How do they do that?9 July 1999
Bringing together Chinese and foreign contractors on the world’s largest construction site is a complex and demanding task. Adrian Greeman went to see how work at the Three Gorges site is organised
For Shiu Fung Yu, one of China’s many female construction workers, it is an honour to be working on the Three Gorges dam project on the Yangtze river. ‘It is very famous’ she says, besides ‘being in a nice environment’. And of course she needs to have a job.
She came there from Sichuan province a month ago, one of thousands of new workers moving onto site as the main concreting and construction begins for the dam itself. She will stay, along with her husband who also works there, for the next five years. A peak workforce of 25,000 is expected, 18,000 of whom are already there.
Behind her, as she walks off the afternoon shift, can be seen a small forest of cranes, conveyors, drills and excavators. They rise from the depths of 85m deep excavations between two giant cofferdams. Completion of these cofferdams last year finally sealed off the main construction area from the Yangtze, the world’s third biggest river and one of its most tumultuous. With completion of phase one of the project, the five-year river diversion was over, and dam construction began.
The sheer cliff of the left bank is where the nearly vertical penstock channels are being concreted for the first of the 14 turbines. At the top, stand the silhouetted shapes of the first steel liner sections. Below, the box-like forms of the powerhouses are nearly complete and ready for installation of mechanical equipment. Western firms have contracts for all fourteen 710MW turbines and generators. Eight of the contracts are with a European group of alstom from France and Switzerland’s ABB, six more are with a transatlantic alliance of GE of Canada, and Germany’s Voith and Siemens.
On the far side, mid-river, is the vertical concrete wall of the lateral coffer dam behind which runs the river in the diversion channel. On top is the steel lattice pylon of German firm Krupp’s ‘Blondin’ or cableway crane system which, despite its capacities, will not be used primarily for concreting but for lifting formwork, machinery and other equipment into place. The task of placing more than 16M m3 of concrete, enough to build a half dozen of the world’s big dams, will be handled by combined tower crane and conveyor systems, four from Rotec of the US and two from a specially developed system from France’s potain and Japan’s Mitsubishi. A host of Chinese goose-necked cranes will assist, along with delivery trucks and much else.
Closing the river
All this is gearing up to begin. Meanwhile, the achievements of the huge project are already enormous. Phase one, beginning in 1993, saw the transformation of the river close to the town of Sandouping from a slow stately bend around a steep headland into a huge diversion channel and the now dry site.
A small island midstream, Zhongbaodao, was lengthened and elongated into a vast longitudinal concrete wall in the middle, while the channel beyond was sealed off, widened to 300m and deepened by removal of some 10M m3 of river bed to accommodate the maximum 50,000m3/sec flood season river flow. Meanwhile through the headland two canyons were blasted, the larger involving an excavation of more than 40M m3 of the solid 1000N/mm2 granite of the area. Work also included a complex series of tunnels and adits for the lock water feed and discharge system.
Concrete construction is now underway for the twin channels for the two-way five-step shiplock that will allow up to 3000dwt vessels to bypass the completed dam. A huge improvement in river navigability is one of the spin-offs of the dam work. The contractor is the Armed Policeman and the 378 Joint Venture. A smaller channel closer to the dam itself was also created in which a shiplift is now being installed.
This channel now serves as the through route for the river traffic during the flood season in July to September, when the main diversion channel is too dangerous for boats.
Phase one work also included some of the concreting for the side wings of the dam, the construction of a 1000m span suspension bridge, the Xiling, which is China’s longest at present, and a motorway covering 26km of the 40km to Yichang city downstream. Ten major domestic contractors were involved, mobilising 20,000 workers, and nine design institutes from around China supervised the construction.
The conclusion to phase one came in November 1997 when thousands of pyramid shaped concrete blocks, and granite boulders from the ship lock channel, were tipped into the river to seal the site, witnessed by then premier Li Peng.
‘Closure of the river was completed successfully on 8 November,’ says Li Jun Lin, senior engineer with the Three Gorges Project Development Corporation (TGPDC), the autonomous body set up as client and operator for the project which also runs the design work.
The initial cofferdam was a 69m high rock and earthfill embankment behind which was built a second 73m high embankment. The area between the two was infilled, mainly with sand and finer material, and a waterproof diaphragm wall was added using equipment supplied by Germany’s bauer.
