In the laboratory determination of California Bearing Ratio (CBR) test on soils as per IS : 2720 (Part 16)-1987, the CBR values are usually reported for penetration values of:
- A5.0 mm, 7.5 mm
- B3.5 mm, 5.0 mm
- C1.5 mm, 2.5 mm
- D2.5 mm, 5.0 mmCorrect
Solution
Concept
CBR is reported at 2.5 mm and 5.0 mm penetration as per IS 2720 (Part 16).
Formula
CBR (%) = (Test load / Standard load) × 100
Step-by-step solution
1. In the CBR test, a plunger is pushed into the soil at 1.25 mm/min. 2. Load is recorded at different penetrations. 3. The standard loads are given for 2.5 mm (1370 kg) and 5.0 mm (2055 kg). 4. CBR is calculated at 2.5 mm and 5.0 mm penetration. 5. Usually the CBR at 2.5 mm is higher and is reported. If the 5.0 mm value is higher, the test is repeated. If it is still higher, the 5.0 mm value is used.
Explanation
The CBR test compares the soil load with a standard crushed-stone load. The standard loads are fixed for 2.5 mm and 5.0 mm penetration. So the CBR is reported for these two values.
Why the other options are wrong
A: 7.5 mm is not a reporting penetration. B: 3.5 mm is not a standard value. C: 1.5 mm is not a standard value.
Shortcut
Remember 2.5 and 5.0 mm with standard loads 1370 kg and 2055 kg.
Common mistake
Mixing up the plunger speed of 1.25 mm/min with the penetration values.
Correct answer: D) 2.5 mm, 5.0 mm