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Separating sediment China's Three Gorges project has taken all the headlines. But on the Yellow River a second large and complex dam is also rising. Report and photographs by Adrian Greeman.

Vast grey semi-cylindrical buttresses tower against a sheer cliff face, with tiny figures moving here and there. Behind the vertical pylons, giant caverns penetrate the sheer rock like the entrance to a Pharaonic tomb in the Valley of the Kings. A wide river runs sluggishly below and plains stretch beyond.

But the painted hieroglyphs here are in Chinese not Egyptian characters, and the towering temple shapes will not be rescued from the river but become part of it. This is the intake structure for China's 'other' major dam project, Xiaolangdi on the Yellow River, known as the 'mother of Chinese civilisation'.

The huge slots and runners in the towers will control flows of water into no less than 16 different tunnels when completed, an extraordinary number reflecting the extraordinary functions of the dam project. It is, uniquely, a dam to control and hold back silt, says Massimo Malvagna of Italian contractor Impregilo, leading the Yellow River Contractors which is building 154m high embankment dam across the river. It will be China's biggest rockfill.

Secondary functions are to generate power, control floodwaters, and on a local scale, feed irrigation. Tunnels and valves, being built by a second European consortium, CGIC JV sponsored by Germany's Zublin, will allow discharge of heavily sedimented water in the rainy season, or deeper level flushing of already settled silt, or feeding of clearer water to the power house, as appropriate. Other tunnels will cope with flood level flows and irrigation and there is also a big open spillway.

A third group, Xiaolangdi JV, led by Dumez and Philipp Holzmann, is building the underground powerhouse where six 300MW turbine generator sets will produce a total 1,800MW output during about nine months of the year.

The Yellow River probably has the world's most heavily sedimented flow, hence its name. It flows through a plain of loess, a fine, wind-deposited material tens of metres deep, and eroded into deep gullies and hillocks of weird and wonderful shapes. The run-off means the river is carrying up to 900g/litre of material at some peak periods, which, as CGIC deputy project director Olivier Colin puts it 'is not water, it is mud'. Even heavily silted rivers usually peak at 200g/litre levels.

Over the centuries the river has been depositing silt in the river bed at an average 100mm per year. Not surprisingly the clogged river often changes course and regularly bursts its banks causing widespread flooding and loss of life. Controlling it by raising hundreds of kilometres of river bank levee costs the Chinese state huge sums.

A dam which holds back the largest part of this silt would be a boon. Although the Xiaolangdi will be limited to some 20 years as far as siltation is concerned, the benefits outweigh the costs according to its designers, the Xiaolangdi Engineering Consulting Company formed by China's Ministry of Water Resources. The World Bank, which is partly funding the $900M (£550M) project, agrees.

Dams have been built before further upstream but these have been aimed primarily at power generation, and have mostly failed, with reservoirs heavily silted even before completion.

Xiaolangdi is located at a key point on the river where the loess hills give way to flat flood plain. Upstream is the major catchment for the river, some 92.3% of its total. A reservoir will form here with a 12,650M.m3 capacity, of which 5,100M.m3 will be 'live'. The remaining dead volume will hold back much of the annual 1.6Mt wash-off sediment.

After 20 years, the Chinese engineers concede, 'we will have to think of something else'. Power, however, should continue to be generated and other functions will continue.

The project, under the control of the client Yellow River Water & Hydroelectric Power Development Corporation, has been under way since the early 1990s when 22 different Chinese construction bureaux were drafted in to prepare the site.

The preparation was 'excellent', according to Malvagna who arrived in late 1994 when the Yellow River Contractors began working on the £195M earth and rockfill dam, one of three sections of the works. Roads, quarries and borrow areas were sorted out and resettlement of the 170,000 population affected by the scheme was well under way. In the immediate area this means people from the small but bustling township of Xiaolangdi itself, built, like many of the villages in the region from cave houses dug into the soft but stable loess. Most of the village has been used for borrow since.

YRC is led by Impregilo, and its 36.5% share is matched by Germany's Hochtief, with the remainder is made up by Italy's Italstrada and China's Water Conservancy & Hydropower Engineering Bureau 14. And it is doing well according to Malvagna.

'We were supposed to have completed the diversion dam by the end of October this year and around 11.6M.m3 of the earthmoving. In fact we have completed nearly 16.7M.m3,' he says, which represents 32% of the task against a programmed 22.5%.

The diversion was carried out on 28 October with a choreographed line of rock trucks completing a sealing dyke for the starter cofferdam, in the presence of Chinese premier Li Peng, himself a civil engineer. A small downstream cofferdam was also finished and the water pumped out of the river bed.

Since then work has proceeded apace completing the 15m high starter cofferdam, and now on building up the main 55m high upstream cofferdam. This must be ready for the flood flows of the river next summer.

The cofferdam will eventually be incorporated into the 154m height of the main dam linked by a final impermeable layer to the inclined core of the main dam. The core is built from the surrounding loess 'which is a good impermeable material', says Malvagna.

The outline of the big structure is already visible on the right bank where much of the 1.665km crest length embankment will run. Impregilo was given a dispensation to push forward construction away from the riverside and has built up the rock and earthfill embankment to within 30m of final height, though the Engineer has stopped further work for fear of slips.

