion (USD 1.5 billion/EUR 1.1 billion) in the new metro project. Once all lines have been completed – sometime between 2015 and 2030 – the entire metro line will stretch 188 km (117 mi).
Soft, powdery soils made up the first part of the drive. Line 1 involved excavation under urban areas of Zhengzhou with cover as low as 7 m (23 ft) for almost half the length of the tunnel. The drive also included a section of tunnel just 7.0 m (23 ft) below Xi Liu Lake, with depths of 1.0 to 2.0 m (3.2 to 6.6 ft). The terrain in this section consisted of permeable soil with underground water below the lake fed by vertical leakage, resulting in challenging ground conditions.
The Robbins EPBs were designed using spoke-type, soft ground cutterheads with tungsten carbide knife-edge bits. The electric, variable speed drives power the cutterheads at 750 kW (1,005 HP), allowing them to rotate at a speed of 0.3 to 2.0 revolutions per minute. To remove liquidized muck, the EPBs are equipped with shaft-type screw conveyors 800 mm (31 in) in diameter, which take the muck to muck cars for extraction from the tunnel. The thrust rate of both EPBs is 36,000kN (8.1 million lb), with a maximum torque of 4,785 kNm (3.5 million lb-ft) and a maximum stroke of 1,950 mm (77 in).
Both machines were designed with active articulation, which allowed them to bore curves on small radii while keeping the segment lining of the tunnel intact. The segment lining was made of reinforced concrete, 300 mm (12 in) thick, in a 5+1 arrangement with each segment 1.5 m in length. To help stabilize the tunnel lining, one-liquid type backfill was injected between the segment lining and the soft ground to fill in temporary gaps and help minimize subsidence.
Each tunnel is 3.6 km (2.2 mi) long, passing through four intermediate stations between Kaixuan and Tongbo areas. The machines used on this project were manufactured at Chengdu CSR Tunnelling Equipment Co., Ltd. and launched in November and December 2010. Structural challenges in the low cover tunnel included a highway interchange bridge and building foundations nearby. Foam and bentonite were injected into the soil to improve its condition and reduce settlement. Following the successful completion of these sections, the machines achieved two intermediate breakthroughs into cut and cover sites in early 2011.
Line 1 of the Zhengzhou Metro was completed in October and November 2011, at least two weeks ahead of schedule. The two 6.3 m (20.7 ft) Robbins Earth Pressure Balance Machines (EPBs) have set a Chinese EPB record in their size class, excavating up to 720 m (2,362 ft) in one month. Daily advance rates of the machines reached 22 rings (33 m/108 ft) in two 10-hour work shifts. These rates additionally mark a project record compared to nine other machines that worked on the job. The two Robbins EPBs already have future projects lined up due to their outstanding performance rates on the Zhengzhou Metro.