Smaller embankments formed a higher top to the wall and in summer 1998 an additional dike was added during the freakishly high river flows that caused the terrible devastation downstream.
In the event, says Li, the extra 5m was not needed as the peak river level only reached around 75m. The work at the 15km2 dam site was barely affected by the dramatic events which saw such heroism on the levees, and the cofferdam was completed on time in August.
Meanwhile, between the 26M m3 of the upstream temporary dam and its smaller downstream sister, de-watering was able to begin in June last year, and the dry area for excavation was ready in September. Pumping was not easy in such a deep pit, says Li.
At the bottom the ground was prepared with removal of river bed material to bedrock. ‘There was a 6m to 8m thick layer of silt, mostly deposition caused by the Gezhouba dam downstream,’ says Li. This smaller hydroelectric dam was completed in the late 1980s, close to Yichang. ‘Below that was harder earth and stones and needed blasting, although blasting of the last metre is forbidden because of the danger of causing rock damage,’ says Li and so a water jet technique was used to loosen spoil. Some 18.2M m3 of excavation was required for the river bed and powerhouse areas.
Base rock quality was as good ‘as we expected’ says Li, pointing out that site investigations had gone on for more than 20 years and ‘we knew what to expect’. The base rock is a granite with limited cracking and reasonably cemented. On this area the main concreting operation has been under way since January.
Beginning phase two
Phase two of the dam comprises the major central section of the dam, the 483m long spillway section, and the powerhouse section, with its 14 turbines, stretching 643.7m on the left bank. Another 584m long section with 12 turbines will be built later in the current diversion channel. Side sections bring the total dam length to 2309.5m.
All this will eventually be a 185m high concrete bastion, a sloping mass of concrete that will total over 16M m3. Some two-thirds of that will be poured in the current phase with peak annual placings of 4.5M m3 and monthly throughputs of up to 550,000m3.
It is a lot of concrete, and will require unparalleled organisation. The TGPDC has brought in a large amount of western equipment for the operation, much of it selected, purchased and then maintained by a special equipment division (see panel).
Plant and machinery is allocated to the major contractors — five groups of companies who have won contracts on phase two works. They are all domestic firms, selected by public bidding in the internal Chinese market, which is becoming normal practice in the country.
For the major concreting works, the choice was between crane–skip placement with tower cranes, and/or a cable way crane, or conveyors. The latter system has been used effectively on several previous dams like the Ertan in Sichuan, and the Xiaolangdi Yellow river dam, though with much smaller quantities. A big Rotec conveyor system was also used for the longitudinal cofferdam in phase one.
Here, finally, the choice has been to use conveyors, mostly a Rotec system from the US, large tower suspended units that swing across the dam blocks as required. But there are also two spectacular giant Potain cranes fitted with conveyors, made by Mitsubishi, which serve the same function.
The 150m high lattice boom towers for a Blondin cableway crane are also being installed by Germany’s Krupp Fordertechnik. Two units are on site, says Martin Michelitsch, the installation supervisor. Each has a 25t capacity which means theoretically a 6m3 skip for concrete can be carried. At times they will be used for awkward corners and the highest points on the walls but mainly they will be used for carrying plant, equipment and formwork.
Concrete will be delivered by fixed conveyors also being supplied by Rotec, though a fleet of Terex concrete skip trucks is also being used. These will help feed concrete from four major batching plants including one from Italy’s Cifa, an American C S Johnson plant and others. Possibly the world’s biggest aggregate crushing plant has also been assembled.
Construction of the powerhouse is being carried out under a separate contract by Gezhouba Group and the Qingyun Joint Venture. In fact concreting for the first six powerhouses has been completed because 378 Joint Venture was able to begin early, carrying out excavation during phase one in 1996 and concreting in January 1997. Excavation for the other seven powerhouses is now almost finished and concreting started. All of this has meant the turbine suppliers have been able to work on site for a few months now, and the shapes of the intakes and tailraces are already becoming apparent. The task of building 710MW units is unique, and in the words of the Three Gorges Corporation ‘very arduous’.
The two sets of contracts were let in 1997, Alstom/ABB’s for US$400M and GE Canada/Siemens for US$300M. Alstom’s component is worth US$212M to design and install eight of these giants, which will utilise an 80.6m fall, the water rotating a 10.6m diameter turbine at 75rpm.
