TBM TUNNELS - ROCK CLASSES FOR GROUND SUPPORT | INTERNATIONAL
in given rock mass classes in such TBM tunnels can be reduced, further noting this is especially important where support quantities are limited and can be installed simultaneously with boring activities.
THE APPROACH The report explains the rock support methodology being pursued are based on stability and safety requirement for different scenarios, and takes the Q-method as a basis, including its Excavation Support Ratio (ESR) approach. The ESR values in the report’s diagrams generally have a correlation with those that Barton suggests for TBM tunnels, it adds. The report then discusses unlined TBM tunnels
having far less installed rock support compared to drill and blast method (based on literature and author experience), and that for TBM tunneling in jointed and faulted rock this reduction is “especially relevant” in some rock classes, and adjustments are required. The methodology chapter then concludes with a
short discussion on the correlation of the Q and RMR systems. Next, the following chapter discusses rock support
diagrams, noting that the guide presents two - for temporary rock support, and for moderately long-term use without secondary lining, respectively. Diagrams then given come from adapting the Q-diagrams and suggested adjustments (Barton & Grimstad, 2024) - but unlike the former do not use a logarithmic scale for the y-axis, and use a set of ESR per diagram.
The report says: “This change was made so that the
curves can be read directly based on the TBM diameter.” The rock support diagrams produced have six Rock
Support Classes (RSC), which are each briefed, and then examples of rock support quantities discussed. Two tables are given, which in this article are combined and presented here. The report on the modified rock support methodology
also presents some special considerations for more challenging TBM drives, such as rock stress induced problems like squeezing and rock bursting, and high pressure groundwater inflows. These special geological considerations are discussed
in the following chapter in four information-packed pages, including two large and detailed tables, dealing with the stress and inflow issues, respectively. In the former, challenges are emphasized at weak zones, including those with clay or chemically-altered rock mass, but also competent rock with high stress and burst risk; and, on the latter, discussion noted the importance of probe-drilling, pre-excavation grouting (PEG) and potential for some use of segmental lining.
CONCLUSION At the end, the ITAtech report says it is a guideline that presents a model that uses empirical data and conclusions from literature to do two things with respect to rock support: to suggest better methodology and also quantities to use, respectively, when undertaking a TBM drive.
Table 1: Example of Q and RMR values, and rock support quantities, for a 19.5-ft (6m)-diameter TBM Rock Support Class
Approx Q Values RSC 1 >4 Approx RMR values >59
Support Type Local, 2m-long rock bolts
Quantity/linear metre Up to 1No bolt/m
RSC 2
1-4
50-59
Local, 2m bolts, mesh, shotcrete
1-3 No Bolts/m 1-1.5m2
0.1-0.5m3
mesh/m shotcrete/m RSC 3 0.06-1 30-50 2.5m bolts, mesh, shotcrete, ribs/ ring beams
3-7 No Bolts/m 5-15m2
0.5-1.5m3 40-150kg steel RSC 4 0.01-0.06 17-30
3m bolts, mesh, shotcrete, ribs/beams
RSC 5 0.001-0.01 5-17
Bolts, mesh, shotcrete, ribs/beams cast insitu
concrete through RSC5 sections Special measures matched RSC 6 <0.001 <5
to conditions, eg forepoling, pre-grouting, jet grouting, cast concrete, etc
Quantities depend on specific local conditions
Source: The table is a combination of Tables 4 & 5 from the report, with rock support quantities as suggested in different rock support classes by Barton and Scolari
6-10 No Bolts/m 15-27m2
1.5-3.0m3 120-300kg steel
Quantities depend on specific local conditions
mesh/m shotcrete/m mesh/m shotcrete/m
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