Classification of Rock Mass
1. Terzaghi's Rock Mass Classification
Terzaghi classified the rock on the basis of presence of joint. He defines the rock having no joint as intact rock and rock that squeezes primarily from
mineral swelling as swelling rock and so on.
2. Rock Quality Designation Index (RQD)
This classification is based on the value of RQD as the following.
RQD % |
Rock Class |
| <25 | Very poor rock |
| 25-50 | Poor rock |
| 50-75 | Fair rock |
| 75-90 | Good rock |
| 90-100 | Excellent rock |
3. Bieniawski's Rock Mass Classificaiton
(Rock Mass Rating [RMR])
This classification is based on total or overall rating obtained from different aspects as mentioned below.
a. Intact rock strength
It is very much similar to the compressive strength of brick. We calculate the strength of the rock in similar manner
b. Spacing
It is based on the perpendicular distance between joints in the rock
c. Rock Quality Designation Index (RQD)
It is mentioned in the above point.
d. Condition of discontinuity
The condition of discontinuities like persistence, aperature, in filling, weathering, roughness of the discontinuity is considered for rock mass rating.
e. Ground water condition
This is based on the amount of water concent in the rock.
| Total Rating | 100-81 | 80-61 | 60-41 | 40-21 | <21 |
| Class number | I | II | III | IV | V |
| Description | Very good
rock |
Good rock | Fair rock | Poor rock | Very Poor rock |
Note :
If 'I' class rock, then it gives 20 years durability without support for diameter of 15m.
If 'V' class rock, then it gives 30 min durability without support for diameter of 1m.
4. Q – Value system
This classification is based on the value of Q, it is given by the following formula.
Where,
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