World's Biggest TBMs at Work at Yangtze Tunnel in ChinaThe People's Republic of China is one of the most dynamic economic regions in the world. With its unprecedented growth, the country has already become the fourth largest economic power in the world. One of the most important engines for growth on the Chinese market is the ongoing urban expansion, along with the extension of associated infrastructure, which has been promoted consistently since the 1980s.With its 20 million inhabitants, Shanghai is now the biggest city in China and one of the largest cities worldwide. This mega city continues to grow and is permanently seeking more space, even beyond the natural borders of the Yangtze River delta estuary. 500 new vehicles are registered each day, although the city can hardly cope with the level of road traffic even now.Two gigantic tunnels are to be opened to traffic in time for the World Expo 2010, which is to be hosted in Shanghai. They will bring Changxing Island closer to Pudong, connecting 600,000 island inhabitants with the Shanghai conurbation. A bridge from Changxing to Chongming completes the connection to the Yangtze estuary with the newly planned Hu Chong Su traffic line (see map). Each tunnel stretches over a length of 7,470 metres and accommodates two levels: a three-lane highway will run on the upper deck, the lower level has space for service and safety facilities and a metro line which may be built later. Since the Yangtze River is one of the country's busiest waterways, it is important that the inland traffic should not be restricted by the construction work. Therefore, mechanized tunnelling was chosen rather than an immersed tunnel or a bridge construction for the very busy south channel of the Yangtze River.GeologyThe ground is a mixture of sand, clay, broken rock and groundwater. Water pressures of up to 6.5 bar must be dealt with during tunnelling. The maximum overburden above the tunnel is 65 metres. The loose geology and the high water pressure can be kept safely under control thanks to the mixshield technology, which is the best solution for long and deep river crossings with large diameters. 21 million cubic metres of ground will be excavated until completion. The daily removal is a logistical challenge and generates busy traffic to and from the construction site.Mixshield technologyShanghai Changjiang Tunnel & Bridge Construction Development Co. Ltd. ordered in spring 2005 two high-tech Herrenknecht mixshields to tunnel beneath the south channel. With a diameter of 15.43 metres, these machines are the largest TBMs in the world. They have been designed by Herrenknecht in Germany where the main components, such as the cutting head, main drive, process technology and hydraulic thrust cylinders were also produced. On 18th May, 2006 the first machine (S-317) was delivered to the customer. Three months later, on 27th August, 2006 the official factory acceptance of the sister machine S-318 took place. Shanghai Tunnel Engineering Co. (STEC) dismantled the TBMs in partnership with Herrenknecht. Click here. Visit www.herrenknecht.com Following shipment, STEC assembled the 132 m-long machines on site with the assistance of Herrenknecht. The first machine S-317 was launched on 23rd September, 2006 at a depth of 26 metres. The second machine S-318 began tunnelling at the end of December 2006 at a distance of 23 metres from the parallel bore.The mixshield works on the hydroshield principle. Instable tunnel faces are supported by bentonite, a mixture of clay and water, so that the cutter head can drive quickly and safely through ground with high water permeability, mostly sands and gravels. The precise control of the support pressure avoids subsidence and heave on the riverbed and the TBMs go ahead safely with the tunnel production under extreme water pressures. In order to guarantee the highest safety standards in such extreme conditions when replacing the cutting tools, the six cutting wheel spokes are accessible to persons under atmospheric conditions from behind the pressure wall. On each track two tools have a tool lock which opens to change the tool. The tool retracts, the lock is closed and the worn tool is changed. There is no need for divers to go under 6.5 bar pressure to change the tools, which greatly reduces time, risk and cost for changing the tools. LiningAt the shield tail, the tunnel is lined with specially manufactured, steel reinforced concrete segments.Both machines will install a total of 7,500 60 cm-thick lining segments. Each individual ring consists of nine segments and one key stone, which weigh up to 16 tonnes each. Both machines are running very well. The first machine S-317 has installed 1,092 rings, two metres wide each, until 29th May, 2007. The second machine S-318 has installed 453 rings at the same date. The best daily advance rate (24 hours) was 12 rings with one machine. The tunnel is built by STEC in joint venture with Bouygues Travaux Publics. The contract is worth a total of EUR185 million (EUR74 million for Bouygues TP). A subsidiary of Shanghai Urban Construction Group, STEC specialises in the construction of tunnels of all sizes, and is currently working on around twenty different tunnels in China. As many as 400 people will work on the tunnel project at peak times. The first shield will reach the target shaft on Changxing Island at the end of 2008. View a video here (164 MB) and pictures here. Click cn/51. Read E-News Weekly 37/2004 & 15/2002. Visit www.stec.net or www.tunnelling.cn and www.bouygues.com 23-24/07.