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Q-system of rock mass classification

Short answer

The Q-system rates tunnel rock mass quality by the index Q = (RQD/J_n)×( J_r / J_a )×( J_w / SRF). Q ranges from 0.001 to 1000, with classes from exceptionally poor to exceptionally good, and support is determined using an equivalent dimension chart.

Describe the Q-system (rock tunnelling quality index) of rock mass classification, its parameters and rock mass classes. How is Q calculated?

The Q-system was developed by Barton, Lien and Lunde (1974) at the Norwegian Geotechnical Institute from many tunnel case histories. It gives the rock tunnelling quality index Q, used to classify rock mass and to design tunnel support (shotcrete, rock bolts, spans).

Calculation of Q

Q=RQDJn×JrJa×JwSRFQ = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF}
TermMeaningRepresentsRange
RQDRQDRock Quality DesignationDegree of jointing0 to 100
JnJ_nJoint set numberBlock size0.5 to 20
JrJ_rJoint roughness numberShear strength between blocks0.5 to 4
JaJ_aJoint alteration numberShear strength between blocks0.75 to 20
JwJ_wJoint water reduction factorWater pressure effect0.05 to 1
SRFSRFStress reduction factorActive stress0.5 to 20

The three quotients mean: RQD/JnRQD/J_n = relative block size, Jr/JaJ_r/J_a = inter-block shear strength, Jw/SRFJ_w/SRF = active stress. Q varies from 0.001 to 1000.

Example: RQD=75, Jn=9, Jr=3, Ja=1, Jw=1, SRF=1RQD = 75,\ J_n = 9,\ J_r = 3,\ J_a = 1,\ J_w = 1,\ SRF = 1 gives Q=(75/9)(3/1)(1/1)=25Q = (75/9)(3/1)(1/1) = 25, i.e. good rock.

Rock mass classes

Q valueClass
400 to 1000Exceptionally good
100 to 400Extremely good
40 to 100Very good
10 to 40Good
4 to 10Fair
1 to 4Poor
0.1 to 1Very poor
0.01 to 0.1Extremely poor
0.001 to 0.01Exceptionally poor

The support is then chosen from the Q chart using the equivalent dimension De=span or height/ESRD_e = \text{span or height} / ESR, where ESR is the excavation support ratio.

✓ Answer checked by Claude Opus 5.5 (high effort)

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