The risks were formidable. APE employees' reputations in China were at stake, says Yang Yun Fu, an APE sales representative. "There was zero time in the schedule to account for any failure or breakdown, and there were very few backup parts."
APE won the bid to build the approximately $20-million hammer—actually eight vibratory hammers combined—in 2011. "The single-most difficult technical obstacle to overcome was to engineer a way for all eight hammers to remain in perfect sync," says David White, APE China managing director. "If even one hammer vibrates out of phase from the others, it would generate so much heat that it would melt the steel in less than 15 seconds." The team had a 1/300th-of-a-second maximum margin of error, he adds. Dubbed the "Octakong," the monster hammer required 9,600 HP in order to drive piles weighing 600 tonnes.
APE installed 61 cylinders for the west island and 59 for the east island. The monster cylinders were fabricated by Shanghai Zhenhua Heavy Industries and shipped to the site. With 40.5 m to 50.5 m heights, 22 m diameters, steel-plate thicknesses of up to 25 millimeters and reaching as deep as 29 m into sandy soils, "piles this big have simply never been driven at all," says White. Once the island perimeters took shape, crews backfilled the interiors and used plastic pipes to dewater them. Potential soil settlement had to be controlled within 30 centimeters, says Dong. Short cut-and-cover excavations allow for the transition from island to underwater tunnels.
Factory Precision
On Guihan, a natural and uninhabited island, CCCC brought in power generators and built a precasting factory and living quarters for some 1,000 workers involved in prefabricating each approximately 38-m-wide, 11.4-m-high, 180-m-long tunnel element. Each element consists of eight 22.5-m-long concrete segments, cast continuously within 33-hour windows. Rigorous standards allowed for cracks no larger than 0.2 mm. Every cu m of concrete has more than 300 km of rebar, about twice as much as average, says Luo. Factory operations have been nonstop since 2011, with crews working in three shifts over 24 hours, Dong notes.
The team built a conveyor belt that delivers ice-cooled concrete to a hydraulic formwork system. Each pair of segments is match-cast on fixed beds, then pushed forward along sliding tracks to make room for the next segment: a huge assembly line. Ultimately, the completed element is pushed out on rails into the dry dock, which is then immersed with water.
CCCC customized a barge for the towing and placing of elements in the open sea, some 11 km from the factory site. Waves, winds, forces created by the immense elements, extensive marine traffic and the white-dolphin habitat make each journey a nail-biter. For the first element, the team used six tugboats and "went against the current to have better control" over navigation, says Dong. The installation took 96 hours, but, after the learning curve, the second element employed 10 tugboats, flowed with the current and took less than half that time, he adds.
Of the 6.7 km total length, the immersed elements total 5,664 m. The rest of the elements include the cut-and-cover sections and associated ramps. Special barge-mounted equipment also dredged the seabed trench, which will hold most of the 74,000-tonne elements. Allowable dredging tolerance was within 0.5 m and needed constant monitoring of sedimentation.
COWI's Olsen says the team developed a computer model to address the gravel bed's settlement differential, which varied from 1 to 2 m in thickness due to the variations in soil layers, with several types of soft, weak soils atop a hard layer of rock. "The material properties could vary by 30%," he notes.
COWI engineers hope for an opportunity to use the modeling on the planned Fehmarnbelt Tunnel between Germany and Denmark. At 18 km, that immersed tunnel will succeed this one as the world's longest.
