Zheng Yan Long, TunnelTalk China Correspondent
China is constructing another large-diameter TBM highway tunnel under the Yangtze River to further ease traffic congestion in Nanjing. This adds to an impressive number of mega-diameter TBM tunnels built in China. Zheng Yan Long, TunnelTalk's China Correspondent, reports.
Fig 1. New river crossing for Nanjing
- Nanjing, the capital city of Jiangsu Province on the east coast of China, is constructing a second highway tunnel crossing of the Yangtze River. The new Weisan Road Tunnel project is another major step towards further relieving severe traffic congestion which continues to affect the city despite three existing bridge crossings and a first tunnel crossing that opened in May, 2010. The new tunnel will be welcomed by Nanjing's eight million inhabitants and boost the economic development of the wider Pukou District.
- The tunnel project is an integral component of an overall plan and is being delivered using the BOT (build-operate-transfer) model. The China Communication Construction Company (CCCC) Investment Company and the Nanjing Municipal Government will contribute 70% and 30% respectively to the RMB 5.25 billion (about US$820 million) budget. Once built, the CCCC will operate the toll highway tunnel for 30 years. It will then transfer back to the Government.
- Rather than the more usual parallel drives, the project comprises two shield drives that cross over one another at the south end. The North line tunnel is 3.54km long and the South line drive is 4.14km long (Fig 2). In addition there are TBM-launch and -reception shafts; the cut-and-cover transition section at the portals, and the intermediate ventilation shafts for each tunnel on Qian Island.
Fig 2. Alignment of the Nanjing Weisan Road Tunnel
- The North line tunnel drive was awarded to the CCCC Second Harbor Engineering Company, and the South line drive to the CCCC Tunnel Engineering Company.
- The project broke ground in November 2010 and is expected to open to traffic in 2014, in time for the start of the Nanjing Youth Olympic Games.
- Unlike the first Nanjing Yangtze River Tunnel, which consists of twin parallel tubes and comprises six lanes (three in each tube for each direction), the two tubes of the Weisan Road Tunnel follow an X-shaped alignment and have a double-decked interior providing eight lanes in total, four in each tube (Fig 3 and 4). Traffic on the two decks of each tube will run in opposite directions. Other parameters are virtually identical.
Figs 3 and 4. Design of the first Nanjing Yangtze River Tunnel (above left) and design of the new Nanjing Weisan Road Tunnel crossing (above right)
The first twin-tube tunnel was excavated using two Herrenknecht Mixshields of 14.93m diameter each. The two new, tunnels are to be excavated by two 14.93m slurry TBMs leased from a joint venture between Chuwa Bussan Co Ltd, Japan Tunnel Systems Co Ltd, and Mitsubishi Heavy Industries Mechatronics Systems Ltd (MHIMS). Manufacture of the two machines is a full co-operation between IHI (Ishikawajimaharima Heavy Industries Co Ltd), MHIMS and the CCCC Tianhe Machinery Manufacturing Co Ltd. The Chinese company will supply components and assemble and test the machines in the Changshu factory in Nanjing. The two Japanese partner companies will provide key components and engineering expertise.
Fig 5. Design of the cutterhead
- The excavation is being carried out in soft river sediments with a very shallow cover especially across the centre part of the river. The maximum water pressure is 7.7bar.
- Each tunnel tube will have an exterior diameter of 14.5m and lined with precast concrete segments to a 13.3m interior diameter. The bolted primary precast concrete lining comprises nine segments and a key in each 2m wide ring. Cross-passages will connect the tunnels to provide safe emergency escape routes.
- The giant slurry TBMs will pass through complex geology beneath the Yangtze River. The permeable strata range from clay to muddy soil; from coarse to abrasive sand; from gravel to mudstone; to mixed faces that include them all; and with boulders to be anticipated of up to 200mm in diameter. After careful consideration the slurry type of TBM technology was adopted for excavation of both tunnels.
The project faces many challenges. Chief among them are:
• Complex geological conditions
The mixed geology is demanding because the traditional cutter bits for soft soil and disc cutters for hard rock are, in some senses, incompatible. A universal cutterhead, dressed with tools to suit the different formation should be developed. With abrasive materials anticipated, bit wear should be detected automatically and the cutting tools should be interchangeable. In this case, special knife-edge bits and side protection bits are designed to protect the scrapers. To avoid blockage of the slurry pipeline a special crusher is developed to break up boulders of a maximum dimension of 800mm.
• High water pressure
Maximum water pressure anticipated is 7.7 bar. This requires special design of the brush tail seal and of the main bearing seal, and development of a system for replacing tools under normal atmospheric pressure.
- Table 1. Technical specification of the TBM
- Progress of this new large-diameter TBM highway tunnel in China adds to the list of mega-TBMs used in the world to date. A total of 13 mega-TBMs of more than 14m diameter have been used on projects in China and there are more in the planning stage.
Tracking the world's mega-TBMs - TunnelTalk, Discussion Forum
Two new mega drives under the Yangtze - TunnelTalk, Sept 2009
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