Sung Bo say breakages go in Korea

Progress continues well on the 2.3km long tunnel, part of Pyeoungtak-Eumsung Highway 40 contract in Korea, with over 1.5km completed and breakthrough planned for October this year.

Sung Bo Development is contractor for a 5km long section of the highway that includes 1,150m of the tunnel, driven from the west. NanSun Construction is contractor for the balance of the tunnel, driven from the eastern portal. The twin dual lane tunnels are being built to a final section of 14m by 8.5m.

A senior member of Sung Bo's site team said the black biotite gneiss being driven through was in very poor condition and non-homogenous. A top and bottom heading was required said the source. The top heading is excavated to a 12m by 7m section. The bottom heading completes the profile with a 14m by 1.5m section.

Sung Bo is using a Tamrock Axera three-boom computerised drill rig. For the top heading, 120 easers of 48mm diameter are drilled to a hole depth of 2.2m in 80 minutes. Contour holes are arranged in two lines 740mm apart and between 600-800mm hole spacing. A v-cut made up of 14 holes angled at 60º was found to give the optimum pull.

The round is charged with dynamite in the cut and easers, with Hinex used for the trimming holes. Twice daily blasts are mucked by a 54 strong fleet of 15t dump trucks. The back is shotcreted immediately after mucking and scaling. The lower section is benched using 20 holes and two lines of trimmers.

The site source said: "The fragmented rock provides extensive problems with under and over break requiring shotcreting, bolting and reinforcement bars along the complete excavation." Sung Bo opted for Sandvik's Alpha 330 drill tools system, which is designed to address bending and breakages associated with conventional R32 threads at the front of the drifter rod when coupled with newer 20kW drills, a problem exacerbated in fractured ground.

Sung Bo reports breakages between the R32 coupling and the rod full section have been eliminated and claims a 30% increased life for its drill tools with the Alpha system.