Rock mass classification systems - Geotechnical Investigation and Design Tables - page 230

Table 18.9 Norwegian Q system (Barton et al., 1974).

Parameter Symbol Description

Rock mass quality Q (RQD/J n ) (J r /J a ) (J w /SRF) = × ×

Rock quality designation RQD (RQD/J n )

=

Relative Block Size: Useful for distinguishing

Joint set number J n massive, rock bursts prone rock

Joint roughness number J r (J r /J a ) Relative Frictional strength (of the least favourable

=

Joint alteration number J a joint set or filled discontinuity)

Joint water factor J w (J w /SRF) Relative effects of water, faulting, strength/

=

Stress reduction factor SRF stress ratio, squeezing or swelling (an “active''

stress term)

18.10 Relative block size
•

The relative block size is based on the RQD and the Joint set number.

•

Number value based on RQD ≥ 10.

Table 18.10 Relative block size (Barton et al., 1974).

Parameter/symbol Description Number value

Quality RQD value

Rock Very poor 0%-10% 10

Quality Very poor 10%-25% 15,20,25

Designation Poor 25%-50% 30, 35, 40, 45, 50

RQD Fair 50%-75% 55, 60, 65, 70, 75

Good 75%-90% 80, 85, 90

Excellent 90%-100% 95, 100

Joint set number Joint randomness

Joint sets No or few joints Massive 0.5-1.0

Number One 2.0

J n One random 3.0 +

Two 4.0

Tw o random 6.0 +

Three 9.0

Three

+

random 12

Four or more random, heavily jointed earth-like 15

+

Crushed rock 20

•

RQD in intervals of 5.

•

RQD can be measured directly or obtained from volumetric joint count.

•

For tunnel intersections use 3.0 × J n .

•

For portals use 2.0 × J n .

18.11 RQD from volumetric joint count
•

The RQD may also be assessed by the volumetric joint count.