In the laboratory determination of the California Bearing Ratio (CBR) of soils as per IS 2720 (Part 16) – 1987, the CBR values are usually reported for penetration values of:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 06/06/2024 (Evening)
Detailed Explanation
The California Bearing Ratio test compares how hard it is to push a standard plunger (50 mm diameter) into a soil sample with how hard it is to push it into a standard crushed stone. It is the basic test for designing flexible (bituminous) pavements, because it tells us how strong the subgrade is.
\[ \text{CBR} = \frac{\text{test load at a given penetration}}{\text{standard load at the same penetration}} \times 100 \]
The plunger is pushed at 1.25 mm/min, and loads are read at several penetrations. The standard loads are 1370 kg at 2.5 mm and 2055 kg at 5.0 mm.
Why “2.5 mm, 5.0 mm” Is Correct
IS 2720 (Part 16) asks for the CBR to be calculated at 2.5 mm and 5.0 mm penetration. Normally the 2.5 mm value is larger and is reported. If the 5.0 mm value comes out larger, the test is repeated; if it is still larger, the 5.0 mm value is reported.
| Penetration (mm) | Standard load (kg) | Standard load (kN) | Unit standard load (kg/cm²) |
|---|---|---|---|
| 2.5 | 1370 | 13.4 | 70 |
| 5.0 | 2055 | 20.2 | 105 |
| 7.5 | 2630 | 25.8 | 134 |
| 10.0 | 3180 | 31.2 | 162 |
| 12.5 | 3600 | 35.3 | 183 |
Standard loads for the CBR test (IS 2720 Part 16). They come from tests on a standard crushed stone; CBR = test load ÷ standard load × 100.
Why the Other Options Are Wrong
- A. 1.5 mm, 2.5 mm: 1.5 mm is not a standard reading point for working out CBR.
- B. 5.0 mm, 7.5 mm: Loads at 7.5 mm are read to plot the curve, but CBR is not reported at 5.0 and 7.5 mm.
- D. 3.5 mm, 5.0 mm: 3.5 mm is not a standard penetration for CBR.
Key Point
CBR is reported at 2.5 mm (normally) and 5.0 mm penetration; standard loads 1370 kg and 2055 kg.
Key Concepts for Students
- The plunger is 50 mm in diameter and moves at 1.25 mm per minute.
- Soaked CBR (4 days in water) is used for design in wet conditions.
- A higher subgrade CBR means a thinner flexible pavement is needed.
Practice more: this is question 1 in Shallow and Deep Foundation MCQs – Part 1. Try the whole set and check every answer instantly.