Stand-by in the Schulwald tunnel: One of four Rocket Boomer 352S rigs waits for an excavator to dig out soft rock.

The joint venture ATAC/KDD, consisting of the contractors Alpine Bau, Ilbau, Beton und Monierbau, Östu Stettin, Hinteregger & Söhne and Thyssen Schachtbau, is responsible for a 12-km stretch of the new rail link.

On this section of the railway is the Schulwald twin-track tunnel, the longest on the entire project. The rock formation in the 4.5 km-long tunnel is soft and excavators are used to dig out the rock. Blasting is carried out only occasionally to break up the rock for the excavators.

Rock support

Four Atlas Copco two-boom Rocket Boomer 352 S rigs are employed solely on rock support work. They are equipped with 14-foot feeds and hydraulic drill supports for extension drilling. And they use Atlas Copco Rock Tools' R32 drill rods and 48, 51 or 57 mm bits for drilling holes up to 12 m long.

The rigs drill 4-6 m-long holes for spiling, in which 28 mm diameter rebars are pushed into the holes, 3-6 m holes for the final support rockbolts, and holes up to 12 m long for face support in poor conditions. Availability of the rigs, which are in operation for eight hours per shift, is more than 90%.

One rig is used at each of the four "attack" points, which have a daily advance of 3-5 m and the total tunnel advance is 12 m per day. The rockdrills are Atlas Copco COP 1838 units, with R32 shanks, that produce a penetration rate of 4-5 m/min with exceptionally low wear on the drill steel and bits. The drilling is carried out with air-flushing and water mist, which prevents excess water from entering the rock and adequately deals with the dust.

Difficult task

To protect the tunnel from water inflows, 60-70 m-deep wells are drilled to lower the groundwater table, 10-15 m from the tunnel.

But it's a difficult task, as the soft rock is spongelike and is reluctant to give up water. However, the final tunnel lining will be plastic-shielded (installation starts next spring) for protection against water, after which the groundwater level will return to normal.

At the Schulwald twin-track rail tunnel: Site Manager Franz Holzfeind listens to Atlas Copco's Karl-Heinz Wennmohs.

Site Manager Franz Holzfeind says: "We are very pleased with the progress we are making with the help of the four Atlas Copco Boomer rigs, which are coping well in the soft rock and generally poor working conditions.

"In the difficult area of rock bolting, they are helping us to stay on schedule. Availability of the rigs is good, and the life of the drilling equipment is more than satisfactory."