Singapore Power Co. awarded Siemens/Samsung Engineering & Construction JV a $400 million project to build a 740-megawatt power station extension. A temporary cofferdam sheetpile wall and two permanent sheetpile lines were required to allow construction of a seawater intake channel.

In this reclaimed area on Jurong Island, hard rock was unexpectedly encountered at approximately 26 feet to 30 feet below ground level, preventing driving of the sheetpiles. The options: drill a slot through the rock, or mobilize a large dredge and remove the total obstruction by grab – at very high cost. A DTH under-reaming proposal was submitted to the customer to drill a secant slot through the obstruction, filling each hole with 3⁄4-inch crushed stone as the casing is removed to hold back the granite boulders and cobbles, and allowing the sheetpiles to be driven through the crushed stone.

Siemens/Samsung accepted the DTH under-reaming method and secured the product from THL Foundation Equipment Ltd in Singapore. It was decided to drill from surface, which would include drilling through sand, boulders and nearly 20 feet of sticky clay. After initial problems trying to drill the clay dry, a water pump was brought to site. Up to 17 gallons per minute of water was injected, which liquefied the sticky clay. This enabled each hole to be drilled with confidence. When fresh water was unavailable, sea water was used, after which the Numa Champion 240 hammer was flushed with air and rock drill oil, then cycled a number of times to ensure oxidation was limited on all internal components of the hammer.

The plan was to drill in the sequence 1-3-2-5-4-7-6 and so on along the line. The holes were filled with 3⁄4-inch crushed stone as the casing was retracted to hold back the granite boulders. This would enable the sheetpiles to be driven through the crushed stone. The concern was that sufficient crushed stone could be ejected while drilling the secondary holes, causing boulders or cobbles to slump into the slot. In fact, only one sheetpile refused in more than 90 holes drilled, which easily was remediated by drilling two additional holes at that location.

Drilling was carried out in one 12-hour shift per day. The best production was five 59-foot holes in a single shift, while the overall average was three-plus holes in a single shift. In this project, more than 90 holes were successfully drilled to an average depth of 59 feet.

Due to the success of this method, the contractor extended the original scope of work of drilling the temporary cofferdam sheetpile slot to include the two permanent sheetpile lines on either side of the intake channel.