Construction Starts at Shiziyang Tunnel in ChinaMore than 300 Chinese workers have begun excavating a cross-river railway tunnel near the estuary of the Pearl River, in south China on 9th November, 2007. The structure will be China's first cross-river rail tunnel and also the country's longest subaqueous tunnel. The twin tube tunnel is a key part of a 146 km high speed rail link that will connect Canton, the capital of Guangdong province and an economic powerhouse of China, with Shenzhen, also in Guangdong, and Hong Kong. The twin tubes of the tunnel will stretch for a total length of 10.8 kilometres (5,550 metres and 5,250 metres) and cross the Pearl River estuary at Shiziyang, near the Humen bridge, to link Canton, situated on the Pearl River, and Dongguan, located in the southeast of Canton. Twenty-three cross passages will connect the twin tunnels, whereof 19 in the sections excavated by TBM.Four large NFM Technologies slurry shield tunnel boring machines, 11.12 m in diameter, will be used to dig the tunnel (4,540 metres and 4,500 metres), with a pair of TBMs per tube and each machine in each tube working towards its sister. To date, two TBMs have already arrived to the site. TBM 1 has already driven 500 metres. TBM 2 commences on 1st February. TBM 3 will arrive to the site in early February while TBM 4 is being tested in the Shenyang plant. Visit www.nfm-technologies.comWhen a tunnel is bored by a single shield boring machine, the shield is usually disassembled outside the tunnel after the breakthrough, when tunnelling is finished. However, for the construction of some long and large diameter tunnels with a tight time schedule, two tunnelling shields may work from opposite directions towards each other and the shields have to be disassembled when they encounter in the tunnel. It must be safe and reliable for the disassembly of the two opposite shields so more consideration must be given during the shield structure design and process workout. For the Shiziyang tunnel, technical measures have been applied to the structural design of the shields that could allow an independent double-layer shield casing, a sectional construction of the main components, weight limiting of the unit, and track type trailing gear. Also, preparation work before disassembly and a method to reinforce the disassembly area with multi-layer concreting will be necessary. The disassembly process will be mainly staggered, from the disassembly of the trailing gear, basic parts of the machine, main drive part and the cutter wheel.   The builders of the project are China Railway Tunnel Group (CRTG) for two drives and China Railway 12th Bureau Group Corporation for the two other drives. The tunnel is scheduled to be finished in April 2009. The budget for the project is CNY2.4 billion. The Canton-Shenzhen-Hong Kong high speed line, with trains travelling at 350 km/hour, is scheduled to begin service in 2010. The line will enable travellers to journey among the three locations within one hour. Click cn/75. 06/08.