tunnelbuilder.com - CT- Bolt company profile

Orsta Staalindustri AS started manufacturing rockbolts 25 years ago at its plant, situated on the Norwegian coast, 50 km south of Alesund. Some 200 people are employed at Orsta, where resin anchored rebars and mechanically anchored bolts have been main product lines for many years. Around ten years ago, the CT-Bolt was developed to provide a 50-year life in aggressive environments, such as those in the tunnels being developed beneath the fjords. It was immediately accepted as the standard rock bolt for fjord crossings in Norway, and has since been used in projects in a number of other countries.

Rapid Installation
The new CT-Bolt can be installed manually or by mechanised rockbolting rigs into a pre-drilled 44-51 mm hole. It is anchored with an expansion shell, and is pre-stressed during installation. The bolt gives immediate support, and is easily grouted. A polyethylene sleeve seals the bolt against corrosion. Mechanised rockbolters can drill and install a 2.4 m-long CT-Bolt in 3.5 minutes, including grouting.

Easy Grouting
The CT-Bolt has a special hemispherical dome at its outer end, which holds the rock loading on the plate, and serves as a grouting chamber. When grout is pumped through the dome, it flows along the bolt inside the polyethylene sleeve to the back of the hole. From here it spreads back between the sleeve and the sides of the hole, uniformly filling the annulus and expelling any trapped air and water. The result is a solid, full-length anchor.

Click on photographs to enlarge

Advantages
The CT-Bolt combines the benefits of immediate point anchor support and a post-groutable fixture. Once the grout is placed and cured, the CT-Bolt becomes a permanent support, providing the user with both early and long term safety. If the area has to be shotcreted before the bolt is grouted, a tube can be attached to the dome and plate to facilitate post-grouting. Once grouted, the polyethylene sleeve seals the CT-Bolt against corrosion, making it an attractive, reliable and long-lived support.

The chart shows the result of tensile tests by independent laboratories. The displacement and load characteristics of point anchored and fully grouted rockbolts is demonstrated. The CT-M22 rockbolt has a yielding load of 230 kN (51,600 lb) and failure load of 290 kN (65,000 lb) after the grout is cured. Bolt lengths range from 1.5 m to 6.0 m.

References
CT-Bolts have been applied in all manner of major tunnels in recent years. Transportation projects include: the railway tunnels at Lotschberg in Switzerland, Romeriksporten in Norway, and Hallandsas in Sweden; and subsea tunnels at Hitra and Froya in Norway, under the Alps at Mont Blanc between Italy and France, beneath Hvalfjordur in Iceland, and along the M5 motorway in Sydney, Australia.

CT-Bolts are also being adopted as reliable, long life support in rock storage caverns for oil and gas in countries such as Singapore, Norway, Sweden and Portugal, and are finding use in mines in Australia, Finland, and Turkey.

For further details of specifications and references contact Orsta Staalindustri AS, N-6100 Orsta, Norway, tel +47 7004 7000, fax +47 7004 7004, e-mail firmapost@orstastaal.no, or visit www.ct-bolt.com