The Hitra portal: Soon to be the start of a 5.3 km undersea journey.

But the drillers, using Atlas Copco equipment, have been so successful that they were able to pull it off early, in September.

And that has brought the opening day of the tunnel forward to Midsummer Eve (June 23) next year - a full year ahead of the original schedule.

The plan to construct the three-lane subsea tunnel between the mainland and the island of Hitra to replace the existing ferry service was the subject of much discussion back in the early 1990s.

Residents in the area feared the project would have a damaging impact on their beautiful surroundings, both during construction and when it came into service. They were worried about the prospect of dramatic change and its effect on both their environment and their lives.

But Statens Vegvesen, the Norwegian highway authority, and the consultation committee of residents co-operated to find a solution and agreement was finally reached. It resulted in the 5.65 km-long tunnel being completed ahead of time and opening in 1994.

The tunnel and its environs quickly became part of the local surroundings - so much so that residents happily supported plans for a similar second tunnel between Hitra and its neighbouring island Frøya.

The same team of Statens Vegvesen and the contractor Selmer got the project under way early last year and achieved breakthrough recently. The tunnel will link the two islands - each with a population of 4,000 - and provide an all-weather route from Frøya to the mainland.

Two-lane road

The 5.3 km-long crossing is being developed from both the Hitra and Frøya portals and 3.6 km of the tunnel is beneath the sea. The tunnel will have a two-lane road and its cross-section is 55m². There will be nine niches for emergency lay-bys and three truck turning bays.

Of the 300,000 m³ being excavated, some 9,000 m³ is in poor ground where 6- 8m x 32 mm rebar spiles are cement-grouted into 55- 64 mm holes drilled by the Atlas Copco Rocket Boomer 353S. The Unigrout system equipped with thenew, user-friendly PUMPAC grout pump, based on the same double-acting principle as the well-known ZB range of pumps, is used for this part of the operations. The spiles are installed over the crown in umbrellas with 2- 3 m overlap. Radial bolting, using 2.4- 4 m-long rockbolts, is also undertaken as required.

It was estimated that 4,000 spiling bolts would be needed for the whole contract, but more than double that amount has been used.

The rocks of the area are of the Precambrian era with gradual transitions between various gneissic rocks such as granite gneiss, micagneiss and migmatite. A few bands of limestone/marble have also been recorded.

Poor conditions

During final design in 1995, core drilling revealed exceptionally poor ground conditions, which resulted in a realignment of the proposed tunnel. Refraction seismic measurements revealed more weakness zones than for any previous undersea tunnels constructed in Norway. Two of the weakest zones are on the alignment and one is in the Tarva fault, which can be followed more than 150 km towards the north-west on the Norwegian mainland.

All face, probe hole and rockbolt drilling has been carried out by a single drill rig at each of the two faces. Both rigs are Atlas Copco Rocket Boomer 353S units equipped with COP 1838 rock drills. Using Atlas Copco Rock Tools, they drill up to 77 holes of 5m x 51mm for a standard round - but the operator can select a variety of patterns down to 73 holes, according to rock conditions. Other Atlas Copco equipment used in the tunnel includes 5.5 m R35 Enduro drill rods and 48 mm ballistic bits, type 77.

The two rigs are equipped with systems for contour profiling and scanning of the face. Using a single laser beam as a reference point, the rig can be accurately positioned at the face to drill the selected hole pattern and the operator can also decide on any trimming holes required to tidy up the previous round. This arrangement affords the facility for fast and accurate drilling with the minimum number of holes, while keeping overbreak within tight limits.

The Hitra portal: Soon to be the start of a 5.3 km undersea journey.

Efficient drilling

Drilling takes approximately 75 minutes per round and five rounds are normally drilled and blasted in each 24-hour cycle. The maximum advance in each heading is 110 metres per week.

The drill rigs are also equipped with RAS mechanised rod-adding systems, which consist of a control unit and two mechanical grippers mounted on the feed beam. All loading is carried out by excavators with a loading rate of 600 tonnes per hour - and it takes about 45 minutes to clean up after a full round.

Drill rig operator Sigvart Bolli has a unique role in the operations - he doubles as a loader driver and is also the shift boss!

Asked his opinion of the Atlas Copco equipment being used at the tunnel site, he replies: "Atlas Copco is the only drilling equipment for me!"

Site Manager Roar Sve, who has played a leading role since 1986 in five projects involving seven tunnels, says: "Because of the difficult conditions, this has been the most exciting project I have been involved in.

"Ground conditions were sometimes so poor that parts of the tunnel excavated themselves! But overall, the project is very successful and I am extremely pleased."