The final 72 ft diameter pile for the creation of the artificial islands for the Hong Kong-Zhuhai-Macau Seaway was driven on December 8. Courtesy of American Piledriving Equipment, Inc.
Possibly the largest-diameter steel piles ever driven have been installed by an eight-hammer pile driver called the OctaKong for the Hong Kong-Zhuhai-Macau Seaway.
January 31, 2012—One hundred twenty steel piles measuring 72 ft in diameter and 180 ft in length have been driven into the South China Sea for the Hong Kong-Zhuhai-Macau Seaway project, a 50 km long bridge and tunnel complex that will connect these major Chinese cities and reduce driving times. The piles—claimed by the pile-driving company to be the largest-diameter steel piles ever driven—will be linked by pile walls to form two enclosed cofferdams, each of which will form the basis of an artificial island.
The piles were driven by American Piledriving Equipment (APE) Inc.’s China division, based in Shanghai, using its eight-hammer, vibratory pile driver, called the OctaKong, which was made specifically to accelerate cofferdam and seawall construction. At the fastest point in the project, the hammer drove six piles in three days, enabling the entire process to take less than seven months.
On December 8 OctaKong drove the final 600 metric ton pile in the series. Sixty piles are grouped together and connected to 37 ft wide U-shaped shell pile walls to form each of the two cofferdams. The cofferdams serve as bulkheads for the creation of the two artificial islands, between which a 4.2 mi underwater tunnel will be built to enable deep-water ships to pass above. Bridge spans from the opposite ends of each of the islands will then connect with bridge segments from Hong Kong to the east and Macau and Zhuhai to the west.
In between each pile is a 37 ft wide U-shaped shell pile wall;
together, the piles and walls form a cofferdam. Courtesy of
American Piledriving Equipment, Inc.
“The man-made islands are the key to the bridge, so those had to be done first,” says David White, the general manager of APE China and the lead technical representative for the OctaKong project.
White says that large-diameter piles were used for the cofferdams because they are stronger than traditional caisson columns or sheet piles and would better absorb impacts from the sea and typhoons. Experts were concerned that sheet piles might fail, would take too long to create, and would require too many hammers and boats to drive economically, White says.
The concept of using individual hydraulic impact hammers on the project was abandoned because they were deemed insufficiently strong; there was also a possibility that they might not be procured quickly enough to meet the project’s deadline, and that they might generate oil runoff, posing a contamination threat to Hong Kong and the South China Sea. The OcktaKong runs on electric power—albeit eight times as much as a regular APE hammer—and can handle wide loads at a fast pace, helping to meet the Seaway project’s tight time window while protecting the environment.
“We researched a way to design a hammer that would take our single-hammer design and shake the pile into the ground,” says White. “That required us to hook eight of our power units together, which was a challenge. These hammers were completely accurate. [The hammers] could not vibrate 1/300th of a second in error from each other, which is difficult to do when one hammer is 72 ft away from the other hammer.”
The eight-hammer pile driver called the OctaKong drove 120 of the
massive steel piles. Courtesy of American Piledriving
Another significant challenge on the project was driving the large piles into the uneven soil along the seabed. In some cases, one side of a pile cut through the soil layer before the other side of the pile hit the same layer, requiring the vibratory hammers—and the OctaKong’s horsepower— to shake the pile up and down as it drove the pile to a perfectly vertical position.
At press time, the cofferdams had been completed and filled with sand; supporting piles will next be installed to create the entrances to the tunnel. The Hong Kong-Zhuhai-Macau Seaway is scheduled for completion in 2016.