Rock classification - Geotechnical Investigation and Design Tables - page 34

3.7 Rock quality designation
•

RQD (%) is a measure of the degree of fracturing. This is influenced also by quality
of drilling, and handling of the rock cores.
- Many variations for measurement of this supposedly simple measurement.
- Drilling induced fractures should not be included in the RQD measurement.
- The domain rather then the core length should be used to assess the RQD.
Different values result if the RQD is measured in a per- metre length or a
domain area. The latter represents the true RQD values while the former
would have an averaging effect.
- RQD is dependent on the borehole orientation. An inclined borehole adjacent
to a vertical borehole is expected to give a different RQD value.

3.8 Rock strength
•

This Table refers to the strength of the intact rock material and not to the strength
of the rock mass, which may be considerably weaker due to the effect of rock
defects.

Table 3.8 Rock strength.

Strength Symbol Field assessment

By hand Hammer with hand held specimen

Extremely low EL Easily remoulded to a material with soil properties.

Very low VL Easily crumbled in 1 hand.

Low L Broken into pieces in 1 hand.

Medium M Broken with difficulty in 2 hands. Easily broken with light blow (thud).

High H 1 firm blow to break (rings).

Very high VH 1 blow to break (rings)

>

Extremely high EH Many blows to break (rings).

3.9 Rock hardness
•

The rock hardness is not the same as the rock strength.

Table 3.9 Field assessment of hardness.

Description Moh's Characteristic using pocket knife

of hardness hardness

Rock dust Scratch marks Knife damage

Friable 1-2 Little powder None. Easily crumbled. Too soft No damage

to cut. Crumbled by hand

Low 2-4 Heavy trace Deeply gouged

Moderately 4-6 Significant Readily visible (after

hard trace of powder powder blown away)

Hard 6-8 Little powder Faintly visible Slight damaged; trace

of steel on rock

Very hard 8-10 None None Damaged; steel

left on rock