The shear strength and stiffness modulus are accepted indicators of the susceptibility of the soil to permanent deformation. A soil with high values of both of these characteristics will be less susceptible to permanent deformation. Both are usually reduced by increases in moisture content. Knowledge of them is essential with the pavement design process in order to determine the required thickness of the pavement.
It is not usually always feasible to establish these two parameters for a soil. So, the CBR test is often used as an index test (in place of them).
The CBR test acts as an attempt to quantify the behavioural characteristics of a soil trying to resist deformation when subjected to a locally applied force such as wheel load. The test does not measure any fundamental strength characteristics of the soil. It involves a cylindrical plunger being driven into a soil at a standard rate of penetration, with the level of resistance of the soil to this penetrative effort being measured. The test can be done on site or in the laboratory
If the test is done in the laboratory, it is important that the moisture content and dry density of the sample being tested should approximate as closely as possible those expected once the pavement is in place. All particles greater than 20 mm in diameter should first be removed.
If done insitu, the test should be performed on a newly exposed soil surface at such a depth that seasonal variations in moisture content would not be expected (see BS 1377, BSI, 1990).
At the start of the test, the plunger is seated under a force of 50 N for a soil with an expected CBR value of up to 30% or 250 N for an expected CBR greater than 30%. It then proceeds to penetrate the soil specimen at a uniform rate of 1 mm/min. For every 0.25 mm of penetration up to a maximum of 7.5 mm, the required loading is noted.
- A graph of force versus penetration is plotted and a smooth curve drawn through the relevant points.
- These values are compared against the standard force-penetration relationship for a soil with 100% CBR.
- The CBR is estimated at penetrations of 2.5 mm and 5 mm. The higher of the two values is taken.
Table 1 shows the standard force-penetration relationships for a sample with 100% CBR value.
Table 1; Standard force-penetration relationships for a sample with 100% CBR
| Penetration (mm) | Load (kN) |
| 2 | 11.5 |
| 4 | 17.6 |
| 6 | 22.2 |
| 8 | 26.3 |
| 10 | 30.3 |
| 12 | 33.5 |
We are looking for the load at penetrations of 2.5 mm and 5mm. since these values are not available in the Table 1, we can only get them by interpolation
For 2.5 mm
| Penetration (mm) | 2 | 2.5 | 4 |
| Load (kN) | 11.5 | x | 17.6 |
(x-11.5)/ (17.6-11.5) = (2.5-2)/ (4-2) » 2 (x – 11.5) = 3.05, then x = 13.02 kN.
For 5 mm
| Penetration (mm) | 4 | 5 | 6 |
| Load (kN) | 17.6 | x | 22.2 |
(x-17.6)/ (22.2-17.6) = (5-4)/ (6-4) » 2 (x – 17.6) = 4.6, then x = 19.9 kN.
Modifying Table 1 above, we have
| Penetration (mm) | Load (kN) |
| 2 | 11.5 |
| 2.5 | 13.02 |
| 4 | 17.6 |
| 5 | 19.9 |
| 6 | 22.2 |
| 8 | 26.3 |
| 10 | 30.3 |
| 12 | 33.5 |
If in a laboratory test on some sample, we have 8.2 kN at 2.5 mm penetration and 13.0 kN at 5 mm penetration, then the CBR values would be calculated as follows (measured against the standard values presented in modified Table 1 above).
At 2.5 mm penetration for our sample, load = 8.2 kN
At 2.5 mm penetration for standard sample sample, load = 13.02 kN
CBR = (8.2 x 100)/13.02 = 63%
At 5.0 mm penetration for our sample, load = 13.0 kN
At 5.0 mm penetration for standard sample, load = 19.9 kN
CBR = (13.0 x 100)/ 19.9 = 65.3% ≈ 65%
Note
The larger of the two values is taken as the CBR value, hence, 65%
CBR values are rounded off as follows (for CBR ≤ 30%, round off to nearest 1%; for CBR ˃ 30% round to the nearest 5%).
