Like a flock of ponderous birds, some two dozen assorted Chinese cranes dip and sway their arched green, red or yellow necks across the deep Yangtse valley. Among their somewhat old-fashioned outlines stand other, slenderer species, like the lattice-boomed crawlers from Germany or here and there the straight tubular bodies of concrete conveyor-placers, or the uprights of big tower cranes.
The story of the Three Gorges dam construction will be dominated over the next five years by logistics, and most of all the movement of materials. Phase two, which is now getting underway, is the first and larger part of the dam itself (see box, p27), an incredible mass concrete wall which will incorporate more than 12m cubic metres of concrete in its 185m-high sloping form.
To make it demands a rate of placement on a titanic scale, with several million cubic metres of concrete poured each year, some 4.5 million cubic metres in the peak year and as much as 550,000 cubic metres in each of the peak months. That is enough concrete each year to build many of the world’s other biggest dams in their entirety.
Concrete must be moved but so must the heavy formwork to shape the giant block pours – and so must the heavy mechanised vibrator units, machines, cooling coils, spare parts, smaller excavators and loaders for the bits of clearing up. So must the giant 7m by 9m low-level sluice gates which are installed 90m up, and the 23 valves for the 8m-wide top spillways at 153m sill elevation; so must the steel for penstocks and the intakes. And so on.
China’s Three Gorges Project Development Corporation (TGPDC), an autonomous body set up by the State Council to carry through the dam project, and therefore the key client and project manager, has gone for a conveyor system for the bulk movement of the concrete. Six big placers are already standing along the main length of the wall; in total, about 4,000m of fixed pier conveyor is also being erected to feed concrete from the four batching plants around the site.
Much of the supply conveyor is from Rotec, the American specialist manufacturer, which also makes the big TowerBelt placers, a hybrid of tower crane support and articulated conveyor. These monsters with their upper booms supporting the conveyors below have already been used on the Ertan dam in Sichuan Province, on the Xiaolangdi intake works on the Yellow river and on the phase one diversion works for this project.
The units have swinging telescopic conveyors, with 800mm belts. A first unit used for the longitudinal wall of the diversion channel can extend outwards for 80m and hinge around to cover almost a full circle. Capacity at such as radius is 30t says Rotec’s managing director, Steve Ledger, though it will carry 60t to a radius of 40m.
Three more units, which will join the first one for phase two, have a reach of 100m. All of them handle concrete at a rate of 400m3/hour. Along with some smaller mobile conveyors and concrete skips, Rotec says it has sold $70m-worth of equipment to the project.
Alongside these stand two Potain Maxi MD 2200 Topbelt towers, derived from the French manufacturer’s largest production line model. Currently being commisioned on site, they will be used to move concrete along 105m-long conveyor belts, supported by the crane and developed in collaboration with Nippon of Japan (China bound May98,p18).
As with the Rotec system, conveyors rise up the Potain tower columns as concreting proceeds. These two units will reach a maximum of 90m above the concrete bedding surface, but, as the dam wall rises around them, the towers will eventually reach an actual height of 200m from top to subterranean foot.
But while these conveyors are the main concrete handling system, there is no shortage of other lifting mechanisms on site. High above the work, for example, hang the cables for not one but two large capacity Blondin cableway cranes, supplied by Krupp Fördertechnik of Germany in an order worth DM30m ($16m). Erection of these began last October and they are now just coming out of the commissioning stage, says supervising engineer Martin Michelitsch.
Cable-stayed masts for these, each 150m tall, sit on the finished 185m-high side walls at the edge of the dry working area for the dam, sealed by cofferdams upstream and downstream. The cable span is 1,406m and each can carry a 25t load. These Blondin cableways will be used mainly to move equipment or formwork, though a 6m3 concrete skip will be used later, especially for reaching awkward points and high areas on the dam wall.
Travel speed for the cranes is 7.5m/sec and hoisting speed 2.6m/sec, according to Micheltisch.
Lower down there are more conventional tower cranes: a smaller Potain and, yet to be erected, a massive Krøll, to cover the movement of materials and formwork on the downstream face of the dam (see above).
Currently two large crawlers are helping to assemble what are mostly rail-mounted Chinese cranes that will surround the dam wall both upstream and down. Information about these is harder to glean but some are designated as model “70” and some “50” and capacities are up to 50t.
Many of the latter cranes are supplied by the contractors themselves, Gezhouba Group and Qingyun Joint Venture on the main dam wall, and 378 Joint Venture for the power plant works. More of the bigger Chinese cranes can be seen over on the huge 90m-deep shiplock channel where a partnership of the 378 Joint Venture and a contractor by the name of Armed Policemen is carrying out concreting for the five stage lock system.
Most of the big western equipment is purchased and maintained by a special-purpose company – called The Equipment Company – set up by the TGPDC. Some 300 engineers of various disciplines talk with western suppliers to find the best items and negotiate purchases, explains TGPDC manager Xiao Chang Zian. Engineers from this team also travel to foreign manufacturers’ plants to get training in the equipment and learn about maintenance needs, he explains. The Equipment Company has its own building and permanent exhibition hall for the purpose at the dam site.