Each unit is 5.5m high and weighs 440t. Main components will not be delivered until between January 2002 and August 2004 but the team is settling in well at site. Initial works have begun well, according the Mathieau Didier, Alstom’s main representative at the dam, who will co-ordinate supervision and commissioning.
|The Three Gorges Project In Brief|
|The Three Gorges dam across the Xiling Gorge at Sandouping will be 185m high and will impound a reservoir of 175m at normal height, and 180m flood when a maximum discharge of 102,500m3/sec is possible. The spillway has 22 bottom outlets (7m by 9m at 90m height) and 22 top sluices (8m wide at 158m). An initial impounding of 135m begins during the second construction phase which began in 1998 and continues until 2003. The primary purpose of the currently estimated 203.9B Yuan project (allowing for inflation and financing cost) is flood control, lent new urgency by last year's disasters. It will be able to take a significant 500mm to 1m from peak water levels downstream, which is the difference between a 1/10year and 1/100 year event. The dam will generate vast amounts of power, with 14 generators on stream in 2004-6 and another 12 installed in the third phase, due to finish in 2009. In some scenarios another six underground units may ultimately be installed, giving a total output of 22,720MW. The reservoir will be a long thin tail stretching 650km, as far upstream as Chongqing, where it will cause a 3m rise in water level. Spin-off benefits will include upgrading river transport capacity. The five stage shiplock will be able to accommodate barge strings of five giving a total 15,000t throughput in one go. Future benefits will also include a large reservoir for possible water transfer schemes to the north where the Yellow river basin is short of water. Fish farming is also expected to be developed. Pollution fears have been voiced, but western engineering teams studying pollution alleviation at Chongqing, for example, have said it can be exaggerated. The reservoir can be seen as a widened river says France's Sogreah, and it has a relatively short retention time of two months. There are losses, not least the inundation of valuable archeological sites and the flooding of homes. The Chinese say that obviously resettlement is necessary, and that they are aware of the difficulties. They say they are spending 40bn Yuan (just over 40% of the total project cost) to create new infrastructure and towns and to create jobs in an area which was poorly developed previously (see IWP&DC, July 1998, p52). Some 50km2 of land will be inundated, with a total 2000km length of coast.|
|The Three Gorges project has been almost entirely carried out by Chinese designers and contractors. There has been western speculation recently that the Three Gorges Project Development Corporation (TGPDC) might appoint western consultancies for supervision on phase two, following widely publicised remarks by Zhu Rongji that China needs to pay closer attention to quality control. Firms in Hong Kong say informally that they are in such discussions. If it happens, and according to senior engineer Li Jun Lin 'nothing has been decided yet', the firms would fit easily into the project structure. The State Council has a special commission for the project which created the TGPDC as an automomous client body with wide ranging powers, charged with completing the whole scheme. It co-ordinated an 'exceptional' number of universities, design bureau, research institutes and manufacturers for the investigations and designs. The work itself has been let to Chinese contractors from across the country, usually in consortia, who tendered for the contracts competitively and are assigned specified contracts. This is a relatively new development in China, replacing state assignment of work. Contract management and control is by the engineering construction division of the Corporation with a team of engineers based at the site. The Corporation's finance, planning, management and power plant divisions are located downstream at its Yichang headquarters. Construction supervision and inspection is carried out by Chinese design institutes specifically appointed for the purpose. Nine institutes were used in phase one. The TGPDC says it is aware of the crucial importance of concrete design placing and quality control and has ongoing research into these questions with outside institutes. But Li Jun Lin angrily rejects allegations that there have been problems with concrete quality to date, declaring the stories to be the result of 'journalistic simplification'. A special equipment company has been set up by the Corporation to examine and purchase key items. Chinese machinery is used where possible and contractors bring in some of their own equipment but there is also substantial quantities of international machinery, with over US$120M spent so far. Suppliers include Caterpillar, Terex, Rotec, Potain, Kroll tower cranes, Demag and Krupp. For assessment and bidding there is a permanent exhibition hall at the dam site for manufacturers to show products. Some 300 technicians, economists and engineers are employed by six divisions, responsible for purchase and then subsequent maintenance of large scale construction equipment such as trucks, cranes and concrete placement items. Specialists visit manufacturers for training in ongoing maintenance.|