On the opposite bank, which is mainly dominated by tunnelling and powerhouse works for Lots Two and Three, the dam outline is also picked out, this time as a concrete layer across the side of the river gorge. Malvagna explains that the underlying rock is a sandstone, usually overlain on flat ground by a 5m layer of claystone under 10m-15m of loess. On the abutment as little as 5m has needed to be taken out. But the Chinese Engineer has asked for a concrete facing to the abutments where they contact the core, says Malvagna.

German engineer Dominik Godde, in the YRC planning department, adds that he believes that a European Engineer would have chosen a direct cleaned rock for core placing rather than concrete. The flexible core material can then accommodate any settlement in the fractured sandstone.

'But the Russian/Chinese philosophy is to make sure of the contact by forming a smooth surface,' he says. Malvagna says a layer of relatively weak 15N/mm2 concrete 200mm to 300mm thick has gone on. 'We have placed 100,000m3 of concrete for that though 40,000m3 was originally calculated,' he adds.

On site now a grout blanket is being installed across the river bed and work is starting on an up to 80m deep cutoff wall. Bachy Soletanche is subcontractor for this and will use hydrofraises to key 1m or so into the rock.

The wall is said by the subcontractor to represent the first use on a dam for a new technology involving 'plastic concrete'. The wall panels are formed in between right angle panels set across the line of the main wall. The transverse panels are made of plastic concrete which means the main panel can first be easily cut into the side panel at either end to form a watertight key. Secondly if the main panel goes off the vertical it still has the full width of the right angled side panel to cut into.

The silt blanket that will immediately form in the reservoir is a major safety factor, he points out, giving the dam an unusual self-healing capacity.

Work so far has been relatively amicable, says Malvagna. 'We are prepared to approach the Chinese mentality,' he says. 'They are very pragmatic and though discussions are very long they never lose sight of the end point. If you are willing to compromise so will they, though you have to gain some respect at first. But if you insist on sticking to principle you will get nowhere. But the paperwork is unbelievable!' he adds.

On the complex DM900M (£310M) Lot Two tunnel works, things have not gone so smoothly, and early on the contractor accumulated a year's delay. Things are going better now and the first critical deadline, completion of the diversion tunnels and their plunge pools, was met 10 days ahead of 28 October date, says Olivier Colin from French firm Spie Batignolles.

Colin is deputy project director for the CGIC JV which is led by Germany's Zublin. Tunnel firm Wayss & Freitag and Strabag, both out of Germany, Salini from Italy and two Chinese firms, Ministry of Water Resources Bureaux No 7 and No 14, make up the group.

Most of the delays are down to exceptionally difficult ground, he says.

Tunnels are driven predominantly through a sandstone, he says, which has a tendency to pull away in blocks. The tunnels were driven by conventional drill and blast methods, and the overbreak can be high which demands extra excavation and extra concrete.

The problem is worsened by a sub-horizontal bedding crossing the axis of the tunnel at around 15. Finally a number of faults intersect the tunnel. 'Once you meet a fault you know you have met it for good,' he exclaims.

CIGC experienced 21 collapses in a six month period. Three were large says Colin, several thousand cubic metres of rock coming down. A big pullback in progress has been achieved, but there is a stand off on the current outstanding £105M in claims. The Engineer maintains that the contractor's risk assessment should cover the situation.

One section of the work is proving particularly interesting. The sediment tunnels, circular and 6.5m in diameter, are being given a prestressed concrete lining. An unusual technique, only twice used before, is being incorporated. 'Instead of using a duct, the prestress strand is cast directly into the concrete,' explains Colin.

'The strand is coated with a polyethylene cover and greased which means it can move. Strands go in as circular 'hoops' every 500mm instead of every 250mm and you can still get a better distribution of forces.'

Meanwhile large claims and difficult rock conditions are also dogging work on the Lot 3 contracts. Some £30M of mainly rock condition claims are being submitted on the £82M job for the construction of the power facilities.

British engineer Maurice Thornton is contract manager for the job as part of the German firm Philipp Holzmann. France's Dumez sponsors the project and Chinese firm the Sixth Bureau makes up the joint venture.

'Technically the difficult bit is the powerhouse, which is both very large and very complicated inside,' says Thornton. And a major additional problem is the steel penstock lining work for the power tunnels where they reach the powerhouse and then drop down vertically to the turbines.

Rock troubles were a major part of the story on the excavation of the powerhouse, says Thornton, especially early on in the roof. There 325 prestressed tendons were ordered by the Engineer for the roof vault once work had begun. The powerhouse is 251.5m long, 25m across and 61.5m high.

Again the bedding and faulting of the rock has proved very difficult. There have been difficulties too with the installation of penstocks which are running behind schedule.

How much of the problems will eventually be accepted as reasonable claims remains to be seen. Thornton says that on much of it the Engineer maintains conditions were foreseeable by the contractor.

One problem perhaps, he says, is that the scale of additional expenditure for machine and expatriate workers is not always appreciated by some of the engineering staff, especially if they have not had previous contact with western firms.

Despite all that, he says relations with the Engineer and the Chinese side are not bad, and the contractor has learned a lot about how to adapt to conditions.

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