What are the adverse effects of joints on rock mass quality?
Answers
Answer:
Classification of the rock mass
The rock mass quality (Q) is a very sensitive index and its value varies from 0.001 to 1000. Use of the Q-system is specifically recommended for tunnels and caverns with an arched roof. On the basis of the Q-value, the rock masses are classified into nine categories (Table 8.9). Rock mass quality varies from Qmin to Qmax, so the average rock mass quality of (Qmax × Qmin)1/2 may be assumed in the design calculations.
Table 8.9. Classification of Rock Mass Based on Q-Values
Q Group Classification
0.001–0.01 Exceptionally poor
0.01–0.1 3 Extremely poor
0.1–1 Very poor
1–4 2 Poor
4–10 Fair
10–40 Good
40–100 1 Very good
100–400 Extremely good
400–1000 Exceptionally good
The Q-values will be higher where a tunnel boring machine (TBM) or a road header is used to smooth the surface of excavation. The Q-value, on the other hand, in the tunnel blast method will be lower because of high overbreaks and the development of new fractures. To minimize the negative effect of blasting on Q, use a controlled blasting technique. The blasting effects are better in the rock masses having a Q-value between 1 and 30.