Question

For a fair rock, the Rock Quality Designation (RQD) in percentage varies from:

• A. 0 to 25
• B. 25 to 50
• C. 50 to 75
• D. 75 to 90

Hint:

Remember: 50–75\% RQD range indicates fair rock quality.

Answer 1

The Correct Option is C

To determine the Rock Quality Designation (RQD) range for fair rock quality, we analyze standard rock mass classification systems.

1. Understanding RQD:
RQD is a measure of rock mass quality calculated as the percentage of intact rock pieces longer than 100 mm (4 inches) in a drill core.

2. Standard RQD Classification:
According to Deere's classification and ISRM standards:

3. RQD Ranges and Quality:
- 0-25%: Very poor rock
- 25-50%: Poor rock
- 50-75%: Fair rock (correct answer)
- 75-90%: Good rock
- 90-100%: Excellent rock

4. Characteristics of Fair Rock (50-75% RQD):
- Moderate fracturing with some intact rock
- Requires systematic support in excavations
- May have localized stability issues
- Typical of many construction sites

5. Engineering Implications:
When RQD is in the 50-75% range: - Rock bolts are generally required
- Shotcrete may be necessary
- Excavation rates are moderate
- Stand-up time is limited

6. Field Identification:
Fair rock typically shows: - 2-4 fractures per meter
- Partial weathering along discontinuities
- Some rock degradation
- Block sizes 10-30 cm

Conclusion:
For fair rock quality, the RQD percentage ranges from 50 to 75.

Final Answer:
The correct option is: 50 to 75.