APE’s 200-6 successfully drove six piles at a time for the 32 meter diameter (104 ft) cofferdams that are being used to extend the Hong Kong airport runway. Prior to the project start, the Chinese government design groups had a tough choice between using either 32m (104 ft) diameter super-large piles, similar to the 22m diameter (72 ft) piles driven for the Hong Kong-Zhuhai-Macao man-made island project, or instead to use sheet piles. Since the project is next to the Hong Kong airport there are many safety regulations for construction ships in the area. To drive 32m (104 ft) super-large diameter piles would require a 4,000 ton boat crane with mast that would exceed the 35m (114 ft) height restriction of the area. The fear was that if the boat crane was too tall it would be a danger to the airplanes trying to land at the airport nearby. Therefore, despite the success of the “APE Octakong Project” that drove 22m (72 ft) super-large diameter piles just 10 months ago, they decided to use sheet piles to create the 32m cofferdams instead.
Driving sheet piles for 32m (104 ft) diameter cofferdams on water is no easy task, but despite all of the challenges the main concern was construction speed. To speed up the project the contractor wanted to drive six sheet piles at a time per hammer, something that again has never been done before on this scale. APE provided four 200-6 hammers with six APE model 150 clamps attached to custom beam. Each hammer has a special manifold that allows independent control of each clamp. After the sheets are put in place the APE 200-6 hammer is placed on a row of six sheets to be driven, however when sheets are first placed in the cofferdam template they are not all sitting at the same height since some sink in the soil farther than others. The contractor needed a way to grab onto several piles and vibrate down while some of the clamps are still open.
APE changed the design of the clamp slightly to withstand the forces of a vibratory hammer even while some the clamps are in the open position while others are in the closed. This week the 200-6 was used for the first time with great success. Currently there are now four working 200-6 hammers on the job site each driving six sheets at a time. To increase the speed of the placing of the sheets the contractor also designed a special boat that could grab 49 sheets at a time and place them in the cofferdam template; effectively placing 25% of the sheets at once. A special barge was created to pre-place sheet piles in a position that the special crane could grab. The project requires 147 cofferdams to be driven with 196 flat sheet piles per dam. Each sheet is 30m long to be driven 20m deep. One cofferdam can be fully driven to grade in 4-5 days, a speed unattainable without modified equipment and unique construction techniques used.
This project demonstrates the cooperation APE has had with the Chinese government to continue to supply pile driving equipment that meets their ever increasing construction speed requirements.