RECENT EXPERIENCES WITH THE Q-SYSTEM OF TUNNEL SUPPORT DESIGN

The Q-system of rock mass classification and support design is based on a numerical assessment of the rock mass quality using six different parameters. The six parameters consist of the RQD, the number of joint sets, the roughness of the most unfavourable joint sets, the roughness of the most unfavourable joint or discontinuity, the degree of alteration or filling of the most unfavourable joint or discontinuity, the degree of water inflow, and the stress condition. Another classification system, the Geomechanics Classification is also based on six parameters. Qualitative differences between the two methods are discussed. The 200 case records that were analysed when developing the Q-system, included more than 30 cases of permanently unsupported openings. An analysis of the rock mass characteristics involved has shown that certain characteristics are essential if an excavation is to be left permanently unsupported. If the maximum unsupported span for a given Q-value is exceeded, the safe life of the excavation may be shortened. A preliminary attempt is made to correlate stand-up time, rock mass quality Q, and span width. The Q-system has been applied on several projects in Scandinavia and abroad since its development in 1973/1974. An example of a recent application is given in detail. The preliminary estimates of permanent support for a 19 metres span underground power house were obtained from an analysis of corelogs. In a subsequent site visit the Q-system was applied in-situ. The final estimates of permanent support were found to compare well with the preliminary estimates. Core logs, seismic profiles and surface mapping were used as a basis for preliminary design of permanent support for the 9 meters span tailrace tunnel, again using the Q-system. This tunnel is presently under construction so comparison of predicted and actual support is not yet possible. /Author/

• Supplemental Notes:
  • Reprinted from Proceedings of the Symposium on Exploration for Rock Engineering, Johannesburg, November, 1976, Vol. I, pp 107-117.
• Corporate Authors:

  Norwegian Geotechnical Institute

  P.O. Box 40 Tasen
  Oslo 8,   Norway 
• Authors:
  • Barton, N
• Publication Date: 1978

Media Info

• Features: Appendices; Figures; References;
• Pagination: 11 p.

Subject/Index Terms

• TRT Terms: Bridge members; Excavation and tunneling; Field tests; Joints; Length; Mapping; Mass; Rocks; Roughness; Seismology; Tunnel supports
• Uncontrolled Terms: Bridge spans; Discontinuity; Rock mass
• Old TRIS Terms: Insitu methods; Joint
• Subject Areas: Bridges and other structures; Geotechnology; Highways;

Filing Info

• Accession Number: 00183661
• Record Type: Publication
• Report/Paper Numbers: Publication No. 119 Proceeding
• Files: TRIS
• Created Date: Dec 3 1978 12:00AM