Engineering Rock Mechanics

Appendix C: Rock mass classification fables - RMR and Q 497

(C) Effect of discontinuity orientations in tunnelling

Strike perpendicular to tunnel axis
drive with dip
dip 45-90 dip 2045 dip 4590 dip 20-45
very favourable favourable fair unfavourable

drive against dip

Strike parallel to tunnel axis
dip 2045 dip 45-90 dip 0-20
fair very unfavourable fair

Irrespective of strike

(D) Rating adjustment for discontinuity orientations

Effect of discontinuity Very fa- Favour- Fair Unfavour- Very unfa-
orientation (from Table C) vourable able able vourable

Ratings:
Tunnels and mines 0 -2 -5 -10 -12
Foundations 0 -2 -7 -15 -25
Slopes 0 -5 -25 -50 -60

(E) Rock mass classes determined from total ratings

Rating 100-81 80-61 6041 40-21 <^20
Class no. I I1 I11 IV V
Description very good rock good rock fair rock poor rock very poor rock

(F) Interpretation of rock mass classes

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Class no.: I I1 I11 IV V

Average stand-up time 20 yr for 1 yr for 1 wk for 10 h for 30 min for

15-m span 10-m span 5-m span 2.5-m span 1-m span

Cohesion of rock mass >400 300400 20@-300 100-200 <lo0

(ma)

(deg)

Friction angle of rock mass >45 3545 25-35 15-25 t15