China's 'other' major dam project, Xiaolangdi on the Yellow River, at 154m in height, will be the country's biggest rockfill.
Uniquely, it is a dam to control and hold back silt, says Massimo Malvagna of Italian contractor Impregilo, leading the Yellow River Contractors. 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 led 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 - and there are 16 in total - will cope with flood level flows and irrigation and there is also a big open spillway.
A third contracting 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 takes its name from its astonishingly heavily sedimented flow, which it picks up as it passes through thick loess deposits.
At peak periods, run-off carries up to 900g/litre of material, which, as CGIC deputy project director Olivier Colin puts it, 'makes it mud, not water'. 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 of 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.
Astonishingly the Xiaolangdi dam has a design life of just 20 years, as far as siltation is concerned, but the benefits will still 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 weird and wonderful deep loess gullies and hillocks give way to flat flood plain. 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, how- ever, should continue to be generated and other functions will continue.
The project, under the control of the client Yellow River Water & Hydroelectric Power Dev- elopment Corporation, has been under way since the early 1990s when 22 different Chin- ese construct- ion 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 major 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.
By last October around 16.7M.m3 of the earthmoving had been achieved representing 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.
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 1,665m 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.
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.
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 technique 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.
Generally leakage is not considered to be a major problem at the site since the silt blanket that will immediately form in the reservoir gives the dam an unusual self-healing capacity.
Work on the complex DM900M (£310M) Lot Two tunnel works, has not gone so smoothly. Early on the contractor CGIC JV accumulated significant delays mostly down to exceptionally difficult ground.
Tunnels have been driven by conventional drill and blast, predominantly through a blocky sandstone. The geological setting is worsened by sub- horizontal bedding crossing the axis of the tunnel at around 15. Finally a number of faults intersect the tunnel.
CIGC experienced 21 collapses in a six month period, says Olivier Colin, the contractor's deputy project director. Three were large involving several thousand cubic metres of rock coming down. Meanwhile large claims and difficult rock conditions are also dogging work on the Lot 3 contracts.
Some £30M of claims have been submitted on the £82M job for the construction of the 251.5m long, 25m across and 61.5m high powerhouse and associated facilities.
Again the bedding and faulting of the rock has proved very difficult, but the extent to which these problems will eventually be accepted as reasonable claims remains to be seen. The engineer is reported to maintain that the conditions were foreseeable by the contractor.