Geomechanics & Ground Control

Rock Mass Classification (RMR, Q)

Adding up the six RMR ratings and multiplying the six Q-system parameters to score a rock mass for support design.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Empirical classifications turn observations into a design score. Bieniawski's RMR *sums* ratings for six parameters (UCS, RQD, joint spacing, joint condition, groundwater) and applies an orientation adjustment, giving a value out of 100 that maps to rock-mass classes. Barton's Q-system *multiplies* six parameters as three quotients: block size (

R Q D / J_{n}

), inter-block shear (

J_{r} / J_{a}

) and active stress (

J_{w} / S R F

The Core Formula Matrix

RMR:

R M R = \sum (6 parameter ratings) + orientation adjustment

(the adjustment is usually negative).

Q-system:

Q = \frac{R Q D}{J _{n}} \times \frac{J _{r}}{J _{a}} \times \frac{J _{w}}{S R F}

J_{n}

= joint-set number,

J_{r}

= roughness,

J_{a}

= alteration,

J_{w}

= water,

S R F

= stress reduction factor.

The ‘IIT Traps’

⚠
RMR adds, Q multiplies. Mixing the operations is the most common error.
⚠
The RMR orientation adjustment is subtracted for unfavourable discontinuities — don't drop it.
⚠
Q's three quotients each have a meaning (block size, shear, stress); keep numerator/denominator pairs together.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

Parameter ratings are

15, 10, 20, 20, 10

with an orientation adjustment of

- 5

. The RMR is:

Q2MEDIUM2 Marks · NAT

A rock mass has parameter ratings

12, 13, 20, 25, 15

and an orientation adjustment of

- 10

. Its RMR is ______. (Round off to the nearest whole number.)

Q3HARD2 Marks · NAT

For

R Q D = 75

J_{n} = 9

J_{r} = 2

J_{a} = 1

J_{w} = 1

S R F = 2.5

, the Q-value is ______. (Round off to two decimal places.)