TUNNELLING IN POOR QUALITY ROCK

DRIFT AV VEGTUNNELER I DARLIG FJELL

The bedrock in Norway normally consists of competent Precambrian and Paleozoic rocks. Road tunnels are blasted with round lengths varying from 2,4 to 4 metres. On the average only 1-3% of the total road tunnel length require lining for roof support. In general, the tunnels are worked with slight problems without the need for the equipment and skill required for tunnelling in incompentent rocks. Tunnelling in competent rocks must be aimed at obtaining a stable rock surface with a minimum demand for rock reinforcement. This aim requires close and accurate drilling and careful blasting. When meeting incompetent rock, problems may arise. On the basis of a survey on existing tunnelling methods in such rocks, remedial measures for Norwegian conditions are discussed. Steel support is not well suited, owing to extensive preparations and costs coupled with a small and uncertain demand. Rock bolting is useful and covers many conditions. Systematic bolting, however, is timeconsuming as drivage support, even when applying quick hardening resins for anchorage. In poorer quality rock it is recommended that shotcreting equipment should be readily available on site. Shotcreting is rapid and comparatively inexpensive when applying the new, high capacity equipment available. The method should not be used uncritically, however, since specifications require that shotcrete linings have to be supplied with a waterproofed concrete lining in moist rock, which normally is the case where reinforcement is needed. Scaling should be the dominant security measure. But when the need for scaling leads to lower progress, it should be replaced by rock reinforcement. Most effort must be concentrated on improving the blasting technique. Close and accurate drilling, combined with small and adjusted charges transfer a minimum of strain to the surrounding rock. In incompetent rock, reduction of the pull of the rounds is recommended rather than two step tunnelling with a pilot bore. Where poor quality rock is predicted form the geological survey, but not investigated by core drilling, some holes in each round should be drilled two to three rounds ahead in order to obtain information on rock quality and leakages. /Author/

  • Corporate Authors:

    Norwegian Road Research Laboratory

    Postboks 8109, Gaustadalleen 25
    Oslo,   Norway 
  • Authors:
    • Gronhaug, A
  • Publication Date: 1975-9

Language

  • Norwegian

Media Info

Subject/Index Terms

Filing Info

  • Accession Number: 00131558
  • Record Type: Publication
  • Report/Paper Numbers: No. 48
  • Files: TRIS
  • Created Date: May 14 1976 12:00AM