Korean tunnel success for new Sandvik rock tool

Sandvik Mining and Construction Tools recently introduced Sandvik Alpha 330 system is providing Korean contractors Sung Bo Development with an extended 30% lifespan in difficult drilling conditions in a 2.3 km tunnel; forming part of South Korea’s Pyeoungtak-West Ansung Highway 40. Running across the country from the port of Pyeoungtak in the west, the new highway will run parallel to the heavily congested Highway 50 approximately 20 km to its north. Traffic currently tends to use Highway 50 from the port to the tourist areas of the east and is a mix of container and tourist traffic. During peak summer periods more than 200,000 vehicles have been recorded using the highway during a 20 day period. It is anticipated completion of the new Highway 40 will take up to 50% of the traffic flow.

Starting at the interchange with the north-south Highway 15, approximately 50 km south of Seoul, the first 32 km of the proposed 64 km section of the new highway has been completed, crossing the major north-south Highways 1 and 35 with interchanges. Construction on the remaining 32 km from West Ansung to Eumsung is underway with Sung Bo responsible for Section 7. Eventually it is planned to extend the highway a further 36 km linking Eumsung to Chungju and onto Taebak in the east.

Sung Bo's 5 km long Section 7 contract includes 1150 m of a 2300 m tunnel and also features three elevated bridges. The 5 km section rises from its 150 m mean level at the Highway 15 interchange. Over the first 1260 m it features a 0.75% gradient, crossing the 105 m and 300 m long elevated sections. The gradient increases to 2.35% over a 630 m long elevated bridge to the West Portal of the tunnel to give a total height increase of 50 m.

Tunnel construction

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. Breakthrough in the middle was anticipated by October 2005.
It is the tunnel that has presented both contractors with problems. Twin dual lane tunnels feature a 14 m wide x 8.5 m high profile. "Right from the start of excavation we recognised we would experience trouble with the rock," says Sung Bo Project Manager, Mr Lee. He adds, "The rock, black Biotite Gneiss is in very poor condition and is non-homogenous." As a result the contractor has opted to excavate the profile 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.

Sandvik Alpha 330

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.
The Sandvik Alpha system was originally designed for the mining industry as contractors continue to exploit the power and speed of more powerful rock drills to increase production the standard for blast holes which continued to be 45 mm.

Sandvik's conventional R32 thread at the front of the drifter rod for 45 mm bits was experiencing problems with the powerful 20 kW rigs, particularly when the drill bit was hitting the rock at an oblique angle – causing the rod to bend. Breakages were occurring typically at the gooseneck between the full rod section and the R32 thread. Sandvik's primary aim was therefore to minimise the effects of the reactive bending forces that cause the drill bit – under the influence of feedforce, impact power and rotation – to deflect when collaring holes into the many oblique surfaces on the average drift or tunnel face, and when drilling into cracks or oblique changes in rock resistance inside the hole.

The Sandvik Alpha 330 system benefits 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. With the Sandvik Alpha 330 system featuring a more robust connection between the rod and bit, the overall frequency of breakages is also greatly reduced. The resulting tool economy, together with more accurate collaring and straighter holes to give better fragmentation and improved profile-control reduces the overall cost of drifting and tunnelling. Today the benefits of the new system are benefiting both the mining and construction areas with tunnelling contractor being able to use 43, 45, 48 or 51 mm diameter bits with the Sandvik Alpha system.

Increased life span

The new system has proved to be highly effective for Sung Bo providing an extended 30% life span compared with the previous R32 system. According to the rig operator, Mr Park, the R32 rods also experienced regular 'gooseneck' breakages. "These have now been eliminated with no breakages to date."
Using 4.9 m long rod type T38-H35 Alpha 330, the system quickly proved successful providing a higher quality and cleaner hole.
Since the start of tunnelling Sung Bo is regularly achieving 4000 m/rod. The contractor is drilling 120 face holes in the top profile using Sandvik spherical Alpha 330, 48 mm diameter bits to a depth of 2.2 m. These are completed in 80 minutes with each taking an average of 1.5 minutes. The contour holes around the outer perimeter are spaced at 600 mm distance with the second line 800 mm apart. Spacing between the lines is 740 mm.

With no reamer holes, Sung Bo opted for a total of 14 V-cut holes at angle of 60° to achieve the optimum pull rate.
On completion of the drilling, the full face is charged within an hour using Hinex for the contour holes and dynamite in other holes. Blasting is carried out twice a day in each tunnel with each blast producing 810 t, excavated and hauled by a fleet of 54 x 15 t dump truck movements. Once blown and cleared the roof is fragmented requiring immediate shotcreting.

For the second lower section 20 holes are drilled across the 14 m wide x 1.5 m high profile and include just two contour lines and 15 holes along the bottom. No V cut holes are required in drilling operations carried out in just 30 minutes; producing 180 t of rock. "The fragmented rock provides extensive problems with under and over break requiring shotcreting, bolting and reinforcement bars along the complete excavation," confirms the Project Manager. He adds, "As a result of the difficult rock conditions, 2.2 m holes were selected as the optimum length, with a total of 16 x 4 m long rock bolts across the profile perimeter using the Tamrock Axera."

"We were not aware that the rock was going to be so difficult. It is so serious that the drilling period has been doubled as a direct result.” He says, adding, “Without the Tamrock rig with its fast penetration rates, the delays may have been longer."
He concludes, “Even though the rock is very soft, it is also extremely abrasive.” Despite this the contractor has been getting good service from the Sandvik drill bits grinding each bit three times on-site.By the end of May 2005, 800 m had been completed in the left portal and 700 m from its right portal.
During excavation of the West portals, Sung Bo used Sandvik 105 mm diameter bit to drill 34 holes to a depth of 22 m around the profile perimeter.