Tool saving

Korean contractor Sung Bo Development is achieving a +30% increased life span of its Sandvik Alpha tool system and has eliminated rod &gooseneck' breakages drilling through diffi cult, fragmented rock on the Pyeoungtak-Eumsung Highway 40 section 7 contract.

Running across the country from the port of Pyeoungtak in the west, South Korea's Pyeoungtak-West Ansung Highway 40 will run parallel to the heavily congested Highway 50 approximately 21 km to its north. Sung Bo's 5 km long Section 7 contract includes 1150 m of a 2300 m tunnel and also features three elevated bridges.

Construction of the 2300 m long tunnel has been split equally between Sung Bo Development from the Western Portal and NanSun Construction Co working from the Eastern Portal. It is the tunnel that has presented both contractors with problems.

“Right from the start of excavation we recognised we would experience trouble with the rock,” said Sung Bo Project Manager, Mr Lee. “The rock, black Biotite Gneiss is in very poor condition and is non- homogenous,” he added. As a result the contractor has opted to excavate the profi le in two sections - the upper one across a 12 m base and 7 m high and the second, the lower section across the 14 m base and a height of 1,5 m.

Both contractors opted to use Tamrock drilling rigs; Sung Bo an Axera 3 boom computerised rig and NanSun a Titan 316 - 150 rig. Sung Bo, with experience of Sandvik's recently introduced Alpha 330 tools system in Korea on a similar tunnel project again opted for the new system.

Sandvik's conventional R32 thread at the front of the drifter rod for 45 mm bits was experiencing problems with the powerful 20 kW drills, particularly when the drill hit the rock at an oblique angle - causing the rod to bend. Breakages occurred typically at the gooseneck between the full rod section and the R32 thread.

The Sandvik Alpha 330 system benefi ts from a stronger and more rigid Alpha 330 thread - just 1mm larger in diameter but with a 14% larger cross section than the R32. To improve rigidity further, Sandvik has introduced a new guiding function between the rod and bit skirt. In addition to reducing the risk of bending, the bit skirt completely covers the thread to reduce the risk of corrosion fatigue.