Rock mass classification and discontinuities

Planes of discontinuity

A discontinuity is any plane of weakness or break in the continuity of a rock mass that has little or no tensile strength. Main types:

• Bedding planes in sedimentary rocks.
• Joints - fractures without displacement.
• Faults and shear zones - fractures with displacement.
• Foliation and cleavage planes in metamorphic rocks.
• Unconformity surfaces and contacts between rock types.
• Boundaries of dykes and sills, and sheeting joints in massive igneous rocks.

Their orientation, spacing, persistence, aperture, roughness, infilling and water condition control the strength and permeability of the rock mass.

RMR system (Bieniawski)

Rock Mass Rating gives a rating (0 - 100) as the sum of the ratings for five parameters, followed by an adjustment for joint orientation:

1. Uniaxial compressive strength of the intact rock (0 - 15)
2. RQD (3 - 20)
3. Spacing of discontinuities (5 - 20)
4. Condition of discontinuities (0 - 30)
5. Groundwater (0 - 15)
6. Orientation adjustment (0 to -12 for tunnels)

Classes: I very good (81 - 100), II good (61 - 80), III fair (41 - 60), IV poor (21 - 40), V very poor (<21). It is used to find the support requirement and stand-up time.

Q system (Barton et al.)

The tunnelling quality index:

$$Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF}$$

where $J_n$ = joint set number, $J_r$ = joint roughness number, $J_a$ = joint alteration number, $J_w$ = joint water reduction factor and SRF = stress reduction factor. The three quotients represent block size, inter-block shear strength and active stress. Q ranges from 0.001 to 1000 on a logarithmic scale: exceptionally poor (0.001 - 0.01), extremely poor (0.01 - 0.1), very poor (0.1 - 1), poor (1 - 4), fair (4 - 10), good (10 - 40), very good (40 - 100), extremely good (100 - 400) and exceptionally good (400 - 1000). Q is used to select the support type in tunnels and caverns.